VOLUME II
OCCURRENCE AND CHARACTERISTICS
OF GROUND WATER IN
THE ROCKY MOUNTAINS REGION, MONTANA
Roger A. Noble, Robert N
Brenda C. Sholes
Montana Bureau of
Bergantino, Tom Patton
and Faith Daniel
Mines and Geology
Report to
U, S. ENVIRONMENTAL PROTECTION AGENCY
Contract. Number GO-082-908-10
Project Officer
William E. Engle
January 1, 1982
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MONTAilA or'r.'j OF
v ED OCT/
preface PBBMX7-S33T TO
EiV»—id
This report, "Aquifer Characterization of Montana", is a two-volume
study; volume one has been compiled for the Great Plains physiographic
province and volume two for the Rocky Mountains physiographic province of
Montana. Because of the complex structural geology of Montana, this division
is necessary in order to describe the various aquifers that occur in each of
the physiographic provinces. This report contains descriptions of thickness,
yield, structural configuration and water quality data for the major aquifers
within each province.
These two volumes contain a comprehensive compilation of existing hydro-
geologic information for the State. Because statewide hydrogeologic investiga-
tions have only recently begun in Montana, there are many data gaps, especially
for the deeper aquifers. Consequently, some information is still conjectural.
Demands on Montana's ground water are expanding because of increasing energy
development and agricultural requirements, especially irrigation. For new
developments, ground water is the only alternative left, as most of Montana's
surface waters are already over-appropriated.
Montana is currently quantifying its water use and consumption through a
water-right adjudication program. This program is being implemented by the
Department of Natural Resources and Conservation through Senate Bill No. 76.
The completion date for the adjudication program is April 30, 1982; therefore,
quantitative statistics for Montana's ground-water use will not be available
until after this date.
The study, "Aquifer Characterization of Montana", was funded by the U. S.
Environmental Protection Agency through Contract No. GO-082-908-10, for the
Underground Injection Control Program. The Safe Drinking Water Act (Public Law
93-523) was enacted by Congress for the purpose of protecting underground
ii
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sources of water from contamination by well injection. This act mandated the
U. S. Environmental Protection Agency to establish the Underground Injection
Control Program to prevent underground injections which endanger ground-water
resources. The Montana Bureau of Mines and Geology's participation in the
Underground Injection Control Program involves the identification and charac-
terization of aquifers for the State of Montana.
iii
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TABLE OF CONTENTS
CHAPTER PAGE
PREFACE i.i
GENERAL STATEMENT 1
Purpose and Scope 1
Description of Montana 1
Previous Investigations 5
I. INTRODUCTION TO THE GREAT PLAINS REGION 6
Physiography 7
Topography 7
Surface Drainage 9
Climate 12
Cultural Geography .-v
Population V
Land Use and Ownership
V "
eulogy '
Stratigraphy . 20
Structure ... 23
II. HYDROGEOLOGY BY AQUTFF.RS 25
Quaternary Unconsolidated Deposits 25
Tertiary Valley-Fill Sediments 28
The Tobacco Plains 33
Kalispell Valley 35
Swan Valley 38
Mission Valley 4 0
Little Bitterroot Valley 43
Missoula-Ninenule Valley 45
Blackfoot Valley 47
Prickley Pear Basin (Helena Valley) 50
Bitterroot Valley 56
Deer Lodge Valley 59
Townsend Valley 63
Three Forks Basin 67
Cold Spring Valley (North Boulder) 71
Little Whi.teta.il and Jefferson Valleys 74
Melrose and Beaverhead Valleys 77
Madison Valley 81
Emigrant Valley 84
Centennial Valley 87
Consolidated Sedimentary Rocks 91
Metamorphic and Igneous Rocks 93
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TABLE OF CONTENTS
PAGE 2
III. GROUND-WATER USE 95
Agriculture 97
Municipal and Domestic 98
Industry 100
IV. WATER QUALITY 102
Data Sources 102
General Water Quality 103
Cenozoic Basin-Fill Deposits 105
Early Tertiary through Precambrian
Consolidated Sedimentary Rocks 107
Metamorphic and Igneous Rocks 108
V. SUMMARY AND CONCLUSIONS 112
VI. REFERENCES 112
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LIST OP FIGURES
Figure Page
11 -1. Major Drainage Basins 4
11 - 2. Mountain Ranges and Intermontane Valleys of the
Rocky Mountains region S
11-3. Mean Annual Runoff of Major Streams 10
II-4. County Census Subdivisions IS
[1-5. Stratigraphic Column 22
II-6. Generalized tectonic map of the Rocky Mountains region . . 24
IT-7. Cenozoic Basins Evaluated 32
11-8. [sopach of Cenozoic fill in the Tobacco Plains 34
11-9. Isopach of Cenozoic filL in the Kalispell Valley .... 36
11-10. fsopach of Cenozoic fill in the Swan Valley 39
11-11. Isopach of Cenozoic fill in the Little Bitterroot and
Mission Valleys 42
11-12. Isopach of Cenozoic fill in the Missoula-Ninemile Valley . 46
11-13. Isopach of Cenozoic fill Ln the Blackfoot Valley 49
11-14. Isopach of Cenozoic fill in the Prickly Pear Basin .... 52
11-15. Isopach of Cenozoic fill in the Bitterroot Valley .... 58
11-16. Isopach of Cenozoic fill in the Deer Lodge Valley .... 61
11-17. Isopach of Cenozoic fill in the Townsend Valley 64
11-18. Isopach of Cenozoic fill in the Three Forks Basin .... 69
11-19. Isopach of Cenozoic fLJL in the Cold Spring Valley ... 72
IF -20. Isopach of Cenozoic fill in the Little Whitetail and
Jefferson Valleys 75
11-21. Isopach of Cenozoic fill in the Melrose and Beaverhead
Valleys 79
11-22. Isopach of Cenozoic fill in the Madison Valley .... 82
11-23. Isopach of Cenozoic fill in the Lmigrant Valley .... 85
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LIST OF FIGURES PAGE 2
11-24. Isopach of Cenozoic fill in the Centennial Valley .... 88
11-25. Frequency of occurrence versus dissolved solids for waters
from Quaternary and Early Tertiary unconsolidated deposits,
Rocky Mountain Region, Montana 106
11-26. Frequency of occurrence versus dissolved solids for waters
from Early Tertiary through Precambrian consolidated rocks,
Rocky Mountains Region, Montana 109
Tl-27.
Frequency of occurrence versus dissolved solids for waters
from igneous and metamorphic rocks, Rocky Mountain Region,
Montana Ill
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1,1ST OF TABLES
Table Page
I-L. Drainage Area in Montana 3
II-2. River Basin Inflow and Outflow 11
I1-3. Population of Counties and County Subdivisions 15
11 - 4 . Well Use by County 96
11-S. Comparison of Selected Elements and Ions in Waters of
the Rocky Mountains Region, Montana, to Drinking Water
Quality Standards 104
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LIST or PLATHS
Plate
DS 100.10 W Quaternary and Late Tertiary Unconsolidated Sediments
DS 100.50 W Cenozoic Basin-Fill
TF 100.50 W Cenozoic Basin-Fill, IV 1/2
DS Consolidated Sedimentary Rocks
DS Metamorphic and Igneous Rocks
Legend
TF - Thickness of Formation
DS - Dissolved Solids
211.11 - Format]on Code
W - Western Half of Montana's 1:500,000 scale map
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GENERAL STATEMENT
A. Purpose and Scope
This report was prepared by the Montana Bureau of Mines and Geology in
order for the State of Montana to comply with federal requirements relating
to the Underground Injection Control Program. Existing hydrogeologic data
were used for the aquifer characterization maps and the descriptive narrative.
The aquifer characterization maps depict: (1) the areal and subareal extent:
(2) surface configuration; (3) thickness; (4) potentiometric surface; and
(5) water chemistry, expressed as dissolved solids, for the major aquifers in
Montana. The narrative describes the lithology, general hydrogeologic parameters
and potential well yields for individual aquifers. The inventory of injection
wells was compiled from information obtained from the Montana Oil and Gas
Commission. The inventory provides a listing of injection wells with locations,
owners, affected aquifers and injection rates. The report also contains a
section delineating well use by county. While broad in scope, this report is
designed to meet the needs of federal regulatory agencies responsible for
writing and implementing regulations for underground injection.
B. Description of Montana
Montana, the third largest state of the forty-eight contiguous United States,
is vast and diverse. It has an area of 147,138 square miles and a population of
786,690 (U.S. Dept. of Commerce, 1980); the average population density is 5.4
people per square mile. Most Montanans live in the major cities that are geo-
graphically dispersed throughout the state. These cities are supported by the
surrounding rural communities. Although Montana is sparsely populated, it is
rich in natural, resources and is a prime producer of agricultural staples for
the nation. Montana's abundant natural resources include fossil fuels, minerals,
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timber and water. These resources, however, are either fully appropriated or
are being exploited rapidly.
In 1980, Montana's low-sulfur coal reserves were estimated to be in excess
of 120 billion tons (U.S. Bureau of Mines, 1980). These coal deposits of the
Fort Union Formation are easily accessible through strip-mining procedures and
supply a substantial part of needed energy for the nation. Total coal production
for 1980 was 29,905,627 tons (Cole and others, 1981), of which 90 percent was
exported to other states. Montana also has projected oil reserves of 248 trillion
barrels, an undetermined reserve of natural gas and unknown potential for uranium
resources (Montana Dept. of Natural Resources and Conservation, 1980).
Montana's mineral resources are of great economic importance to the state.
Montana ranks among the top five states in the production of antimony, silver,
copper, talc, vermiculite and bentonite (U.S. Dept. of Interior, 1979). In
addition to these commodities, Montana has significant deposits of lead, zinc,
tungsten, chromium, manganese, nickel, titanium, vanadium, platinum-group metals,
molybdenum, arsenic, iron, antimony, thorium and other rare earths. Metallic
and non-metallic exploration activity in the state is increasing every year.
Most of western Montana is heavily forested and most of these forests lie
within designated state and national forests or parks. Timber harvesting occurs
on selected tracts within these forests and on privately-owned land. The volume
of timber harvested in Montana from 1976 to present (1982) has decreased because
high mortgage rates have substantially reduced the number of buildings being
constructed.
Montana's water, both from ground-water reserves and surface-water flow,
is one of the state's most valuable resources because it is vital to agriculture,
mining and power production. More than forty-three million acre-feet of water
flow from the state each year; 65 percent of it originates in Montana (Montana
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Department of Natural Resources and Conservation, 1976) . Three major river
basins, the Columbia, Upper Missouri and Yellowstone, account for 97 percent of
this flow. Statistics concerning the drainage areas of the major river basins
are presented in Table 1-1 with the major drainage basins displayed in Figure II-l.
TABLE II-l
DRAINAGE AREA IN MONTANA
River Basin Area (sq. mi.) Percentage of Percentage of
Montana's Area Montana's Water
Columbia 25,152 17% 59%
Upper Missouri 82,352 56% 17%
Yellowstone 35,890 24% 21%
Little Missouri 3,428 2% 1%
St. Mary 648 1% 2%
147,470 100% 100%
Of the fifteen million acres of cropland in production in the state, 12.5
million acres are dryland and the remainder are irrigated. Montana's major
water use is the irrigation of these 2.5 million acres of cropland from both
surface-water and ground-water diversions. Agricultural demands, hydro-electric
generating facilities and instream-flow reservations have already claimed most
of the surface water. This surface-water demand has resulted in over-appropria-
tion of these waters, placing additional demands on ground-water resources.
Sources of potable ground water in certain areas are now limited.
For the purpose of this report, the state has been divided into the Rocky
Mountains region and the Great Plains region. Because geology, climate and
aquifer characteristics of the Great Plains region are significantly different
from those of the Rocky Mountains region, this natural physiographic division
was used to facilitate the aquifer descriptions in this report. The line sep-
arating the two divisions is not precisely the same as that used by geographers
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MAJOR DRAINAGE BASINS
FIGURE 11-1
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because it follows the eastern edge of rocks that were severely disturbed by
the Laramide Orogeny rather than the actual mountain front except where the
two coincide. The following is a compilation of data for each of the major
aquifers of the Great Plains region.
C. Previous Investigations and Sources of Information
The collection of data for this report was made possible by the cooperation
of the U. S. Geological Survey, especially Richard D. Feltis and William R.
Hotchkiss, who furnished essential information on particular aquifer units.
Other data were compiled from oil well logs and the Montana Oil and Gas Commission,
various Montana Bureau of Mines and Geology and U. S. Geological Survey publica-
tions, numerous theses and dissertations and unpublished information generated
from water-well logs and records.
Water quality data in this report were obtained from Montana Bureau of Mines
and Geology files. Additional analyses were collected from the U. S. Geological
Survey.
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I. INTRODUCTION TO THE ROCKY MOUNTAINS REGION
The Rocky Mountains region of Montana is predominantly an area of rugged
mountain ranges and intervening valleys constituting the western one-third of
the state. This region extends from the eastern front of the Disturbed Belt
(a northwest-trending zone 25 miles wide where mountain-building forces deformed
the rocks, but did not result in a mountainous terrain) west to the Montana-Idaho
border. Latitude 49° north establishes the northern border, and the state
borders of Idaho and Wyoming with Montana delineate the region's southern extent.
Linear mountain ranges form the Continental Divide separating this region into
two major drainage basins. The headwaters of the Columbia River drain the
northwestern portion of this region, whereas the tributaries of the Missouri
River drain the southeastern portion. Most of the state's large-scale hydro-
electric generating sites arc .located along these major rivers or their
tributaries. Although the rugged mountains are especially scenic features within
this region, the intermontane valleys or basins are important areas for habita-
tion, agriculture and ground-water usage.
The economy of the Rocky Mountains region is based on forest products, mining,
smelting, agriculture, governmental and educational activities and light industry.
Glacier and Yellowstone National Parks and numerous designated wilderness areas
account for a substantial amount of the seasonal tourism. Oil and gas explora-
tion is increasing rapidly, with the current interest generated along Montana's
Overthrust Belt.
These economic operations have already placed a significant demand on the
region's water resources. As the population continues to grow, additional sources
of potable water will be needed. Because of the geologic nature of this region,
there exist only limited areas suitable for ground-water development. Volume II
of this report is an examination of: the occurrence and characteristics of ground
water in Montana's Rocky Mountains.
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PHYSIOGRAPHY
Topography
The Rocky Mountains region of Montana covers approximately 54,750 square
miles of the Northern Rocky Mountains physiographic province or roughly the
western one-third of Montana. The region consists of contrasting steep mountain
slopes and flat river valleys which often contain some well-defined terraces,
but relatively few foothills or prairie expanses. About three-fourths of the
region is occupied by a series of 40 or more individual mountain ranges that
are 25 to 75 miles long. In general, the mountain fronts rise abruptly from the
valley floors to peaks which vary in altitude from 10,448 feet above sea level
at Mount Cleveland in northern Glacier National Park to 12,799 feet above sea
level at the summit of Granite Peak in the Beartooth Mountains, northeast of
Yellowstone National Park. Floors of the intermontane valleys which separate
the mountain ranges lie at elevations ranging from 2,800 to 6,700 feet above sea
level (the Mission and Centennial valleys, respectively). These valleys, generally
containing a river, may be from a few to as much as 20 miles wide and 10 to 50
miles long. Elevations in the Rocky Mountains region range from 1,825 feet above
sea level, where the Kootenai River flows out of the state, to 12,799 feet above
sea level at Granite Peak. The locations and names of many of the mountain
ranges and intermontane valleys are shown in Figure II-2. Terraces or pediments
often adjoin the mountain fronts.
With some exceptions, the mountains of western Montana are dominantly com-
posed of metasedimentary rocks of Pcecambrian age; marine sandstones, shales and
carbonate rocks of Paleozoic and Mesozoic age; marine strata of Jurassic and
Cretaceous age; and andesitic volcanic rocks of late Cretaceous and early
Tertiary ages. The Boulder Batholith and its associated satellites in the
center of the region are principally composed of quartz monzonite of Cretaceous
age. The intermontane basins have been filled with Tertiary and Quaternary
7
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M rip of urvtcrn Monlnnu nml mlj.uvnt nn-np, h ! i o w I n l,' relations of the mountain rnn^'os and Jntermontnnc vnlleyR (stippled):
J, I'urcell Trent Ji . l'l:i Micad Vnlley. KalKpell Valley; 4, Mule Ulttrroot Vnllry ; 5, Cnmuft Prairie Basin , G, MlsHlon Valley; 7, Jocko
Valley: S, Blnekfuot "\ alley; Missoula Valley; 10, Cjuikih l'rnlric , II, Nevada Valley; 12, Hltterroot Valley; 13, Flint Creek Valloy ; ]4,
Avon Valley ; 1 fi. l'rlckly lVnr Vullcy; l(i, l'li II Ipsbu rg Vnlley; 17, Di'Cr Lodtfc Valley; IS, Townsend Valley; 10, .Smith Klvor Valley;
20, Silver How Valley; 21, I'.iv; Hole Uusln , 22, \ Spuml l'jirk, 2H, JcfforHon Valley; 2-1, Gallatin Valley; 25, lleiiverliend Vnlley, 20, Madison
Valley , 27, Centennial Vallej
MOUNTAIN RANGES AND INTERMONTANE VALLEYS OF THE ROCKY MOUNTAINS REGION
FIGURE 11-2
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sediments derived from the surrounding mountains.
Mountain glaciers extended over most of the region, leaving either some
various glacial deposits or erosional features. These glaciers were responsible
for such erosional features as the jagged peaks and U-shaped valleys in Glacier
National Park and the Beartooth Mountains. They also produced the hummocky kame
and kettle topography around Ovando and the low hills (which are moraines) in
the Kalispell area. Many of the mountain lakes which dot mountain slopes and
valley plains mark the occurrence of glacial activity. Because of the geologic
diversity, topographic variability and structural complexity of this region,
isopach, potentiometric surface and structural configuration maps could be
produced only for specific aquifer units.
Surface Drainage
Three major river systems in North America have their origins along the
Continental Divide in the Rocky Mountains region of Montana. These three river
systems are the Columbia, Missouri and Saskatchewan Rivers. The common point
of juncture for these rivers is located at Triple Divide Peak in Glacier National
Park. Because of the westward deflection of the Continental Divide, most of the
drainage area for the Columbia River occupies the northwest portion of the
region, while the watershed of the Missouri River lies within the southeastern
portion of the region. Tributaries of the Saskatchewan River drain only a small
percentage of the region located in Glacier National Park. The mean annual runoff
of the major streams for the Rocky Mountains region is presented schematically in
Figure II-3, with a breakdown of drainage basin inflow and outflow values shown
in Table II-2.
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MEAN ANNUAL RUNOFF OF MAJOR STREAMS
Width of stream line corresponds to top width of channel. Mean annual discharge, in
thousands of cubic feet per second, is represented by channel cross section.
FIGURE 11-3
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TABLE 11-2
RIVER BASIN INFLOW AND OUTFLOW (TN ACRE-FEET)
Drainage
Inflow
Originating in Leaving Percentage Originat-
the region the region ing in the region
Clark Fork
Kootenai
Missouri
Hudson Bay
Yellowstone
694,800 15,515,200
6,600,000 2,510,000
0 4,913,000
0 510,000
2,259,000 475,000
16,210,000
10,110,000
4,913,000
510,000
2,734,000
96
25
100
100
17
The Columbia River Basin comprises all land in Montana west of the Contin-
ental Divide. This area has a substantial volume of surface water compared with
its total land area. While containing only 17 percent of the land mass of
Montana, this basin is the source of 59 percent of the state's total surface
water outflow. The Clark Fork River and the Kootenai River are the two major
tributaries of the basin. The Clark River heads in Silver Bow basin south of
Butte, originating as Silver Bow Creek. The Clark Fork River joins other major
tributaries, the Bitterroot, Blackfoot and Flathead Rivers at their confluences
to become the Pend d'Oreille River in Idaho. A small area in the northwestern
corner of the region adds to the watershed of the Kootenai River. The Clark Fork
has an average annual flow of 16,210,000 acre-feet per year near Cabinet, Idaho,
at the Montana-Idaho border as compared to the Kootenai's average outflow of
10,110,000 acre-feet per year at Leonia, Idaho. Major tributaries of the Kootenai
River are the Yaak and Fischeries Rivers.
The Columbia River Basin has almost 12 million acre-feet of storage. Lake
Koocanusa is the largest reservoir with a storage capacity of 5,850,000 acre-feet.
Hungry Horse Reservoir and Flathead Lake are the other major storage sites, with
capacities of 3,468,000 and 1,791,000 acre-feet of total storage, respectively.
The Missouri River Basin drains the eastern slopes of the Continental Divide.
At the eastern border of the Rocky Mountains region, the Missouri River has a
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drainage area slightly smaller than the Columbia River, yet has about 1/3 of
the Columbia River's average discharge. The Missouri River proper begins at
the confluence of the Jefferson, Madison and Gallatin Rivers below Three Forks.
Canyon Ferry, with a total storage capacity of 2,051,000 acre-feet on the main-
stream of the Missouri River, is the largest reservoir in this portion of the
region.
The Hudson Bay drainage in Rocky Mountains region consists primarily of
the St. Mary River and its tributaries draining the northeast corner of Glacier
National Park. The river flows northward to join the Saskatchewan River in
Canada.
A portion of the Yellowstone River's watershed arises also in the south-
eastern corner of the Rocky Mountains region. Only a few hundred cubic feet
per second are produced from its small watershed.
Climate
Because western Montana has a great amount of topographic variation, it
also has a great variety of climate. This climate diversity is such that most
small-scale climate maps show western Montana as having merely a "highland"
climate. More detailed climate maps using the Ko'ppen classification system
would show that the intermontane valleys have a "steppe" climate (BSk) , that
the mountains would have various microthermal or "snow forest" climates such as
Dbf, Dbs, Dcf and Dcs and that the summits of the higher mountains would have
"tundra" climates (ET). The Thornwaite climate classification shows the climate
of the intermontane valleys and some of the mountains in southwestern Montana
to be, "subhumid, microthermal, precipitation deficiency in all seasons," (cc'd).
The mountains of northwestern Montana and those in the southeastern part of the
Rocky Mountains region are shown to have the following climates: subhumid,
microthermal, precipitation adequate in all seasons, (cc'r); humid, microthermal,
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precipitation adequate in all seasons (Bc'r) and taiga (d1). Not shown on the
Thornwaite map, but present on many of the higher peaks, would be "tundra" (E').
That a map has yet to be prepared for Montana accurately showing the extent of
these climatic provinces is not surprising, considering that many other factors
besides topography are important in determining mountain climates. Some of
these other factors are: rainshadow effect, direction and strength of the pre-
vailing wind, slope angle, air drainage, latitude, longitude, valley width and
valley orientation. Long-term temperature and precipitation records are generally
available only for major cities, most of which are located in intermontane valleys.
Supplemental climate data from snow survey sites on the mountains are now adding
greatly to the understanding and quantification of the climate of western Montana.
Climate records show that the valleys of extreme west-central Montana have
the warmest July temperatures. July average maximum temperatures of 88°F occur
at Thompson Falls along the Clark Fork River. Warm July temperatures in this
part of the state result from long, clear days, lower altitudes and reflective
heating from the north valley slopes (south-facing slopes). The July average
maximum in Butte is 80°F and at West Yellowstone, 75°F. Maximum shade tempera-
tures on mountain summits in July often average 65°F or less. Average minimum
temperatures in July are as low as 40°F in northwestern Montana and the high
valleys of southwestern Montana. Because of the low humidity, radiative cooling
begins as soon as the sun sets. January average minimum temperatures are gen-
erally as much as 15°F warmer than those in northwestern Montana and generally
keep the temperature more moderate compared to the Great Plains region where
Arctic air masses are dominant in winter. Extreme low temperatures occur in
western Montana when Arctic air masses spill across the mountain barriers.
Average annual precipitation amounts in western Montana range from less
than ten inches in the intermontane basins of southwestern Montana to more than
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100 inches in Glacier National Park. Precipitation amounts generally increase
with altitude except where severe rainshadow conditions exist on the lee side
of the mountains. Monthly weather records show that two precipitation maxima
occur in western Montana, one in mid-winter and the other in late May to early
June. Average annual snowfall amounts range from 25 inches in the area near
Townsend (along Canyon Feri:y/Reservoir) to more than 1,000 inches on the summits
in Glacier National Park. The average snowfall in most of the major cities is
between 40 and 90 inches. The snowpack on the mountains of western Montana acts
like a great storage reservoir for the many streams that have their headwaters
in this area. Melting snows release water slowly to these streams, keeping
them flowing long after the late spring rains have ceased.
CULTURAL GEOGRAPHY
Population
The Rocky Mountains region encompasses roughly one-third of Montana's
land area, yet one-half of the state population inhabits the region. According
to 1980 census figures, 393,625 persons are living in the region, yielding a
population density of 7.12 persons per square mile. Because the region is pre-
dominantly rugged mountain ranges, approximately 90 percent of the people live
in the intervening valleys. Major cities exceeding 10,000 people account for
37 percent of the region's population. The 1980 census defined a city as an
incorporated place and according to this classification the major cities in order
of their size are: Butte, Missoula, Helena, Bozeman, Anaconda and Kalispell.
The population distribution for the Rocky Mountains region is summarized in
Table II-3 with the county census subdivisions represented in Figure II-4.
Butte is the largest incorporated place within the region attributing its
size to a prosperous copper mining industry of the past. Butte typifies a boom
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TABLE II-3
POPULATION OF COUNTIES AND COUNTY SUBDIVISIONS
OF THE ROCKY MOUNTAINS REGION, MONTANA
County/County Subdivision 1980 1970 % Change
Beaverhead County
8,186
8,187
Big Hole Basin Division
740
Clark Canyon-Horse Prairie Division
426
Dillon Division
6,567
Lima-Centennial Valley Division
453
Broadwater County
3 ,267
2,526
29.3
Townsend East Division
2 ,522
2,016
25.1
Townsend West Division
745
510
46.1
Cascade County
80,696
81,804
- 1.4
Sun River Valley Division
3 ,258
2,558
27.4
Deer Lodge County
12 ,518
15,652
-20.0
Anaconda Division
10,403
Deer Lodge Valley Division
2 ,115
Flathead County
51,966
39,460
31.7
Bad Rock-Columbia Heights
Division
2 ,793
Columbia Falls Division
6,574
Creston-Bigfork Division
4,114
2,315
77.7
Glacier Division
105
153
-31.4
Kalispell Division
22 ,860
Kalispell Northwest Division
1,939
Kalispell Southwest Division
2,700
Lower Valley-Somers Division
1,183
South Fork Division
2,000
Whitefish Division
7 ,698
Gallatin County
42 ,865
32,505
31.9
Belgrade Division
5 ,884
Bozeman Division
28,604
Gallatin Gateway Division
1,949
Manhattan Division
3,057
2,448
24.9
Three Forks Division
1,997
1,839
8.6
West Yellowstone Division
1,374
1,099
25 .0
Glacier County
10,628
10,783
- 1.4
Blackfeet Division
6,039
Glacier National Park Division
49
Granite County
2 ,700
2,737
- 1.4
Drummond Division
1,092
1,141
- 4.3
Philipsburg Division
1,608
1,596
0.8
Jefferson County
7 ,029
5,238
34 .2
Boulder Division
4,5.18
3,350
34.9
Whitehall Division
2 ,511
1,888
33 .0
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TABLE II-3 (Continued)
POPULATION OF COUNTIES AND COUNTY SUBDIVISIONS
OF THE ROCKY MOUNTAINS REGION, MONTANA
County/County Subdivision 1980 1970 % Change
Lake County
19,056
14,445
31.9
Big Fork-Swan Division
1,998
Charlo Division
1,242
1,111
11.8
Poison Division
7,492
Ronan Division
4,875
St. Ignatius Division
3,449
2,797
23.3
Lewis and Clark County
43,039
33,281
29.3
Augusta Division
847
854
- 0.8
Helena Division
38,853
Lincoln Division
2,234
Wolf Creek Division
1,105
Lincoln County
17,752
18,063
- 1.7
Eureka Division
3,727
3,558
4.7
Libby Division
10,960
12,045
- 9.0
Troy Division
3,065
2,460
24.6
Madison County
5,448
5,014
8.7
Harrison Division
762
800
- 4.8
Madison Valley Division
1,466
1,179
24.3
Sheridan Division
1,525
1,337
14.1
Twin Bridges Division
1,387
1,437
- 3.5
Virginia City Division
308
261
18.0
Meagher County
2,154
2,122
1.5
Martinsdale-Ringling Division
377
White Sulphur Springs Division
1,777
Mineral County
3,675
2,958
24.2
Alberton Division
587
600
- 2.2
Superior Division
2,126
1,580
34.6
West End Division
962
778
23.7
Missoula County
76,016
58,263
30.5
Frenchtown-Enaro Division
3,665
1,547
136.9
Lolo Division
4,871
1,747
178.8
Missoula Division
65,476
Seeley Lake-Blackfoot Valley
Division
2,004
1,201
66.9
Park County
12,660
11,197
13.1
Gardiner-Cooke Division
860
845
1.8
Shields Valley Division
1,471
Upper Yellowstone Valley
Division
10,329
16
-------
TABLE II-3 (Continued)
POPULATION OF COUNTIES AND COUNTY SUBDIVISIONS
OF THE ROCKY MOUNTAINS REGION, MONTANA
County/County Subdivision 1980 1970 % Change
Pondera County
Blackfeet West Division
Powell County
Avon-Elliston Division
Deer Lodge Division
Helmville Division
Ravalli County
Darby Division
Hamilton Division
Stevensville Division
Sula-Edwards Division
Victor Division
Sanders County
Flathead Division
Plains Division
Thompson Falls-West End Division
Silver Bow County
ButteiDivision
Silver Bow Northwest Division
Silver Bow South Division
Toole County
South Toole Division
Sunburst Division
Yellowstone County
Yellowstone National Park Division
6,731 6,611 1.8
473
6,958 6,660 4.5
1,002 1,018 - 1.6
5,473
483
22,493 14,409 56.1
1,718
11,467
6,516
950
1,842
8,675 7,093 22.3
1,887 1,907 - 1.0
2,553 1,938 31.7
4,235 3,248 30.4
38,092 41,981 - 9.3
36,817
491
784
5,559 5,839 - 4.8
3,932
1,627 1,904 -14.5
108,035 87,367 23.7
275 64 329.7
17
-------
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COUNTY CENSUS SUBDIVISIONS
FIGURE 11-4
18
-------
town which had a population of over 100,000 around the turn of the century and
has now receded to 35 percent of that early-day size. This trend in population
decline roughly parallels the decline in employment for copper production.
According to Rand-McNally's 1977 Atlas, Butte has a basic trading area of
91,300 people.
Missoula and Helena are respectively the second and third largest incor-
porated places of the region, however, Missoula's metropolitan area now exceeds
80,000. Missoula initially owed its growth to the timber industry; once being
the home of five major lumber mills for the area. The population of this city,
however, has increased 13.2 percent since 1970 as small industry and the University
of Montana expand. Serving in excess of 118,000 residents, Missoula has the
largest basic trading area in the Rocky Mountains. Helena is the capital of
Montana and is dominantly supported by governmental employees.' Its population,
though, continues on an upward trend.
The overall population of the Rocky Mountains has increased 16.6 percent
since the 1970 census, yet particular counties deviate greatly from this trend.
An example of this is the 56.1 percent increase of Ravalli County which is due
to an influx of people throughout the Bitterroot Valley. The other extreme is
the 9.3 percent decline of Silver Bow County due to a depressed copper mining
industry.
Land Use and Ownership
The primary land use in the Rocky Mountains region is forest. Roughly
one-half of this region is considered forest land of which 60 percent is class-
ified as commercial forest. However, timber harvesting has been reduced due to
the associated decline in construction. Most of the forest land in the region
is under federal control and managed by the U. S. Forest Service of Bureau of
19
-------
Land Management. The remainder is either under state jurisdiction or private
ownership.
The next largest land use is rangeland for livestock grazing. Rangeland
consists of grazeable forest land, tame pasture and native rangeland. These
lands are vital not only for cattle and sheep production, but are grazed also
by big game and other wildlife. Approximately 35 percent of total land area of
the Rocky Mountains region is used for rangeland and is owned dominantly by
private individuals or corporations.
Agricultural cropland accounts for approximately eight percent of land use
of the region. This land use includes irrigated and non-irrigated cropland and
irrigated and non-irrigated pasture land. With agriculture being ranch oriented,
hay production is the major use of cropland acreage. Most of this acreage occurs
along the flat and gentle slopes of the river bottom land and terraces and along
the foothills of the mountains. Nearly all cropland is privately owned.
The remainder of land use for the Rocky Mountains region spans from
recreation and wildlife refuges to community facilities. Indian lands, national
parks and wilderness areas comprise the majority of land acreage, while municial-
ities and subdivisions are increasing rapidly. For the most part, the former
areas are federally owned and the latter are under private ownership.
GEOLOGY
Stratigraphy
A composite stratigraphic section ranging in age from Precambrian to
Holocene is present in the Rocky Mountains region of Montana. These formations
constitute many of the rugged mountains of western Montana.
The oldest rocks are the gneisses and schists of the early Precambrian
20
-------
Era (1.7 billion years old) of southwestern Montana. The majority of the bed-
rock formations exposed are the Precambrian metasediments of the Belt Series
which dominantly cover the northern half of the region. These formations were
mainly shelf and marginal shelf sediments deposited in a geosyncline. They were
then altered by burial metamorphism to argillites and quartzites of their present
configuration. Often they exist as assemblages tens of thousands of feet thick.
Seas again spread over the area during the Paleozoic Era depositing the
shales and limestones of the Cambrian period. A temporary hiatus occurred during
the Ordovician until mid-Devonian leaving a stratigraphic gap. Widespread
shallow seas then deposited carbonates from late Devonian through Mississippian
time. Fractures in the Madison limestone of this period serve for both recharge
and sources of ground water in bedrock aquifers. Tectonic activity accompanied
the invasion of Pennsylvanian and Permian seas, producing the clastic sediments
of those periods. During the Jurassic and Cretaceous times, seas again moved
into the region leaving alternating transgressive and regressive sedimentary
sequences. These Paleozoic and Mesozoic formations exist in large bedrock
assemblages of shales, carbonates and sandstones, with the carbonates and sand-
stones being the primary sources of ground water.
Tertiary and Quaternary deposits of the Cenozoic Era mainly occur in the
intermontane valleys. For the most part, they are fluvial sediments derived from
the surrounding mountains. These unconsolidated sediments serve as a primary
ground-water source for most municipalities throughout the region. Mountain
valley glaciers covered much of western Montana during the Pleistocene Epoch.
These glaciers produced till and outwash deposits which now form a veneer over
some valleys and mountain fronts and are sources of nominal amounts of ground
water. Figure T.I-5 portrays the generalized stratigraphic sections for the
Rocky Mountains region.
21
-------
F IGURE 11-5
-------
Structure
The structural geology of the Rocky Mountains region is extremely
complex and variable throughout the area. Deformation occurred in several
phases beginning in Late Cretaceous and extending to the-end of the Paleocene.
This deformation through folding, faulting and igneous activity has produced
the mountain ranges and valleys of western Montana. Particular ranges are
oriented so that they can be categorized into distinct geologic provinces.
Through a combination of geographic, structural and lithologic character-
istics, the region can be divided into three separate provinces. The northern-
most is the Belt province which lies north of the Montana Lineament and extends
to the eastward margin of the Disturbed Belt. The area is characterized by
northwest-southeast trending mountain ranges and accompanying high-angle normal
faults. Precambrian Belt sediments outcrop across most of the area, while
imbricate thrusts of Paleozoic and Mesozoic age comprise the Disturbed Belt.
Northwest-southeast trending intermontane valleys filled with Cenozoic sediments
are also characteristic of the region. Lying south of the Montana Lineament and
north of the Basement Province is the Batholithic Province. This area contains
numerous late Cretaceous and Tertiary igneous plutons, the best of which is the
Boulder Batholith. Extrusive igneous rocks and irregularly• shaped basins are
also abundant in this province. The Basement Province is the third province of
the region and is typified by its pre-Belt metamorphic assemblages. Northeast-
striking, high-angle faults in this area demonstrate the regional stress
orientation active during the Laranride Orogeny. The generalized tectonic map
showing these provinces for the Rocky Mountains region is represented in
Figure II-6. Because the Rocky Mountains region is so structurally complex and
active, it has been placed in seismic risk zones of 2 and 3, moderate and major
damage, respectively. Fault lines usually serve as good water conduits and are
occasionally tapped by wells.
23
-------
—^
0\ul BELT d\u\GLACIER "CTA"
\ V \ f\
v.
IGNEOUS INTRUSlVES
¦ PROVINCE BOUNDARIES
MOOIFIED FROM McMANNIS (1065)
46°
] 09°
YELLOWSTONE PARK
Generalized tectonic map of the Rocky Mountains region
Figure 11-6
24
-------
Since deformation of individual formations beyond meaningful aquifer units
has occurred, four separate hydrologic units were identified for the Rocky
Mountains region. The four units defined for the presentation are: consolidated
sedimentary rocks—of all geologic ages; Tertiary basin-fill deposits; and
Quaternary unconsolidated sediments. Also, because of the structural complexity
of the region, no attempt was made to construct isopach or structure contour
maps with the exception of Cenozoic basin-fill. Thicknesses of Cenozoic basins
were determined largely by interpretation of gravity data because of the
paucity of drill holes penetrating bedrock.
II. HYDROGEOLOGY BY AQUIFERS
QUATERNARY UNCONSOLIDATED SEDIMENTS
Alluvium and glacial deposits comprise most of the Quaternary unconsolidated
aquifers, while other aquifers consist of colluvium and terrace gravel deposits.
All of these sediments are composed of unconsolidated gravels, sands, silts and
clays. Water availability from these deposits is widely variable and is depend-
ent upon the characteristics of the deposits.
Alluvial aquifers border present-day streams. These aquifers consist of a
variety of sedimentary sequences such as pointbars of cross-bedded sands, gravel
lag deposits and finer-grained materials that form natural levees. The stream
is hydraulically connected to the alluvial aquifer and there exists a definite
surface-water--ground-water interaction between them. For the most part, the
alluvium is a water-table aquifer and ground-water movement normally follows the
topography in a downstream direction. An alluvial aquifer may also be a confined
or semi-confined system when clays!form impervious boundaries. Because these
aquifers adjoin a stream, they tend to have an elongated surface expression.
The increase in thickness and areal extent of an alluvial aquifer is usually
directly proportional to the stream's average annual discharge except where the
25
-------
stream is constricted by resistant geologic formations.
The transmissivity and storativity of alluvial aquifers may vary consid-
erably from one location to the next, reflecting depositional variations in the
sediments themselves. However, an alluvial aquifer imparts a stronger horizontal
than vertical conductivity. Transmissivity and storativity values for an alluvial
aquifer are generally large and these aquifers will produce yields of up to
1,500 gallons per minute (gpm). Recharge to alluvial aquifers in western Montana
is primarily from rainfall and snow-melt water, while additional amounts result
from irrigation return flows and influent streams. Wells, effluent streams,
evapotranspiration and leakage to underlying aquifers are the primary means of
discharge. The ground water in alluvial aquifers has a generally dissolved
solids content of around 350 mg/L and is highly sought for domestic and municipal
use. Alluvial aquifers are one of the most important sources of ground water in
the Rocky Mountains region.
Glacial aquifers are the other primary source of ground water among uncon-
solidated sediments. These Pleistocene-age deposits occur as till and glacio-
fluvial or lacustrine sediments that mantle bedrock and Tertiary sediments.
They range from a few to hundreds of feet thick, depending upon their location
and mode of deposition.
Because glacial till is a heterogeneous mixture of boulders, gravel and
sand within a matrix of silt and clay, it has a relatively low hydraulic con-
ductivity. Well yields from till are usually small and discharges range from
5 to 20 gpm. Near rock outcrops, the till contains an abundance of boulders and
gravels and progressively becomes more clay-rich farther from the outcrops. When
running water reworks the till, it sorts the glacial materials, removing the
finer-grained deposits. The remaining deposits often resemble alluvium, but
because of their mode of formation, are termed glaciofluvial deposits. These
deposits are paleodrainages that were once Pleistocene river channels. This
26
-------
type of deposit covers a substantial area near Kalispell. Although glaciofluvial
deposits may often be masked by a blanket of till deposited by an advance of
glacial ice, their linear form often is surficially expressed on aerial photo-
graphs. Well yields increase measurably in glaciofluvial deposits because
fluvial action has removed most of the silt and clay increasing porosity and
permeability to produce a highly conductive aquifer. Wells and ground-water
pits that have been completed in glaciofluvial deposits have yielded as much
as 1,500 gpm with only a few feet of drawdown throughout the irrigation season.
In areas where glacial drift is thick and stratified, a number of aquifers
can be found. Some areas.have a deep artesian aquifer, a shallow artesian
aquifer and a perched aquifer. Wells developed in the deep artesian aquifer
have been found to be capable of yielding 3,500 gpm with almost no drawdown.
Yields for the shallow and perched aquifers are primarily used for domestic
wells, but yields of 500 gpm in some areas are possible.
The glacio-lacustrine sediments are a less common aquifer and are composed
primarily of silt- and clay-sized materials that were deposited as glacial lake
sediments. Well yields from these aquifers are exceedingly small. They are
usually passed over for a better source due to their aquitard characteristics.
Depending on location throughout the Rocky Mountains region, there may
exist only a single aquifer or a combination of these aquifers contingent upon
the extent of glaciation. Recharge to the deep aquifers is dominantly from
precipitation infiltrating along the mountain fronts, whereas the shallow
systems receive direct infiltration. Minor sources of recharge are irrigation
return flow, aquifer leakage and stream and lake seepage. Wells and springs
account for most of the discharge with the remainder from evapotranspiration
and effluent streams.
Water quality of glacial aquifers is generally very good, having an average
27
-------
dissolved solids concentration of 450 mg/L. This figure often varies though as
a factor of depth and locality. The glacial deposits are the most important
source of ground water in the northern half of the Rocky Mountains region be-
cause of their quality and quantity.
A number of wells are drilled in colluvium and terrace gravel deposits
which are geomorphically expressed as fans and benches, respectively. Both
colluvial fans and terrace benches are juxtaposed next to mountain fronts and
are usually incised by ephemeral streams. Coarser-grained materials, such as
cobbles and gravels, lie nearer to the mountain front, while finer-grained
materials such as sand, silt and clay increase toward the center of the valley.
These sediments interfinger laterally and show a marked decrease in hydraulic
conductivity with distance from the mountain front until the valley stream is
reached. Well yields of up to 200 gpm have been recorded, but values of
20 to 50 gpm are more representative of average well yields. Water from these
wells is of good quality and primarily used for domestic and stock water pur-
poses. Recharge is dominantly from precipitation, whereas springs and wells
are the major medium of discharge. Colluvium and terrace gravels serve as a
reliable source for small capacity wells.
TERTIARY VALLEY-FILL SEDIMENTS
Tertiary-age sediments comprise most of the basin-fill deposits found in
western Montana's intermontane valleys. Originally, these terrestrial deposits
were referred collectively to as the Bozeman Group (Robinson, 1963). Recent
investigations by Kuenzi, Fields, Richard, Petkowich and others have divided
the Bozeman Group into various formations dependent upon lithologic and paleon-
tologic relationships. The Tertiary deposits in the basins of southwestern
Montana are composed of a distinct upper and a lower sequence of sediments.
28
-------
Although two separate formations are recognized, it appears detritus from the
surrounding mountains infilled the basins under similar climatic conditions,
but different drainage systems account for the varied lithologies. Volcanic
ash seems ubiquitous to both formations and, according to Kuenzi (1966), the
composition of the volcanic glass and clay mineral suite cannot be used to
distinguish rock units or geologic age. Correlating the different formations
from one basin to the next is a recognized problem that has yet to be unraveled.
Unconformably underlying the Quaternary unconsolidated sediments is the
Six Mile Creek Formation and its equivalents of middle Miocene to Pliocene age.
The formation is topographically expressed as pedimented slopes which dip toward
the center of the valley. This formation is lithologically characterized by
coarse-grained sediments of higher energy environments such as perennial and
ephemeral streams. Typical lithologies include interbedded sandstones, channel
conglomerates, tuffs and siltstones. Because of the coarse-grained nature of
this formation, it represents a viable source for ground water. Wells penetrat-
ing this upper formation usually have yields of 5 to 35 gpm depending upon
locations. Values for transmissivity and storativity for these sediments are
generally unknown because only a few wells have been aquifer tested. Wells
completed in Tertiary sediments are drilled to varying depths, but usually
extend from 100 to 300 feet below the ground surface. The water quality of the
Six Mile Creek Formation is fair to good and is suitable for domestic and stock-
watering purposes. Values for dissolved solids range from 83 ing/L in the
Bitterroot Valley to 1,268 mg/L in the Deer Lodge Valley, with an average of
about 500 mg/L.
The lower sedimentary sequence, the Renova Formation, unconEormably
underlies the Six Mile Creek Formation. The Renova ranges from late Eocene
to early Miocene age. This formation is dominantly comprised of finer-grained
29
-------
sediments indicative of restricted depositional environments such as ponds,
lakes and floodplains. Common lithologies found in the Renova Formation include
alternating layers of thin-bedded claystones, siltstones, poorly-sorted mud-
stones and tuffaceous deposits. Although these sediments probably contain a
large amount of ground water in storage, the nature of the clays prevent it
from being withdrawn. Because this formation generally occurs at substantial
depths in the basins and does not readily produce ground water, it is seldom
used as an aquifer.
Recharge to Tertiary valley-fill sediments is derived from inter-aquifer
seepage from the overlying stream alluvium and alluvial fans, infiltration from
precipitation and irrigation return flows. Wells, springs, seeps and evapo-
transpiration account for most of the discharge from these sediments.
The thickness of the Tertiary sediments and the configuration of the
basement bedrock of Montana's intermontane valleys have been largely unknown
variables. In order to compile an aquifer thickness map for the UIC project,
the valleys were computer-modeled where sufficient gravity surveys have been
completed. The modeling program integrates Bouguer gravity values versus depth
using a predetermined value for the difference between bedrock density and
valley-fill density. Because very little is actually known about the degree
of compaction or alteration of deeper sediments, a single density contrast
value is used for the total depth of the sediments.
Inasmuch as this is a two-dimensional program, numerous gravity profiles
across the valley were needed to construct isopach contours. The result is an
interpretation of the total thickness of the Cenozoic sediments within the
intermontane basin. Seismic investigations and scant drill-hole data were
utilized to add credence to the predicted bedrock depths.
Many hours were spent obtaining the extensive information necessary to
30
-------
evaluate the basins. Additional computer time was also logged to generate
supplementary data. Nevertheless, inaccuracies exist and revisions will be
made as new information becomes available. It should be noted that many of
the previous estimates of Cenozoic valley-fill thickness appear to be on the
conservative side compared with projections in this report.
The following is a basin-by-basin analysis of the Cenozoic valley-fill
thickness of selected intermontane valleys in western Montana and a summary
of the ground-water occurrence in these valleys. Figure II-7 is a schematic
diagram showing the locations of the intermontane valleys of western Montana
and those which were evaluated.
31
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CENOZOIC BASINS EVALUATED
Figure 11—7
32
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The Tobacco Plains
The Tobacco Plains valley is located in the extreme northwest corner of
the state. The valley lies within the Rocky Mountain Trench and is bordered
by major longitudinal gravity faults (Coffin and others, 1971). The Whitefish
Mountain Range forms the eastern wall of the valley, whereas the Salish and
Purcell Mountains delineate its western border. The northern limit of the
basin is the international border, although the valley extends into Canada.
Rocks outcropping along the valley margin, for the most part, belong to
Precambrian Siyeh Formation with others being lower Piegan and Ravalli Group
rocks. These rocks are inferred to underlie the Cenozoic fill in the valley.
-------
CANADA
T37N
T36N
4 ^
!> ¦¦dl
& , T35f
Lp„V'N a
likl ^-r T34N
A
R29W
R28W
R27W
R26W
R25W
R24W
R23W
Isopach Interval: 1000 ft.
Map Scale 1*500,000
Refer to basin no 3
Isopach of Ccnozoic fill in the Tobacco Plains
Figure 11 -8
34
-------
yields ranging from 5 to 25 gpm, sufficient for domestic and stockwater use.
Overall, the ground water of the basin is of good chemical quality. Though
the water is somewhat hard, it is suitable for domestic, stockwater and irriga-
tion uses. Dissolved solids have an average value of around 300 mg/L and the
major constituents are calcium, magnesium, sodium and bicarbonate. Precipitation
accounts for most of the recharge to the aquifer system, with a minor amount
contributed by irrigation runoff. Discharge occurs dominantly from wells,
springs and evapotranspiration. Ground water also maintains a base flow for
most of the streams in the drainage basin. The Tobacco Plains basin is sparsely
populated, and to date has not placed substantial demands on the ground-water
system.
Kalispell Valley
The Kalispell Valley lies in the southern portion of the Rocky Mountain
Trench. The valley is bounded on the west by the Kalispell Fault which is
located along the east base of the Salish Mountains. The Swan-Whitefish Fault
forms its eastern border. The north shore of Flathead Lake is considered the
southern limit of the valley, while the northern end progressively pinches out
at Whitefish Lake. The structural framework of the valley is apparently con-
trolled by a series of north-northwest-trending subparallel faults and two
associated cross faults (see Figure IT.-9) . The area north of the Creston Fault
is a graben. These structural features contribute to spatially render the
valley an elliptical bowl. T.sopach contours show that the Cenozoic fill has
a maximum thickness of A,000 feet near LaSalle, Montana. South of the Creston
Fault, gravity data indicate that the area around Big Fork is an upthrown
block which is bounded by two smaller grabens. Both of these smaller valleys
contain approximately 2,000 feet of valley fill (sec Figure TT-9) . According
to Konzeski, 1968, unconsolidated to semiconsolidated Tertiary rocks occur in
many northern Rocky Mountain intermontane basins of comparable size, but none
35
-------
T31N
T30N
T29N
T28N
/ 1 Win J
. W / >/ L v
:y j ^
surtHji m*' / "*1 <
ta7u>_ ^ ^riO"
j-~ -¦< T26N
R23W
R21W
Isopach Interval: 1000 ft.
Map Scale. 1:500,000
Refer to basin no. 4
Isopach of Cenozoic fill in the Kalispell Valley
Figure 11-9
36
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are exposed in the Kalispell Valley. It is assumed that Tertiary fill overlies
the same Precambrian bedrock which crops out along the valley margins. The fill
is probably comprised of Miocene and Oligocene age gravels, sands, silts and
clays. An unknown thickness of Pleistocene glacial deposits of Wisconsin age
then overlie the Tertiary sediments. The north-central and western parts of the
valley are mostly morainal deposits composed of till, whereas the south end of
the valley contains well-bedded clays and silts of glaciolacustrine origin. In
some locales, dune sand or glaciofluvial deposits cover the area. Holocene
alluvium overlies the glacial sediments along valley bottoms and borders the
major streams. Point bars and floodplain deposits characterize alluvial deposits
and generally are only a few feet thick. The hydrogeology of the Kalispell
Valley is exceeding complex because of the heterogeneity of the glacial sediments.
The discontinuity and interfingering of these deposits make it virtually im-
possible to predict aquifer parameters. Konizeski and others (1968) have
delineated three distinct aquifer systems for this area: (1) the Holocene
floodplain aquifer; (2) the Pleistocene systems comprised of a perched aquifer,
a shallow artesian aquifer and a deep artesian aquifer; and (3) the Precambrian
bedrock aquifer. Values for hydraulic conductivity contrast sharply depending
on the nature of the sediments. Glaciofluvial deposits have high values for
conductivity and are capable or producing yields exceeding 3,000 gpm, as is the
case of a 400-foot well drilled in SE^NW^ sec. 27, T. 29 N. , P. 22 W. Wells
completed in till that has poor hydraulic conductivity, however, usually yield
less than 5 gpm.
The availability of potable ground water in the Kalispell Valley is very
good. Wells capable of yielding large amounts of water for irrigation or
municipal supplies can generally be found in the deeper artesian aquifer. Do-
mestic and stockwater wells producing from 10 to 20 gpm are common throughout
the valley at shallow depths.
Values for dissolved solids average approximately 400 mg/L. The general
37
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water quality of glacial deposits is a dominantly calcium bicarbonate type
which results in hard water. Recharge to the hydrogeologic system is primarily
from rainfall and snowmelt water infiltrating along the mountain fronts with a
minor amount from irrigation return flow and leakage from overlying aquifers.
Most discharge is from evapotranspiration and wells, with the remainder occur-
ring as springs and effluent streams.
Currently, the area along Highway 93 between the cities of Kalispell and
Whitefish is rapidly developing, thus placing increased demands on the ground-
water resources. The potential problem of lowering the potentiometric surface
in the area is becoming more and more relevant. A comprehensive study of this
area should be undertaken to determine the effects development is creating and
to evaluate the ground-water resources for future development.
Swan Valley
The Swan Valley is located along the eastern branch of the Rocky Mountain
Trench at its southern end. The Precambrian Belt strata of the Mission Range
on the west and Swan Range of the east dip gently eastward forming the valley
margins. The valley manifests an asymmetry about its north-south axis which is
a surface expression of the controlling Swan Fault. Gravity data, however,
demonstrates unequivocally that major faulting occurs at the valley's boundary
with the Mission Mountains, although maximum depth to bedrock is nearer the
Swan Range (Crosby, 1968). The valley fill attains a maximum thickness of
6,500 feet approximately five miles north-northwest of Condon (See Figure 11-10).
The valley began initially filling with material derived from the adjacent
mountains during early Tertiary time. Tertiary or early Pleistocene sediments
along the Swan River (T. 25 N.) are described by Alden as being rusty, clayey
sand and buff sandy-clay with some gravel in which some of the pebbles are badly
decomposed. They are overlain by lighter colored, grayish glacial drift con-
taining striated pebbles. Elsewhere, Tertiary sediments are not exposed because
38
-------
¦s°°° 4\ T"!
¦I) ^
( -7r^fW MjS-y 'jt£
T >, ! ,
T2SN
T24N
~- p*- '¦& ' V ' Vj
. ,7r ,8."Tr^
> .-?=¦ K °! iV
-.,<.- "'\\"" t\> ! ,V
^iS»-.£»^_CsaX
:\
- 1/ > ""• v- I ~ ^
• !-' '.J \( / T16N
> ;«V,. ,'V,'
Bof» i I
- U'
R18W
R1 7W
R16W
Isopach Interval 1000 ft.
Map Scale- 1 500.000
Refer to basin no. 7
Isopach of Cenozoic fill
Figure 11-10
39
R 15 W
I in
the Swan Valley
-------
they have been eroded, reworked or buried by glacial deposits. Retreating
glaciers of Wisconsinan . age have mantled the floor of the Swan Valley with a
substantial thickness of glacial drift. These glaciofluvial and till deposits
may be several hundreds of feet thick.
The hydrogeology of the Swan Valley is rather complex as a result of
glaciation. Pleistocene and Recent alluvial sediments form the principal aquifers
of the valley. A review of well logs within the valley determined that most
wells were completed in till. Well yields are generally small, ranging from
1/2 to 50 gpm; an exception is the Forest Service's 330 gpm well at Condon.
The average well yield is approximately 12 gpm. Ground water from the till is
of good chemical quality and used dominantly for domestic purposes. The glacial
till has low permeability in the center of the valley where most of the wells
are drilled, but because sorting of deposits by proglacial streams occurred
along the eastern margin, well yields would likely be higher there The alluvium
bordering the Swan River serves as the other reliable source of ground water.
Well depths are shallow and sufficient quantities for domestic and stockwater
use are easily obtained, making the alluvium a desirable source. North of
Summit Lake, shallow ground water moves northward toward Swan Lake; south of
the lake the flow is in a southerly direction.
Because of the paucity of information on the Tertiary deposits and scarcity
of deep lithologic well logs, very little is known on the composition, water-
bearing potential and general nature of these sediments.
Recharge to the aquifer system is from precipitation and snowmelt infiltra-
tion along the mountain fronts. Discharge occurs through springs and wells, and
ground water maintains a base flow for the Swan River during periods of low flow.
Mission Valley
The Mission Valley lies in the southernmost extension of the Rocky Mountain
Trench. The eastern side of the valley is extremely linear which is indicative
40
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of the Mission Fault. This fault is a high-angle, normal fault with an
apparent stratigraphic throw of about 15,000 feet down to the west. The southern
border of the valley also has a linear expression which represents the St. Mary's
Fault zone. Because of the Mission Fault, the valley has a distinct north-south
alignment. Cenozoic fill is deepest near the east-central margin of the valley
(approximately 4,000 feet to the Precambrian basement) and progressively thins
to the west. Isopach contours have an elongated pattern, and this trend probably
continues under the southern shoreline of Flathead Lake (see Figure 11-11).
Gravity data west of the large Mission Valley manifests a smaller valley, struc-
turally independent of the main basin. This valley is also elongated along a
north-south axis and attains a maximum depth of 3,000 feet. Preliminary geologic
mapping by Harrison and others (1974) revealed that an anticlinorium coincides
with the axis of this fault-bounded valley.
The oldest rocks in the valley are of Precambrian age. They are probably
overlain by Tertiary sediments; however, no reference has been found of Tertiary
outcroppings or presence on well logs. Overlying the Tertiary strata are
Quaternary glacial and lake bed deposits and Holocene alluvium. The Precambrian
rocks, for the most part, are argillites with some quartzites and limestones
present. Wells or springs which tap these sediments along fractures usually
yield less than 10 gpm. The potential for ground water in Tertiary sediments
is unknown because of its uncertain presence.
Glacial deposits are the most important source of ground water in the
Mission Valley, but well yields are unpredictable because of the heterogeneity
of the aquifer material. Wells drilled in morainal material generally yield
small amounts of water, but there are large capacity wells of more than 300 gpm
tapping glacial deposits near "Ronan and Poison (Boettcher, 1980). In some areas
a confining layer overlies glacial or alluvial deposits creating flowing artesian
conditions. Flowing wells yield as much as 600 gpm near Ronan (Boettcher, 1980).
Dissolved solids have an average value of around 350 to 400 mg/L and the chemical
41
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T24N
T23N
R23W
R21W
R19W
Isopach Interval. 1000 ft.
Map Scale. 1 500,000
Refer to basins no. 8 and 9
Isopach of Ceno2oic fill in the Mission and Little Bitterroot Valleys
Figure 11-11
42
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quality of ground water derived from glacial deposits is generally good.
Well yields range from 10 to 400 gpm for the alluvial aquifer. These
yields are largely dependent on location arid well completion with the average
yield approximately -40 to 50 gpm. The alluvial aquifer serves as a
potable source for numerous domestic and stockwater wells. Overall water
quality of the alluvium is good, but the water is somewhat hard. Recharge to
the aquifer system is from precipitation, snowmelt runoff, influent streams and
irrigation return flows. Discharge occurs from well pumping, springs and evapo-
transpiration. Ground water is a valuable resource for the inhabitants of the
Mission Valley. Whereas all the towns in the valley depend partly or entirely
on ground water for their supply, the rural residents are totally dependent on
wells and springs for their livelihood.
Little Bitterroot Valley
The Little Bitterroot Valley is a structural depression, probably bounded
by high angle normal or listric normal faults of Tertiary age (Donovan and
Sonderegger, 1981). Geological mapping by Harrison and others (1974) shows a
series of north-northwest trending normal faults transecting the valley. En-
compassing the valley are various Precambrian-age formations. A geothermal test
well near Campaqua encountered what was thought to be Precambrian Ravalli Group
rock at 264 feet below land surface. Gravity data demonstrates a bedrock high
at this well site and also suggests that the Cenozoic fill rapidly deepens
approximately three miles due west. The fill attains a maximum thickness of
more than 2,000 feet in the south-central part of the valley based on gravity
data calculations (see Figure T.I-11) .
Although Tertiary sediments were absent in the geothermal test well, there
are Tertiary outcroppings of volcanoclastic sandstones and conglomerates, ash
layers and fluvial sediments along the northern margins of the valley. Et is
likely that similar Tertiary deposits overlie the Precambrian bedrock floor in
43
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the deeper segment of the valley, but may have been removed by Pleistocene
glacial erosion in localized shallow areas. Glaciofluvial and glaciolacustrine
deposits unconformably overlie Tertiary and Precambrian rocks and can be con-
tinuously correlated over the entire basin. Donovan and Sonderegger and others
(1981) describe a permeable Pleistocene gravel bed of an estimated 20 to 60 feet
thickness occurring extensively throughout the valley. The top of the gravel bed
appears to be nearly planar and is overlain by 200 to 300 feet of homogenous
silty clays of glacial Lake Missoula. These silts are surficially present over
most of the valley. The Little Bitterroot River has deposited an alluvial veneer
along the eastern margin of the valley.
The hydrogeology of the Little Bitterroot Valley is extremely complex be-
cause of the interrelationship of the separate aquifers. Deep fractures in the
Precambrian rocks provide conduits for the circulation of hydrothermal waters.
Localized hot and warm springs issue from these fractures. Wells reaching this
aquifer have yields of up to 800 gpm and water temperatures of approximately 45°C.
Although the water is somewhat mineralized by sulfate, manganese and iron, it is
generally softer because of the higher sodium concentration. Average values for
dissolved solids are approximately 400 mg/L. The potential availability of ground
water in Tertiary sediments is presently unknown, though Boettcher (1980) believes
water from these deposits has high iron concentrations.
Locally, the aforementioned Pleistocene gravel aquifer appears to be
hydraulically interconnected with the Precambrian system and probably recharged
through vertical leakage and infiltration. This aquifer produces flowing artesian
wells and is ubiquitously used for irrigation in the valley. Water levels in
wells penetrating this aquifer decline during the irrigation season and rise the
rest of the year. Long-term records (8 years) show a net water-level decline in
the area, probably owing to the large number of flowing irrigation wells (Boettcher,
1980). It appears this gravel aquifer has reached its appropriation limit and
further exploitation may result in lowering of water pressures to the point that
44
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wells will no longer flow. The overall water quality is generally good. Ground
water from the alluvium of the Little Bitterroot River is moderately used. Wells
tapping the alluvial aquifer are producing substantial yields, but it is dominantly
used for domestic and stockwater purposes.
Recharge to the ground-water system is from precipitation, snowmelt runoff
and irrigation return flow. Discharge is primarily the result of irrigation
wells, springs and evapotranspiration.
Missoula-Ninemile Valley
The Missoula and Ninemile Valleys form an elongated northwest-trending
trough approximately 42 miles long that is commonly referred to as the Missoula
Basin. Geomorphically represented as a linear succession of truncated spurs
along the southern margin of the Reservation Divide Mountains, the Ninemile Fault
delineates the northeastern edge of the valley. The southwest side of the valley
is formed by the Bitterroot Range and the Ninemile Divide Mountains. The basin
was formed by extensional faulting which downdropped the bedrock floor during
early Tertiary time. Contemporaneous with downfaulting, the basin began filling
with detritus eroded from the surrounding mountains into which were interbedded
layers of volcanic ash. The resultant deposits of interbedded shale, ash and
conglomerate were subsequently mantled by a few hundred feet of well-sorted
channel gravel and sand of Pliocene age (McMurtrey and others, 1964). The
Clark Fork River has dissected the valley deposits and has mantled the glacial
sediments with an alluvial veneer along its course. The sediments have a cum-
mulative thickness of more than 3,000 feet near the airport (see Figure 11-12).
There are three basic aquifer units within the Missoula Basin: Holocene to
Pliocene-age unconsolidated deposits forming the floodplain of the Clark Fork
River and the remainder of the valley floor; Tertiary sediments of the Oligocene
age which underlie the alluvium or border it as terrace deposits; and a Precam-
brian bedrock aquifer. The alluvium is composed of discontinuous layers of
45
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T1 7N
T16N
T15N
T14N
T13N
T12N
R24W
R23W
R22W
R21W
R20W
R18W
Isopach Interval: 1000 ft.
Map Scale: 1.500,000
Refer to basin no, 14
Isopach of Cenozoic fill in the Missoula-Ninemile Valley
Figure 11 -1 2
46
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gravels, sand and clay that range in thickness from a few feet to 250 feet; the
maximum thickness is near the mouth of Grant Creek (Geldon and Curry, 1978) .
Well yields vary depending upon use; some large irrigation and municipal wells
have yields in excess of 4,000 gpm. The chemical quality of water from the
alluvium is generally excellent; the average dissolved solids concentration is
175 mg/L. Alluvial ground waters are generally a calcium bicarbonate type, and
therefore, moderately hard. Recharge to this system is from precipitation,
irrigation return-floxi7 and losing streams. Aquifer discharge is from well
pumpage, evapotranspiration, seepage to underlying units and losses to maintain
stream baseflow.
The Oligocene sediments are characterized as semiconsolidated bedded deposits
of sand, silt, clay, ash and gravel; these rocks underlie Holocene to Pliocene-
age sediments, but are best exposed as sloping sediments. Well yields from these
deposits are generally small, ranging from 1 to 20 gpm, because of the fine-grained
nature of the sediments. Tertiary rocks account for the majority of the valley
fill and may be up to 3,000 feet thick. Dissolved solids content of ground water
from Tertiary sediments averages 300 mg/L, which is quite low. Because well
yields are small, the ground water is generally used for domestic and stockwater
purposes. Recharge is from precipitation and infiltration from the overlying
alluvium. Discharge occurs as springs and well pumpage.
The Precambrian bedrock aquifer is of only minor importance because it is
relatively impermeable. Fracture systems within the bedrock yield small quantities
(1-5 gpm) of water. Little is known of the water quality of this aquifer, but
the water appears to be potable.
Blackfoot Valley
Although gravity data suggest that the Blackfoot and Nevada Valleys are
separate basins, they are topographically continuous. For this reason they are
jointly referred to as the Blackfoot Valley in this report. This intermontane
47
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valley, located in the northern part of Powell County, has a general northwest
trend. The Blackfoot Mountains, composed of Belt Supergroup rocks, form the
northeastern border of the valley. At the base of this range is an unnamed valley-
margin fault. It is a high angle normal fault dipping to the southwest (Witkind,
1975). The northwestern part of the Garnet Range forms the southwestern border
of the valley. These mountains are also composed of Precambrian Belt Supergroup
rocks. Small bodies of quartz monzonite occur locally in the strata and extensive
Tertiary basalts and andesites outcrop near Helmville.
Block faulting and tilting occurred intermittently during deposition and
continued in late Oligocene and (or) early Miocene time, after which the region
was deeply eroded (Cantwell, 1980). Again, during late Miocene and Pliocene
time, block faulting recurred and detritus eroded from the surrounding mountains,
infilling the basin. During Pleistocene time, glaciers advanced and retreated
across the valley. Deposits of glacial outwash and till coyer most of the
Tertiary valley-fill deposits north of Helmville. The low rolling hills which
dominate the present valley floor were formed by the glaciers (Cantwell, 1980).
Quaternary stream and fan alluvium mantle the glacial deposits in some locales.
Gravity data indicate that the Tertiary and Quaternary deposits attain a maximum
thickness of more than 6,000 feet near the center of the valley (see Figure 11-13).
It is assumed that similar Belt Supergroup rocks underlie Cenozoic sediments.
Ground water in the Blackfoot Valley is derived mainly from the Quaternary
alluvial and glaciofluvial deposits. These deposits are composed of unconsolidated
gravels, sands, silts and clays that are moderately well-sorted. The average
thickness of the alluvium along the Blackfoot River is probably 80 to 100 feet.
Wells completed in alluvial and glaciofluvial sediments generally produce good-
quality water that has concentrations of dissolved solids ranging between 150
and 250 mg/l,. Well yields for the alluvial aquifer are usually about 20 to 25
gpm. Glacial till covers much of the Blackfoot Valley. The till consists of a
heterogeneous mixture of unsorted and unconsolidated gravels and boulders in a
48
-------
1 frV'^" n
5 !
> :\ry.,, V .;!
fca^" TO. J '0 #n
fTAM-Js?
T16N
T15N
T14N
T13N
^ T12N
R13W
R12VV
R11W
R10W
Isopach Interval. 1000 ft.
Map Scale- 1*500,000
Refer to basin no. 18
Isopach of Cenozoic fill in the Blackfoot Valley
Figure 11-13
49
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jilty to clayey matrix. The till can be up to 150 feet thick but has low well
fields ranging from 5 to 15 gpm. Underlying the glacial and alluvial deposits
Ls Tertiary valley-fill. The Tertiary sedimentary rocks in this area are a
semiconsolidated light-gray, sandy clay containing interbedded lime marl and
;onglomerate. These sediments have a high percentage of fine-grained materials
md are therefore not very permeable. Well yields are small from Tertiary
sediments, averaging about 5 gpm.
Precambrian meta-sediments and Tertiary volcanic rocks serve as another
source of ground water. Generally, wells completed in these rocks yield only
small quantities of water from fractures. However, there are two large capacity
tfells in T. 12 N., R. 12 W., section 23 and section 28 that produce 3,000 and
350 gpm, respectively. It is believed the wells are completed along a fracture
netxTOrk in Tertiary basalts.
All the geohydrologic units in the Blackfoot Valley are recharged directly
or indirectly by precipitation. Rain, snowmelt-runoff and influent streams
account for most of the recharge. Ground water is discharged to springs,
effluent streams and to the atmosphere by evapotranspiration. Discharge by wells
is minimal even though most residents use water supplied by wells.
Prickly Pear Basin (Helena Valley)
The Prickly Pear basin, or Helena valley, is roughly a nearly-circular
basin surrounded by mountains: the Big Belt Mountains on the north; Scratch-
gravel Hill on the west; Elkhorn Mountains on the south; and Spokane Hills on
the east. Drainages between each of these mountain ranges lead to other Cenozoic
basins. Most of the bedrock exposed in the mountains east, north and west of the
Helena valley belong to the Belt Supergroup. Paleozoic and Mesozoic rocks out-
crop along the southern valley margin. Cranod.Lorite and related rocks of the
late Cretaceous Boulder batholith intruded and metamorphosed sedimentary rocks
along the south and west margins of the Helena valley. At the southern edge of
50
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the valley, the Elkhorn Mountain Volcanlcs overlie sedimentary and intrusive
rocks in the northern Elkhorn Mountains.
The Helena valley is a northwest-trending structural and topographic basin
which began to form in early or middle Tertiary time by block faulting, possibly
along pre-existing zones of basement weakness. Two major northwest-trending
fault zones bound the valley. The Prickly Pear fault zone (northeast side down)
roughly parallels the southwest valley margin. A 9.5 mile segment, half the
total trace length of the Prickly Pear fault zone, lies buried beneath young
alluvial deposits in the western part of the valley. A part of the Lewis and
Clark line, the Helena valley fault zone (southwest side down) consists of five
segments which form the northeast valley margin. The western segments are
remarkably linear and appear to offset middle Pleistocene deposits. Along the
southwest side of this fault zone, basin-fill deposits reach a maximum thickness
of 6,000 feet and average over 3,000 feet thick along most of the fault's length
(see Figure 11-14). The sediments gradually thin in the western and southern
parts of the valley. Numerous small faults near the southern Scratchgravel Hills
and along the northwest valley margin offset deposits as young as late Pleistocene,
but do not appear to define an extensive fault zone.
The oldest recognized Tertiary clastic deposits include well-bedded olive-
gray to yellowish clay, tan siltstone, light gray, poorly sorted arkosic sand,
rounded to subangular pebble gravel and thin lignite beds of probable Oligocene
age. Rocks of similar age outcrop along the southern valley margin and include
white to gray well-indurated volcaniclastic rocks containing pumice fragments
and rhyolite pebbles. The Tertiary deposits covering 80 percent of the eastern
Helena valley consist of tan, micacious siltstone with interbedded sandy pebble
and cobble gravel. Probably middle Miocene to Pliocene in age, this siltstone
is generally coarser-grained than the Oligocene deposits. Early Pleistocene and
possibly latest Tertiary alluvial deposits, cap ridge tops along the southern
valley margin and form eroded and faulted hills along the northeast portion of
51
-------
^^6. in,. .£ .¦ .
'T-vlh v-Q^
>A'i>
^i0OO{/' If-f 7
¦ ' , » \
¦ V -
lcenN>rr\»M\o (
f"* J A
T13N
I T12N
TUN
T10N
r^» *V lOro/fcJln ( .V^'T 1 ' ?Ul '
T9N
T8N
R5W
R3W
R2W
Isopach Interval: 1000 ft.
Map Scale: 1:500,000
Refer to basin no. 21
Isopach of Cenozoic fill in the Prickly Pear Basin
Figure 11-14
52
-------
the Helena valley. A veneer of poorly-sorted, silty gravel covers extensive
pediment surfaces developed along the northwest and southwest perimeter of the
valley. Late Fleistocene to Holocene alluvial plain deposits which overlie the
western Helena valley probably do not exceed 150 feet in thickness. Other uncon-
solidated deposits outcropping over relatively small areas include channel and
terrace alluvium, loess, strath terrace remnants along the Missouri River and
fine sand, silt, clay and minor gravel deposits of Glacial Lake Great Falls
(Stickney and Bingler, 1981).
Ground water in the Helena valley is derived from three separate aquifer
units, distinction of these units is based on their relative geologic ages and
lithologic characteristics. The units are: 1) a bedrock aquifer; 2) Tertiary
age sediments; and 3) various deposits of Pleistocene and Holocene times.
The bedrock aquifer consists of a variety of sedimentary rocks ranging
from Precambrian to Cretaceous age and late Cretaceous and early Tertiary age
igneous rocks. The oldest rocks of the valley are the Precambrian meta-sediments
belonging to the Belt Supergroup. They are composed of red, green and brown
argillites and red and white quartzites, which outcrop along the east, west and
north margins of the valley. Paleozoic and Mesozoic rocks made up of brown to
white quartzite and sandstone, black to brown shale, and bluish-gray to tan
limestone and dolomite are exposed along the southern valley border (Stickney,
1981). In some locales, the eastern extension of the Boulder Batholith has
intruded and metamorphosed the pre-existing sedimentary rocks of the south-
western margin. Andesites and tuffs of the Elkhorn Mountain Volcanics have also
interdivided and overlie sedimentary rocks in the southern portion of the valley.
For the most part, these rocks are well indurated and contain no interstitial
water. Wells drilled along the periphery of the Helena valley are completed in
this bedrock aquifer and attempt to intersect bedrock fractures of joints to
obtain sufficient yields. Well yields usually average 5 to 10 gpm and are of
good chemical water quality for domestic and stock use.
53
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Tertiary-age sediments comprise another aquifer unit in the valley, but
also have limited use. These sediments, which overlie bedrock and underlie
Quaternary deposits, account for the major portion of basin-fill deposits.
Tertiary sediments outcrop in the southern part of the valley and cover a
large area in the eastern part of the valley. In general, the basin-fill de-
posits are composed of detrital materials eroded from the surrounding mountains.
The oldest recognized Tertiary deposits include well-bedded, olive-gray to
yellowish clay, tan siltstone, light gray, poorly-sorted arkosic sand, rounded
to subangular pebble gravel and thin lignite beds of probably Oligocene age
(Stickney, 1981). Interbedded volcanic ash and flows of similar age are exposed
along the southern border of the valley. Overlying the Oligocene deposits are
Miocene and Pliocene age sediments. These coarser-grained sediments are com-
posed of gravels and sands in a silty-clay matrix and are laterally discontinuous.
Exposed Tertiary sediments are generally unconsolidated and tend to become semi-
consolidated with depth of burial.
The fine-grained nature of these sediments limits the permeability of this
aquifer unit. Most wells obtaining ground water from Tertiary sediments have
yields of 15 to 30 gpm, but some of the deeper wells have yields in excess of
200 gpm. In some cases, wells have tapped confined water-bearing zones which
yielded water under artesian conditions. These wells are in the lower part of
the valley and include wells at the Masonic Home in T. II N., R. 3 E., section 2
and at the Montana State Vocational School in T. 11 N., R. 3 W., section 34
(Lorenz and Swenson, 1951). The overall ground-water availability from this
aquifer unit is highly variable and dependent upon location and the composition
of the deposits. This ground water is somewhat hard, but it is suitable for
domestic, stockwatering and irrigation purposes. Some wells produce water with
iron and manganese concentrations higher than recommended limits for potable
supplies. Although these concentrations are not detrimental to health, they
give the water an undesirable taste and stain fixtures reddish-brown. A possible
54
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source of these chemicals is the solution of iron and manganese oxides that
have formed coatings on the gravel and boulders in the basin-fill deposits
(Wilke and Coffin, 1973) .
Pleistocene and Holocene-age deposits serve as the most prolific and
readily available source for ground water in the Helena valley. Late Pleistocene
to Holocene-age unconsolidated alluvial plain deposits mantle the floor of the
Helena valley. These deposits are composed of a heterogeneous mixture of gravels,
sands, silts and clays, and are usually moderately-sorted from fluvial processes.
A large portion of the finer-grained material is carried off downstream. This
loss of fine-grained material enhances the permeability of these sediments. The
sand and gravel layers of this aquifer yield water freely to wells, but often
interfinger with impermeable clay beds and, for this reason, are laterally dis-
continuous. Because of the heterogeneous nature of the sediments, the layers of
sand and gravel form a complex, but generally interconnected, system of aquifer
zones that are considered as one multiple-aquifer system. Several large-capacity
wells (pumping in excess of 500 gpm) have been constructed, and most irrigation
wells derive their supply from these sediments. A transmissivity of about 10,000
gallons per day represents a reasonable estimate for the alluvial aquifer pene-
trated by most shallow wells in the southern part of the valley (Moreland and
Leonard, 1980).
A thin veneer of poorly-sorted gravel covers extensive pediment surfaces
of middle Pleistocene age. These surfaces are extensively developed along the
northwest and southwest sides of the Helena valley. Most of the city of Helena
is built on such deposits. These deposits have a moderate degree of permeability
and supply ample water for domestic and stockwater uses. Other unconsolidated
deposits that outcrop over relatively small areas throughout the basin include
loess deposits, strath terrace remnants along the Missouri River and lacustrine
silts and clays of Glacial Lake Great Falls. They are only of nominal importance
because their well yields are very small.
55
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With few exceptions, ground water from this aquifer is of good-to-excellent
quality. Chemical analyses show the water is dominantly a calcium bicarbonate
type, however, a well's proximity can influence the type. Although there is
variation in some instances, the concentration of dissolved solids in water from
most wells sampled in the valley is less than 400 mg/L.
Infiltration from rainfall, snowmelt-runoff and irrigation return flows
account for most of the recharge to the aquifer system. Other sources of re-
charge are influent stream losses and inter-aquifer leakage. Ground-water
discharge in the valley occurs from evapotranspiration, wells, springs, seeps
and losses to effluent streams.
Bitterroot Valley
The Bitterroot Valley is topographically expressed as a wedge-shaped inter-
montane basin in west-central Montana. The valley's asymmetry about its north-
south axis is a surface expression of the controlling Bitterroot Fault along the
western margin of the valley. This high angle normal fault dips steeply to the
east and forms the triangular facets on the eastern face of the Bitterroot Range.
Activity along the fault may have been as recent as historic time (Witkind, 1975).
The Bitterroot Range is a high-grade metamorphic complex derived from lower
Beltian sedimentary rocks which border the Idaho batholith (Wehrenberg, 1968).
The Sapphire Mountains form the eastern border of the valley. The northern
portion of the Sapphire Range is composed of Precambrian Belt sediments, whereas
the southern portion is Cretaceous to early Tertiary-age granitic rocks.
The surface of the Bitterroot valley is largely an alluvial floodplain, but
also has low-to-moderately high gravel-veneered Tertiary terraces along the
eastern margin. Except for an area of Tertiary volcanic rocks south of Hamilton,
the west side of the valley appears to be underlain by later (Quaternary)
alluvium including glacial moraines and outwash from the Bitterroot Range (Pardee,
1950). The alluvium, glacial deposits and Tertiary tuffs and sediments together
56
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compose the valley-fill. Gravity data suggest that these Cenozoic sediments
attain a thickness of more than 3,000 feet between the towns of Woodside and
Corvallis (Figure 11-15). The isocontours also show the valley-fill thins in
the vicinity of the town of Victor. This apparent bedrock high is probably
related to a thinner section of valley-fill rather than a bedrock density change.
It should be noted the contour pattern along the east margin of the valley is
very irregular, indicating that the eastern wall of the Bitterroot valley has
a different structural origin than the western margin (Lankston, 1975).
Although the Bitterroot valley has been besieged by residential development,
there still exists only sparse data concerning ground water deeper than 100 feet.
This is because the alluvial sediments of the Bitterroot River are widespread
and capable of sufficient yields of good-quality water for domestic and stock
use. The alluvium is composed of unconsolidated gravels, sands, silts and clays.
These sediments dominantly have a high percent of coarse-grained materials be-
cause the finer fraction is carried downstream. This results in a higher degree
of permeability. The alluvial aquifer of the Bitterroot River is thickest in
the center of the valley and progressively thins towards the valley margins.
Wells in the alluvium generally yield between 15 and 25 gpm, but the potential
for larger capacities is readily available. Concentrations of dissolved solids
in the alluvium range from 40 to 705 mg/L for springs and wells.
Glaciolacustrine and morainal deposits along the western and southern
borders of the valley are grouped as another aquifer. These deposits have a
large percentage of fine-grained materials and, therefore, have low permeabil-
ities. The thickness of the glacial sediments varies considerable depending
upon location, however, well yields are consistently small—usually averaging
5 gpm.
Underlying and bordering the Quaternary-age deposits are unconsolidated
to semiconsolidated Tertiary sediments. These sediments consist of arkosic
channel-sand containing thin lenses of gravel eroded from the surrounding
57
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R22W
R19W
R21W R20W
Isopach Interval' 1000 ft.
Map Scale 1 500,000
Refer to basin no. 25
Isopach of Ccnozoic fill in the Bittcrroot Valley
Figure 11-1 5
58
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mountains, occasional lacustrine silts and clays and some beds of volcanic ash.
Within short distances, materials of different textures interfinger and inter-
grade both laterally and vertically in accordance with changes in the original
environments of deposition, and with the degree of volcanic activity in the
region (McMurtrey and others, 1959). Tertiary deposits contain a large percent-
age of fine-grained materials which greatly inhibit the permeability of the
sediments. A preliminary evaluation of, numerous deep aquifer zones was recently
completed. Transmissivities ranged from low values of 25 and 122 gpd/ft. to
higher values of 1,650 and 3,750 gpd/ft. Calculated values for storage co-
efficients varied from 0.00005 to 0.35 (Norbeck, 1980). Wells completed in
Tertiary sediments usually have yields of 8 to 12 gpm, but there are some
large-capacity municipal wells in the valley. The city of Stevensville has a
well drilled to 460 feet; this well produces 400 gpm. Tertiary ground waters
are generally of good chemical quality, but often have moderate concentrations
of iron. A geothermal study has also been completed on numerous deep Tertiary
test wells in the valley. The study determined there is no evidence for hydro-
thermal discharge (Leonard and Wood, 1980).
Annual precipitation for this area is approximately 16 inches per year,
which contributes to recharging the ground-water system. Other forms of recharge
are irrigation return flows, influent streams and inter-aquifer leakage. Ground
water in the valley is discharged by evapotranspiration, effluent streams,
springs, seeps and wells.
Deer Lodge Valley
The Deer Lodge valley is located west of the Continental Divide (locally
known as the Deer Lodge Mountains) and east of the Flint Creek Range. To the
north is the Garnet Range, and to the south, the Anaconda Range. In general,
Precambrian through Mesozoic sediments outcrop on the west and north sides of
the valley, and Tertiary volcanics and Cretaceous intrusions outcrop to the
59
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south, east and north. The Anaconda and Flint Creek Ranges contain Precambrian
through Cretaceous sedimentary rocks with numerous Cretaceous intrusions. To
the east, the Boulder Batholith quartz monzonite and Lowland Creek Volcanics
make up the mountains of the Continental Divide (Sonderegger and others, 1980).
Extensive andesite flows are found on the east side and at both ends of the
valley (Cremer, 1966).
The Deer Lodge valley probably began forming as a shallow topographic low
in response to batholithic intrusion and doming of country rocks. Later, block
faulting increased the topographic relief. Fault scarps of the Powell fault
zone are evident along the western margin of the valley (Cremer, 1966). At the
base of the Continental Divide mountains, gravity data indicate another major
fault along the eastern edge of the valley (Sonderegger and others, 1980).
The basin began filling with sediments during the Oligocene Epoch. Some
uncertainty exists concerning the actual thickness of basin-fill deposits in
the Deer Lodge valley. Gravity profiling of the valley indicates a maximum
thickness of 6,000 feet near the center of T. 5 N., R. 10 W., but in the center
of the valley south of Deer Lodge an exploratory oil well recently penetrated
10,300 feet of Cenozoic sediments (Montana Oil Journal, 12/31/81). Original
gravity profiles for the same region indicate a maximum depth of 3,000 feet to
bedrock. Figure 11-16 shows the Deer Lodge basin-fill thickness contoured from
available gravity data. The location of the exploratory well and the basin-fill
thickness are included to give an idea of the uncertainty of the gravity infor-
mation. A reason gravity data does not coincide with drill-hole data is the
large amount of Tertiary volcanics interbedded with basin-fill deposits through-
out the valley. These volcanics increase the density of the Deer Lodge basin-
fill as a whole and influence gravity measurements in the field. Unless the
increased density is taken into account during computer modelling of the gravity
data, the resulting basin depths are also influenced. As a part of the same
problem, gravity profiles show a bedrock high in the center of the valley. This
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TUN
T10N
T9N
T8N
T7N
T6N
T5N
T4N
T3N
R14W
R12W
R11W
P10W
R9W
R8W
Isopach Interval: 1000 ft.
Map Scale. 1.500,000
Refer to basin no. 28
Isopach of Cenozoic fill in the Cfeer Lodge Valley
Figure 11-16
61
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high is a reflection of lava flows concentrated in the center of the valley
(Konizeski and others, 1968). Before contouring the basin-fill isopach, the
gravity profiles were 'smoothed' to eliminate the false high.
The basin is filled with unconsolidated to consolidated sediments and
interbedded volcanics ranging in age from Oligocene to Holocene. Oligocene
bentonitic conglomerate and arkose of Oligocene age are overlain by Miocene age
unconsolidated to well consolidated fluvial clays, silts, sands and pebble con-
glomerates. Pliocene deposits include cemented colluvium and fan deposits near
the valley margins. The deposits grade into floodplain and channel deposits
toward the center of the valley. These Pliocene alluvial deposits consist of
interbedded limestone, shale, sandstone and gravel with minor pebble and cobble
conglomerates and varying amounts of bentonitic clay. Three-fifths of the Deer
Lodge valley is mantled by Quaternary floodplain and fan deposits. Other
Quaternary deposits include glacial moraines, travertine (near Warm Springs) and
boulder fields near Warm Springs Canyon (Konizeski and others, 1968).
Ground water in the Deer Lodge valley is derived mainly from the alluvium
of the Clark Fork River. Wells completed in this aquifer are generally shallow,
ranging from 10 to 150 feet deep. Water in these Quaternary sediments is gen-
erally unconfined and the water table fluctuates seasonally. Overall water
quality is good and the water is suitable for household and stock uses. Well
yields can vary from 5 to 150 gpm with the average being about 25 gpm.
Tertiary sediments are the other primary source of ground water in the
valley. These rocks either underlie the Quaternary alluvial deposits or flank
the alluvium as deeply incised pediments. Tertiary rocks are composed of finer-
grained sediments which become more consolidated with depth. The water in wells
completed in the Tertiary sediments is generally confined resulting in artesian
conditions. Because of the low permeability of the Tertiary sediments well
yields on the average are generally small (15-20 gpm), however, there are excep-
tions. A well drilled to 436 feet in T. 6 N., R. 9 W., section 7 is recorded
62
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to produce 2,400 gpm. The city of Deer Lodge is also reported to have com-
pleted a 900 gpm test well. These large capacity wells are used for irrigation
and municipal water systems, respectively.
The ground-water system is recharged partially from precipitation, irriga-
tion return-flow and from influent streams. Discharge occurs from springs,
wells, evapotranspiration and effluent streams.
Townsend Valley
Located along the southern end of the Lewis and Clark line, the Townsend
valley trends roughly northwest between the Elkhorn Mountains (west) and the
Big Belt Mountains (east). Precambrian through Mesozoic sedimentary rocks are
found on all sides of the valley. On the west and south sides, Cretaceous and
Tertiary volcanics and intrusive rocks outcrop.
The Townsend valley is a graben formed by crustal extension. Faults are
evident along the mountain fronts to the east and west (Reynolds, 1979). In
the north, the graben splits into two parts around the Spokane Hills horst
(Kinoshita and others, 1964) . To the east of the horst, the graben comes to an
abrupt end against faults of the Lewis and Clark line. On the west side of the
Spokane Hills, the Townsend valley extends into the Helena valley. These basins
developed together in early Tertiary time; later, the drainage between them be-
came blocked by a broad bedrock ridge (Davis and others, 1963). Tertiary deposits
located between the two valleys are less than 1,000 feet thick, but thicken
quickly toward the center of either basin. The Townsend valley-fill is more
than 8,000 feet thick in the northeast and gradually thins to the west and
south (see Figure 11-17). A major fault (east side down) extends southeast down
the center of the valley from the Spokane Hills. At the valley's southern
margin, bas.in-fill deposits form an irregular contact with bedrock. This contact
may be a depositional feature or may have been formed from a complex of small
fault blocks (Kinoshita and others, 1964).
63
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Isopach Interval: 1000 ft.
Map Scale: 1'500,000
Refer to basin no 30
Isopach of Cenozoic fill in the Townsend Valley
Figure 11-17
64
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The graben floor originally dropped with little or no tilting (Davis and
others, 1963). Block faulting, with the major amount of movement along the
Lewis and Clark line and with tilting of the graben floor to the east, con-
tinued intermittently from late Oligocene through Miocene. Throughout the
valley, Tertiary strata have been displaced by small northwest-trending faults
and show varying degrees of dip to the east.
The Tertiary deposits in the Townsend valley include the early Oligocene
Climbing Arrow Formation, middle Oligocene Dunbar Creek Formation, and Miocene-
age Sixmile Creek Formation (Kinoshita and others, 1964). The Climbing Arrow
Formation consists of light-colored, fine-grained tuffaceous sediments with
small amounts of interbedded sand and gravel. Locally, it contains thin beds
of coal or diatomaceous earth. Above the Climbing Arrow beds, the Dunbar
Creek Formation contains coarse sediments mixed with a large amount of tuffaceous
material. Unconformably overlying the Dunbar Creek Formation, the Sixmile Creek
Formation is a light to buff colored sandy clay with some sand and gravel beds,
locally overlain by conglomerate. The thin layer of Quaternary alluvium in the
valley consists of fan and floodplain deposits and a gravel mantle on the
benchlands (Lorenz and McMurtrey, 1956).
The Townsend valley has the most copious ground-water resources of western
Montana's intermontane basins. Numerous large-capacity wells and significant
spring-flows issue from the valley sediments. Whereas most intermontane basins
are underlain with Precambrian metasediments or crystaline igneous rocks, this
valley is partially underlain with a stratigraphic sequence of Paleozoic sedi-
ments. It is believed that a direct relationship exists between ground-water
availability and the Paleozoic strata.
Ground water in the Townsend valley is derived from three distinct aquifers :
(1) unconsolidated Pleistocene and llolocene-age deposits; (2) unconsolidated to
semi-consolidated Tertiary sediments; and (3) bedrock.
The Pleistocene and Holocene age deposits are primarily composed of
65
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alluvium of the Missouri River and its tributaries and alluvial fans along the
valley margins. Alluvial deposits are composed of a heterogeneous mixture of
cobbles, gravel, sand, silt and clay. They have been moderately sorted by
streamflow which carried the finer-grained materials downstream. Although sand
and gravel beds interfinger with clays and silts making them discontinuous, the
aquifer is hydraulically interconnected. Alluvial deposits of variable thickness
extensively cover the bottom lands from Townsend to Toston and veneer pediment
slopes in the Radersburg area. Most wells completed in the alluvial aquifer are
of a small diameter for domestic and stockwater purposes. These wells generally
range from 25 to 50 feet deep and have sustained yields of 15 to 30 gpm. The
city of Townsend has three wells completed in the alluvial aquifer which are
50, 60 and 93 feet deep; they produce 600, 650 and 440 gpm, respectively. Ground
water from the alluvium is usually of good to excellent quality. Generally the
water from the alluvium is a calcium bicarbonate type. Though the water is
somewhat hard, it is highly suitable for irrigation and potable uses.
Unconsolidated to semi-consolidated Tertiary sediments underlie the
alluvium and mantle the remainder of the valley floor. The deposits are geomor-
phologically expressed as a series of terraces that slope toward the center of
the valley. At the north end of the valley near Canyon Ferry Dam, these sedi-
ments are comprised of gravels and cobbles in a sandy-clay matrix representative
of broad channel deposits. In other locales, Tertiary sediments are considerably
finer-grained and contain interbedded tuffaceous layers. These fine-grained
beds are relatively impermeable and act as confining layers which produce
artesian conditions. Artesian pressures occur in Tertiary beds that underlie
the southern end of the valley and the area along the west flank of the Dry
Creek anticline east of Townsend (Lorenze and McMurtrev, 1956).
Many large-capacity irrigation wells have been completed in this Tertiary
aquifer. Well depths generally range between 200 and 400 feet. The areas east
and southeast of Canyon Ferry Reservoir have numerous wells with yields in
66
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excess of 1,000 gpm. Wells penetrating Tertiary sediments west of the reservoir,
however, have yields of 20 to 50 gpm. The ground water is of good chemical
quality and is suitable for domestic, irrigation and stockwater purposes.
Numerous springs with substantial flows also issue from alluvial and
Tertiary sediments. The waters are dominantly a calcium sulfate type, represen-
tative of the Madison Group. It is supposed that fractures in the Tertiary
sediments act as conducts for water discharging from the Mission Canyon Lime-
stone, a member of the Madison Group. Spring flows range from seeps to roughly
20,000 gpm.
The bedrock aquifer consists of Paleozoic and Precambrian age sediments
and Cretaceous age igneous rocks outcropping along the periphery of the valley.
These contain little interstitial water, and ground water is derived from
secondary permeabilities such as fractures, solution voids and joints. Well
yields are generally less than 10 gpm.
Recharge to the ground-water system of the Townsend valley occurs through
a variety of means. The major source of recharge are canal losses and irrigation
return flow. A large portion of the valley is inundated by Canyon Ferry
Reservoir and seepage from it and influent streams likely contribute to the
system. Inter-aquifer leakage, precipitation and spring seepage account for the
remainder of the recharge.
Large-capacity irrigation wells and evapotranspiration are the primary
means of discharge. Minor amounts occur through effluent streams, seeps and
domestic and stockwater wells.
Three Forks Basin
The Three Forks basin is the largest of the Tertiary basins in Montana.
Most intermountain basins in Montana trend north-south, but the Three Forks
basin is elongated east-west because of the Willow Creek fault which runs
west-northwest along the northern part of the valley. Concealed by basin-fill
67
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deposits, the fault's presence has been inferred from gravity data and from
the marked change in bedrock lithology and regional structure across the fault
zone (Davis and others, 1965a). Precambrian through Mesozoic sedimentary rocks
crop out in the low hills to the north of the Willow Creek fault, whereas Pre-
cambrian metamorphics are the most common rocks in the mountain ranges to the
east, west and south of the basin. Elkhorn Mountain Volcanics and Tertiary
intrusives also crop out to the west of the basin, and the Cretaceous-Tertiary
Livingston Formation is found to the northeast.
Erosion and tectonism probably played interrelated parts in the origin
of the basin (Robinson, 1963). On the western side, sinuous basin-fill bedrock
contacts indicate a depositional origin, but on the eastern side, movement
along the Bridger Creek fault played a major role in the basin's formation
(Mifflin, 1963). The Bridger Creek fault (basin side down) zig-zags along the
west front of the Bridger Range and is unlike most mountain front faults which
run fairly straight. The valley fill is over more than 6,000 feet thick east
of Bozeman along this fault zone (see Figure 11-18). On the southeast side of
the Three Forks basin, a high-angle normal fault (northwest side down) forms
the Gallatin Range front. Another fault (southeast side down) runs roughly
parallel to and two miles northwest of the Gallatin Range fault, forming a
narrow trough about 3,000 feet deep (Davis and others, 1965b). Mifflin (1963)
mapped a series of block faults along the Madison Range front to the southwest
of the basin. Farther to the west, gravity and magnetic data suggest a fault
trending west-northwest along the edge of the valley north of Harrison (Davis
and others, 1965a). Gravity data also shows the Jefferson Canyon thrust and
Lombard thrust connected beneath the basin-fill. These thrust faults are
exposed in bedrock on the west and north sides of the Three Forks basin.
In general, the bedrock beneath the basin is an eastward-tilted slab with
a series of troughs and broad, low ridges roughly trending east-west (Davis
and others, 1965b). Lower Tertiary rocks dominate the western portion of the
68
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T3S
T4S
R2E
Isopach Interval. 1000 ft
Map Scale: 1.500,000
Refer to basin no. 33
Isopach of Cenozoic fill in the Three Forks Basin
Figure 11 -18
69
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basin and have been deformed into broad, gentle folds, and upper Tertiary beds
dominate the eastern half of the valley (Robinson, 1961). In the north, the
Tertiary beds have a southeasterly dip (Wantland, 1953).
The basin fill in the Gallatin valley ranges from late Eocene to Holocene
in age (Robinson, 1961). The lower portion of the Tertiary beds consists of a
limestone conglomerate overlain by light-colored, fine-grained, tuffaceous
strata including limestone, siltstone, mudstone and bentonitic clay with inter-
bedded channel sandstones and conglomerates. Robinson (1963) divided these beds
into four formations: Sphinx Conglomerate; Milligan Creek; Climbing Arrow; and
Dunbar Creek. Separated from the Dunbar Creek formation by a Miocene unconformity,
the upper Bozeman Group beds are a similar sequence of light-colored, fine-grained
tuffaceous deposits with some interbedded conglomerates and sandstones. Above
the Tertiary deposits, a thin layer of Quaternary terrace and floodplain gravels,
cemented fanglomerates and wind-blown silt has been laid down (Hackett , 1960).
The Quaternary alluvial veneer covering the floor of the Three Forks valley
serves as the principal aquifer within the area. This aquifer is composed of
unconsolidated deposits of gravel, sand, silt and clay. Although it is a pro-
lific aquifer, agricultural and subdivision development is rapidly approaching
the appropriation limit. The aquifer is characterized by generally high values
of transmissivity—100,000 to 300,000 gallons per day per foot—and, in many
places yields ample water for irrigation (Hackett and others, 1960). Well depths
vary from 10 to 120 feet and well yields range from 10 to 2,000 gpm from the
aquifer. Chemically, water in the alluvium is a calcium-magnesium-bicarbonate
type, and is suitable for domestic, stock and irrigation uses. Values of dis-
solved solids average around 250 to 300 mg/L.
Underlying the alluvial veneer are semi-consolidated Tertiary sediments.
Characteristically, these sediments have a low permeability as a result of their
lithologic nature. The Tertiary strata have low values of transmissivity
(generally less than 6,000 gallons per day per foot) and yield sufficient water
70
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for only stock and domestic use (Hackett and others, 1960). Domestic and stock
well yields usually average about 15 gpm. However, recent deep wells completed
in the Tertiary are now producing yields in excess of 1,500 gpm. The water
quality of Tertiary sediments varies from one locale to the next as well as
being a function of depth. Sodium appears to be a common constituent of ground
water from Tertiary deposits and there are generally higher concentrations of
other minerals.
Recharge to the ground-water system of the Three Forks valley is dominantly
from irrigation return flow and seepage from losing streams. Rainfall and snow-
melt runoff account for only a small portion of the recharge. Ground-water
discharge occurs mainly as evapotranspiration, springs and well pumpage. During
seasonal periods of low stream flow the alluvial aquifer maintains a baseflow
of the rivers.
Cold Spring Valley (North Boulder)
The North Boulder valley is located southwest of the Elkhorn Mountains
and east of Bull Mountain. At the southwest end of the valley, the North
Boulder River joins the Jefferson River. At the southeastern end, Tertiary
deposits in Nigger Hollow extend into the Three Forks basin. Pre-basin rocks
surrounding the valley include the Precambrian LaHood Formation, Paleozoic
and Mesozoic sedimentary rocks and early Tertiary intrusives. Burfeind (1967)
found the valley fill to be a maximum of 4,500 feet thick. At the northwest end
of Nigger Hollow, the Tertiary sediments may be as thick as 1,000 feet (Parker,
1961), but at the east end, they thin to about 300 feet (Wilson, 1962).
The Red Lane fault bounds the North Boulder valley on its western side
(see Figure 11-19). The fault extends from the Jefferson River along the east
side of Red Hill and through T. 4 N. Alexander (1955) mapped this fault as a
reverse fault with the basin side (east) upthrown. Burfeind's (1967) gravity
data suggests the fault may be normal with basin side downthrown. Another
71
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-""•''V?' t 7/^rieVic^) C?ty'
11 ' ¦' ; -v^V- "•!- ' H f< L N*
S „8uit»cS (JWA
' ' /1 /
A
, ii i ^ n^rttei (
....... .foal/'.
: X\cJ\ r
T7N
T6N
T5N
T3N
Isopach Interval: 1000 ft.
Map Scale' 1'500,000
Refer to basin no. 34
Isopach of C enozoic fill in the Cold Spring Valley
Figure 11-19
72
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possible fault (east side down) runs along the North Boulder River across the
northwest border of Nigger Hollow.
Oligocene and late Miocene strata of the Bozeraan Group fill the basin.
In this area the Bozeman Group consists of soft, light-colored, silty, sandy
and conglomeratic vitric tuffs (Alexander, 1955). Large amounts of volcanic
glass are present in various stages of devitrification. Sorting is generally
poor, though a few beds are made up entirely of silt or sand. An unconformity
of early Miocene age separates the Oligocene and late Miocene strata.
The Cold Springs valley is so named because of the cold springs (approxi-
mately 12°C) which issue from the alluvium, probably discharging from the
Madison near the center of the valley. Presently the springs are nonconsumptively
utilized to support an aquaculture project.
Ground water in the Cold Spring valley is principally derived from the
alluvial aquifer which borders the North Boulder River. This aquifer is lat-
erally quite extensive because of the coalescing floodplain deposits created by
the river's meandering. The aquifer is composed of gravel, sand and some silt
and may be as much as 80 to 100 feet thick. The variability of the sedimentary
deposits directly relates to variations of aquifer transmissivities. Shallow
wells tapping the alluvium are capable of yields ranging from 10 to 50 gpm and
are used for domestic and agricultural purposes. There are very few irrigation
wells within the valley, but the aquifer appears to be capable of large yields.
A well drilled to 95 feet below the land surface in T. 4 N., R. 3 W., section 1
is recorded to have a sustained yield in excess of 1,000 gpm. Ground water of
the alluvium is generally hard, potable and of good chemical quality. Based on
water levels in wells along the valley and on the hydrogeological setting, it
appears that the direction of ground-water flow is from north to south.
Underlying and flanking the alluvial aquifer are Tertiary age sediments.
These deposits are generally composed of poorly-sorted, fine-grained sediments
and characteristically have little storage capacity. Ground-water yields from
73
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these Tertiary sediments are small and only used for domestic and stockwater
purposes. Recharge to the aquifers of the Cold Springs valley is from precipi-
tation and snowmelt, whereas discharge occurs from wells and springs and as in-
fluent streams. .
Little Whitetail and Jefferson Valleys
The Little Whitetail Creek and Jefferson River valleys cover about 250
square miles in Madison, Jefferson and Silver Bow Counties. These Tertiary
basins are bounded by the Tobacco Root Mountains and Bull Mountain to the east,
the Highland Mountains or Boulder batholith on the west and Bull Mountain to
the north. At its southern limit, the Jefferson valley borders the Beaverhead
valley.
Faults form the eastern boundaries of the basins (see Figure 11-20). The
Tobacco Root fault (basin side down) extends along the west front of the Tobacco
Root Mountains. Gravity data (Wilson, 1962) suggests faulting in the subsurface
one mile west of and roughly parallel to the mountain front fault. Kuenzi (1966)
extended the Tobacco Root fault across Tertiary deposits near Whitehall to inter-
sect the Bull Mountain fault on the east edge of Little Whitetail valley. East
of the Tobacco Root fault, the Mayflower Gulch fault (west side down) forms the
east and southeast boundary of a small basin in the Parrot Bench region. Gravity
data from Parker (1961) suggest another fault-along the east edge of the shallow
depression at the eastern end of the Jefferson River valley near Cardwell.
Cenozoic movement along this series of faults (each upthrown on the east)
plus erosion and doming of the Boulder batholith to the west, produced the
Jefferson-Little Whitetail valley, Parrot Bench depression, and the small valley
at Cardwell. In early Tertiary time, an irregular erosion surface formed across
the Belt rocks, Paleozoic and Mesozoic sediments, Elkhorn Mountain Volcanics
and Boulder batholith granites of these valleys. Starting in early Oligocene,
Tertiary sediments Pilled the basins to varying depths (see Figure 11-20) . The
74
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j yfisw
rp
T5N
T2N
Isopach Interval' 1000 ft.
Map Scale: 1.500,000
Refer to basins no. 35 and 36
Isopach of Cenozoic fill m the Little Whitetail and Jefferson Valleys
Figure 11-20
75
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maximum thickness of Tertiary sediments in the Jefferson-Little Whitetail
basin is 7,000 feet (Burfeind, 1967; Petkewich, 1972). Burfeind (1967) gives
a maximum depth of 3,700 feet for the Parrot Bench depression, but the re-
constructed thickness of Tertiary deposits there by Kuenzi and Fields (1971)
indicates a maximum depth of 4,500 feet. Unknown structural complications in
the subsurface may account for the differences. To the east, the small valley
at Cardwell is only about 850 feet deep.
The basins are filled with about 6,000 feet of Tertiary deposits that
make up the Bozeman Group. The lower formation of the group, the Renova,
consists of light-colored, fine-grained strata unconformably overlying pre-basin
rocks. Ranging from 0 to more than 3,500 feet thick, the formation contains
alternating limestones, mudstones, siltstones, sandstones and gravels with a
few conglomerates. More than 70 percent of these sediments are composed of
very fine sand or finer-grain size fraction. Deposition of the Renova Formation
ended in middle Oligocene (or later). A period of erosion followed this deposi-
tion and removed a large volume of Renova strata. Currently, the youngest
sediments to be identified as Renova are middle Oligocene in age.
Deposition of the upper part of the Bozeman Group, the Sixmile Creek
Formation began in late Miocene and ended in middle to late Pliocene. Generally
darker than the Renova Formation, the Sixmile Creek Formation consists of from
0 to more than 2,400 feet of coarse-grained sediments. Kuenzi (1966) describes
the lithology of the 900-foot type section of the formation as: sandy, gritty,
medium-to-coarse sand (30%), very fine-to-medium sandstone (21%), sandy siltstone
(11%), mudstone (3%) and marl (7%). Since the late Pliocene, an unknown amount
of Sixmile Creek strata has been removed and locally veneered by Quaternary
deposits.
Most of the wells drilled in these valleys are completed in the alluvium
of the Jefferson or Little Whitetail Rivers. These alluvial aquifers serve as
a reliable ground-water source that can continually produce yields of 50 to 100
76
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gpm. Higher yields necessary for irrigation do not appear to be possible around
the town of Whitehall; however, there does exist a 1,000 gpm irrigation well in
the southern part of the Jefferson valley. The water quality of the alluvium
is generally good, with the exception of high iron concentrations in some areas.
It also tends to be slightly hard from the calcium carbonate concentration.
Recharge to the system during the summer and fall months results from influent
streams, whereas during the rest of the year recharge is from precipitation.
Beneath and adjacent to the alluvium are Tertiary sediments which form gently-
sloping terraces. As previously mentioned, more than 70 percent of these sedi-
ments are composed of very fine sand or finer grain-size fraction which results
in a lack of void spaces. A review of well-appropriation data showed only a
few wells completed in Tertiary sediments, and their average yield was 10 to
15 gpm. Although there is not any water quality data on these Tertiary wells,
the water is generally considered suitable for domestic and stockwater use.
The terraces are recharged primarily from precipitation and discharge occurs
through wells, springs, seeps and evapotranspiration.
Melrose and Beaverhead Valleys
The Beaverhead valley is an irregularly shaped basin with one arm extending
southeast between the Tobacco Root and Ruby Ranges, and another arm, the Melrose
valley, extending northwest between the Highlands and McCartney Mountain. A
third extension runs along the south side of McCartney Mountain. To the south,
the Beaverhead; valley is bounded by the Blacktail Range, and to the west, by
the Pioneer Mountains. The Jefferson valley on the north and the Blacktail
valley on the southeast adjoin the Beaverhead valley.
Tertiary basins in this region formed by block faulting with some basins
bounded on both sides by faults (Chandler, 1973). In the Beaverhead valley,
the east side has been tectonically active, whereas little evidence of activity
is seen to the west (Hoffman, 1972). Tertiary beds in the valley have a gentle
77
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eastward dip and the thickness of basin-fill deposits is greater on the east
side than the west.
A high-angle normal fault (west side down) on the west flank of the Ruby
Range forms the eastern boundary of the Beaverhead valley (see Figure 11-21) .
To the north beyond the Alder valley, another northeast-trending fault runs
along the west flank of the Tobacco Root Mountains. Six miles west of Sheridan,
the basin's depth is over 8,000 feet (Petkewich, 1972). The Blacktail fault
(north side down) forms the southern boundary of the Beaverhead basin. This
high-angle normal fault cuts Miocene beds, but has not moved since the begin-
ning of the Pliocene. Tertiary deposits thicken to 8,500 feet near the junction
of this fault and the Ruby Range fault. A high-angle normal fault (south side
down) is inferred to be on the northeast border of the Melrose valley (Chandler,
1973; Witkind, 1975). The Melrose valley is over 2,000 feet thick near its
northeastern boundary. On the south side of McCartney Mountain, the valley fill
is over 3,000 feet thick. Along the sides of the Alder valley, Petkewich (1972)
mapped three faults trending northwest-southeast. The graben has two faults
parallel to the south border of the Tobacco Root Mountains, each with south side
downthrown, and another fault (north side down) along the northeast border of
the Ruby Range. The Alder valley fill is over 4,000 feet thick near Alder.
Bedrock that outcrops along the basin's borders consists of Precambrian
metamorphics, Paleozoic and Mesozoic sediments, Creatceous-Tertiary intrusives
and early Tertiary volcanics. It is presumed that unconsolidated Tertiary
deposits of the Bozeman Croup are underlain by rocks similar to those out-
cropping along the mountain fronts. The lower portion of the Bozeman Group
(the Renova Formation) contains alternating limestones, mudstones, siltstones,
sandstones, gravels and a few conglomerates. Over 70 percent of the Renova
Formation is very fine or finer sediments. Deposition of the formation ended
in middle Oligocene; an episode of erosion that removed a large volume of
Renova beds followed. Deposition of the upper part of the Bozeman Group, the
78
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1 ,
I . Vhtwp Wtn' > " \\
r 'M'' if*-
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Isopach Interval. 10Q0ft.
Map Scale 1 500,000
Refer to basins no. 40 and 43
Isopach of Cenozoic fill in ihe Melrose and Beaverhead Valleys
Figure [1-21
79
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Sixmile Creek Formation, began in late Miocene and ended in middle-to-late
Pliocene. Generally darker and coarser than the Renova Formation, the Sixmile
Creek Formation consists of 60 percent medium sand and coarser sediments, 20
percent fine-to-medium sand and 20 percent silt and finer sediments. Since
the late Pliocene, an unknown amount of Sixmile Creek strata has been eroded
and locally a veneer of Quaternary deposits laid down.
Within the Melrose and Beaverhead valleys are three distinct aquifer units :
Cretaceous to Precambrian bedrock; semi-consolidated Tertiary sediments; and
Quaternary alluvium. Fracture networks in the bedrock of the surrounding
mountains create a bedrock aquifer. This aquifer has only minor importance
because it is only capable of small yields and has limited access in the moun-
tains. Overlying the bedrock are Tertiary deposits primarily composed of
fine-grained materials of silt, clay and some volcanic ash. This Tertiary ¦>
aquifer probably has a large volume of ground water in storage, but because the
aquifer contains so much fine-grained material, this water is not able to be
released from storage. Therefore, wells penetrating the Tertiary aquifer have
low yields ranging between 5 and 10 gpm. The ground water from this aquifer is
usually hard and values for dissolved solids average about 400 mg/L. The
Tertiary sediments receive recharge principally from streams and irrigation
water; probably very little direct recharge to Tertiary deposits occurs from
rainfall and snowmelt (Botz, 1967).
The alluvium bordering the Beaverhead River and its tributaries serves as
the most valuable aquifer in these valleys. It is composed of interlayered
gravels, sands, silts and clays and has a maximum total thickness of 200 feet
along the Beaverhead River (Botz, 1967). Yields of more than 900 gpm have been
obtained from this aquifer. The water is dominantly a calcium-magnesium
bicarbonate type, and because of this is quite hard.
Recharge to the aquifer is from snowmelt-runoff, rainfall and leakage from
the Tertiary sediments. Discharge occurs from evapotranspiration, wells and
80
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springs and as baseflow for the Beaverhead River during periods of low flow.
Madison Valley
The Madison valley is located west of the Madison Range, east of the
Gravelly Range and southeast of the Tobacco Root Mountains. Bedrock around the
valley includes Precambrian gneisses to the west and north and Precambrian
dolomite and schist to the southeast (Gary, 1980). Paleozoic and Mesozoic
sedimentary rocks outcrop northeast of the valley, and a Tertiary granitic
intrusion is found to the northwest in the Tobacco Root Mountains. To the
southwest and south of the Madison valley, Pliocene basalt and tuffs and
Pleistocene tuff cover a large area.
On the east side of the valley, the Madison fault (west side down) extends
for 55 miles along the west flank of the Madison Range. Movement occurred
along this irregular north-trending fault in 1959. The fault block beneath the
Madison valley is tilted 5 to 10 degrees eastward into the Madison fault (Pardee,
1950); and maximum thickness of basin-fill strata is 9,000 feet along the east
side of the valley (see Figure 11-22). Along the north side of the valley,
another fault (south side down) trends west-northwest and intersects the Madison
fault. In the upper Madison River valley, gravity data indicate a trough filled
with low-density material, such as basin-fill sediments, beneath the Tertiary
volcanics (Schofield, 1980). This trough is about 3,000 feet deep and extends
from the Madison valley to the Centennial valley. Detailed mapping in the area
has shown block faults (northwest side down) breaking the Tertiary volcanic
rocks along the southeast side of the trough (Gary, 1980).
Basin-fill in the Madison valley consists mainly of unconsolidated con-
glomerate with rounded boulders and cobbles in a sandy, silty matrix and is
late Tertiary of early Pleistocene in age. Quaternary deposits include moraines,
landslides, fan deposits and glacial outwash. In the upper Madison valley,
Tertiary and Pleistocene sediments are interbedded with basalt flows and tuff
81
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itfl
'frh k~v^ ¦ -v •¦;¦
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Isopach Interval- 1000 ft
Map ScjIc 1 500,000
Refci to basin no. 47
Isopach of Cenozoic fill in the Madison Valley
Figure 11-22
82
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beds (Gary, 1980). However, in the lower Madison valley, the interbedded
volcanics do not occur, and the Tertiary sequence becomes more like that in
the Three Forks basin; conglomerates and gravels with a large portion of
tuffaceous and fine-grained material and some interbedded sands.
The alluvium bordering the Madison River is the most prolific source of
ground water in the Madison valley. The alluvial deposits are comprised of
unconsolidated gravels, sands, silts and clays, however, a large percentage of
the finer-grained materials has been carried away by stream flow. The result
is a fairly well-sorted deposit of coarse-grained sediments that have a high
degree of hydraulic conductivity. The thickness of the alluvium is quite
variable, and may be as much as 100 feet thick near Jeffers. Well yields from
this alluvial aquifer generally average about 30 gpm, but the aquifer has the
potential for yields in excess of 100 gpm. Alluvial ground water is of good
chemical quality, suitable for domestic and stockwater uses.
Ground water is also derived from Tertiary sediments in the Madison valley.
Tertiary deposits are composed of a large percentage of fine-grained material
such as tuffaceous and clay-size sediments. These sediments inhibit the move-
ment of ground water and are the reason for the aquifer's relatively impermeable
nature. A well located in section 4, T. 11 S., R. IE. is reported to have a
transmissivity value of only 6,800 gallons per day per foot. This figure appears
to be fairly representative of these sediments. Wells drilled in Tertiary
sediments usually range from 100 to 250 feet deep and have yields of 15 to 20
gpm, but a number of deeper wells have with yields greater than 50 gpm.
Geothermal waters occur near the town of Ennis. These springs issue from
a localized fault system there and their hydrothermal potential is yet unknown.
Recharge to aquifers in the Madison valley is from influent streams, rain-
fall and snowmelt-runof f infiltration. A small percentage, also is derived Crom
irrigation return flows, however, the valley is dominantly dryland farmed.
Evapotranspiration, effluent streams, springs and wells account for the ground-
83
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water discharge in the valley. A comprehensive hydrogeologic study of the
Madison valley should be undertaken in order to evaluate the ground-water
resources, determine the hydrochemistry of the system and the potential for
development of these resources.
Emigrant Valley
The easternmost intermountain basin in Montana, the Emigrant valley, lies
between the Callatin Range on the west, the Snowy Mountains on the southeast
and the Absaroka Range on the east. This basin began forming during the Miocene
with movement along the Emigrant fault (west side down) and tilting of bedrock
and basin-fill strata to the east (Bonini and others, 1972). Seen along the
western flanks of the Absaroka Range and Snowy Mountains, this fault constitutes
the eastern limit of basin and range faulting in Montana (Reynolds, 1979).
Tertiary basin-fill deposits and the Tertiary volcanic bedrock of the basin
floor dip 10 to 20 degrees east into this fault. The combination of faulting
and tilting has resulted in a maximum basin-fill thickness of 3,000 feet and an
average thickness of 2,000 feet along the east side of the valley (see Figure
11-23). Movement along the fault has continued up to Holocene time, as shown
by broken Pleistocene deposits and hot springs aligned along the fault trace.
To the east of the valley, Precambrian metamorphic rocks predominate and
have been intruded by a few Tertiary granites. Paleozoic and Mesozoic sedi-
mentary rocks occur on both sides of the valley at its northern end and also
outcrop along the Mill Creek fault zone, an east-west trending fault which
intersects the Emigrant fault at Mill Creek. On the west side of the valley,
Tertiary volcanics conceal the Paleozoic and Mesozoic rocks that outcrop else-
where in the Gallatin Range (Bonini and others, 1972).
Late Miocene to early Pliocene deposits in the basin consist of tuffaceous
silts and clays with some interbedded sands and gravels. Above these deposits
are Pliocene-age stream gravels with well-rounded cobbles in a sandy matrix.
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T6E T7E T8E T9E T10E
Isopach Interval: 1000 ft
Map Scale. 1:500,000
Refor to basin no. 48
Isopach of Ccnozoic fill in the Emigrant Valley
Figure 11 -23
85
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In the southern part of the Emigrant valley, late Tertiary basalts overlie the
gravels. Quaternary deposits in the valley consist of terrace deposits and
glacial drift (Horberg, 1940).
Ground water in the Emigrant valley is derived from a variety of geologic
sources. North of Pray, Montana the valley floor is veneered with alluvial
deposits of the Yellowstone River. These deposits are of an unknown thickness
and probably overlie similar Wisconsin-age till which is exposed in the side
canyons of the valley. The Yellowstone River alluvium is composed of a hetero-
geneous mixture of sand, gravel, silt and clay that readily yields water to
wells. The Montana Department of Fish, Wildlife and Parks has two campground
wells in section 28, T. 4 S., R. 9 E. which are completed in the alluvium. The
wells are 41 and 37 feet deep and have yields of 110 and 120 gpm, respectively.
General well yields for this aquifer range from 20 to 40 gpm.
South of Pray, Montana the valley lowlands are largely ground moraine of
the Yellowstone Clacier of early to late Wisconsin' age. These glacial deposits
are flanked by Tertiary age terraces. The till is composed of a combination of
cobbles and gravels in a silty-clay matrix. The Tertiary sediments are com-
prised of interbedded fluvial sediments and tuffaceous deposits. Both the
glacial till and Tertiary deposits exhibit similar hydraulic properties and, for
the most part, can be considered as an aquifer unit. These deposits are semi-
permeable and, as such, have a limited degree of hydraulic conductivity. Well
yields are usually small and average 5 to 15 gpm.
Ground water in the Emigrant valley is of good chemical quality and is
used for domestic, recreation and stockwater purposes. It is unknown if there
are sufficient yields for irrigation use, as no large capacity wells (in excess
of 200 gpm) exist in the valley.
Recharge to the ground-water system is from precipitation and losses from
influent streams. Discharge occurs through evapotranspiration, effluent streams,
wells, springs and seeps.
-------
Centennial Valley
Unlike the other intermontane basins of western Montana which trend
northwesterly, the Centennial valley trends east-west. Situated between the
Centennial Mountains (south side) and the Snowcrest and Gravelly Ranges (north
side), the Centennial basin is more closely related to Snake River-Yellowstone
Plateau structures than to the basin-range faulting activity seen in the rest
of southwestern Montana (Myers and Hamilton, 1964). The Centennial fault
(north side down) extends for 40 miles along the south side of the valley.
From gravity data, other faults (north sides down) are seen to parallel the
Centennial fault beneath the basin-fill (Schofield, 1980). Across the valley,
a series of faults (south sides down) form the basin fill-bedrock contact.
Gravity data indicate more faults may parallel these beneath the valley sediments.
In the center of the valley, the basic graben structure is complicated by small
northwest-trending faults. To the east of these, other faults trend northeast
and align with faults along the trough between the Madison and Centennial valleys
(Schofield, 1980). Farther east, the Alaska Basin, a small, roughly circular
valley, is bounded on its north, east and southwest sides by faults, each with
the basin side downthrown. This basin is rather deep for its size and is filled
with about 3,000 feet of Cenozoic sediments. The Centennial valley contains a
maximum of 7,000 feet of basin-fill deposited over its irregularly faulted
bedrock floor (see Figure 11-24).
Paleozoic and Mesozoic sedimentary rocks outcrop in the mountains north
and south of the Centennial valley and are overlain in places by abundant
Tertiary volcanics. To the east around Alaska Basin, Precambrian schist out-
crops. Cenozoic rocks in the Centennial valley consist of basalts, travertine
and tuffs interbedded with semi- and unconsolidated sediments. These sediments
include a middle Miocene channel sandstone and pebble-rich, poorly-sorted
sandstone, a Miocene freshwater limestone, alluvial fan deposits and colluvium,
glacial outwash, silts and sands with local interbedded gravels and dune sand
87
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f /?* ^ '. |.^-*r-vi- i v /•
T11S
IDAHO
R3W
R2W
R1W
R1E
R2E
R3E
R4E
Isopacli Interval' 1000 ft.
Map Scale: 1:500,000
Refer to basin no. 51
Isopach of Cenozoic fill in the Centennial Valley
Figure 11-24
88
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(Sonderegger and others, 1980; Hcmkala, 1949).
To date, there exists a paucity of ground-water data concerning both
shallow and deep aquifers in the Centennial valley. A review of wells drilled
in the valley found only a few well logs recorded for the area. The wells
generally ranged between 50 and 100 feet deep and had yields of 15 to 30 gpm.
These wells probably attain their water from a number of hydrogeologic units
in the valley.
Most of the valley floor is mantled with a veneer of Quaternary alluvial
deposits. They are composed of a heterogeneous mixture of gravel, sand, silt
and clay of unknown thickness. The northern portion of the valley has been
surficially mapped as dune sand, conglomerate and tuffaceous deposits. These
deposits normally produce limited amounts of ground water, and it is assumed
that their potential yields are 5 to 10 gpm. Ground water is used only for
domestic supply and stockwater wells since large capacity irrigation wells are
non-existent. The upper thousand feet of valley-fill materials probably have
an effective porosity of at least 15 percent; thus, the ground water in storage
in this zone is about 150 acre-feet per acre (Sonderegger, 1982). Dissolved
solids concentrations in the water are generally less than 400 mg/L and the
water is thus suitab]e for potable use.
Underlying the Ouaternary-age deposits are interbedded Tertiary sediments
and basalt and rhyolite flows. Tertiary sediments have been previously described
and their potential as a ground-water resource is unknown since wells of
sufficient depth to penetrate them have not been drilled. There also exist a
number of thermal springs in the Centennial valley. Possible heat sources for
these springs could be either an intrusive body or deep circulation of water
along fractures.
The main source of recharge for the aquifers in the valley is rainfall and
snowmelt-runoff and, to some extent, return flow from surface-water irrigation.
Ground-water discharge occurs as springs, seeps and wells and is accompanied by
89
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evapotranspiration and effluent stream flow. A thorough ground-water resource
evaluation should be made for the Centennial valley.
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CONSOLIDATED SEDIMENTARY ROCKS
Consolidated sedimentary rocks in the Rocky Mountains region represent,
excepting the Silurian, all geologic time periods from Precambrian through
Cretaceous (see Figure II-4 for a stratigraphic time scale). These formations
have been faulted, folded and occasionally overturned throughout various
orogenic intervals. Because individual formations are frequently structurally
separated and discontinuous, this entire stratigraphic section is considered a
single aquifer system for the ease of evaluation and interpretation. Deposi-
tional environments for individual formations have been discussed in section
C.l. of this report.
The Precambrian formations, comprising most of the northern half of the
Rocky Mountains region, consist of red and green argillites with interventing
black, maroon and pink quartzites. There are 15,000 to perhaps as much as
45,000 feet of Precambrian sediments deposited in this portion of the Rocky
Mountains region. These sediments are roughly 0.8 to 1.4 billion years old
and are collectively known as the Belt Supergroup. The yield from wells
completed in Precambrian rocks is variable, but generally small, because these
rocks are "tight" and water must be obtained from secondary openings such as
joints, fractures, faults and weathered zones of the bedrock. Yields range
from 1 to 35 gpm, with 10 gpm being about the average. Water quality from
Precambrian sediments is of very good quality, usually having less than
300 mg/L of dissolved solids.
Paleozoic strata are mostly made up of carbonate sediments and shales with
some clastic formations such as the Flathead and Ouadrant Formations at the base
and the top of the section, respectively. Mountain-forming stresses have warped
and bent the formations into a series of folds and thrust sheets. As in the
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Great Plains region, the Madison Formation is important not only as a source
of ground water, but as a place for recharge infiltration. Well yields from
Paleozoic strata are highly variable and are dependent not only on the forma-
tion drilled to, but also proximity to outcropping of the formation. Water
quality is generally good, but can vary depending on the formation drilled,
depth and distance from the recharge area. Wells completed in Paleozoic sedi-
ments are used primarily for domestic and stockwater, although some larger
capacity wells in the Madison Formation are used for irrigation.
Mesozoic strata demonstrate a gradual transition from the chiefly marine
beds of the lower formations to mostly terrestrial sediments found in the upper
part of the section. These formations have also been subjected to the same
tectonic stresses which deformed the underlying Paleozoic sediments. Deforma-
tion has not only folded, inclined and overturned the strata, but often
vertically and laterally displaced beds hundreds to tens of thousands of feet.
The major water-bearing units (aquifers) within the Mesozoic svstem are the
Jurassic Swift Sandstones, the basal Cretaceous Kootenai Sandstone and the
Eagle Sandstone where erosion has not removed it. Impermeable shale formations
are interbedded among the sandstone units and act as confining beds. Faulting,
however, has often juxtaposed different formations and the extent of the aquifers
are hydraulically interconnected is uncertain. There exists practically no
water-quality data for wells drilled in Mesozoic Formations of the Rocky
Mountains region, but moderate values of dissolved solids would be expected.
Well yields for these aquifers are widely variable, ranging from 5 to 100 gpm
and are contingent upon location, attitude of the bedded rocks and proximity
to recharge areas. Rainfall and snow-melt water account for nearly all of
the recharge for "consolidated sedimentary rocks." Evapotranspiration from
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dense forests, springs and wells discharge the ground water to keep the
system in balance.
The aforementioned Precambrian, Paleozoic and Mesozoic strata comprise
all of the mountain ranges north of the "batholithic province" and a substantial
number of scattered ranges in the central and southern portions of the Rocky
Mountains region. The inaccessibility of the mountain ranges has deterred and
prevented development and drilling, thereby yielding only scant ground-water
information for this aquifer unit.
METAMORPHIC AND IGNEOUS ROCKS
Metamorphic and igneous rocks comprise a large area of the south-central
portion of the Rocky Mountains region. The Boulder and Idaho batholiths of
Cretaceous age are the largest plutons, while others such as the Tobacco Root,
Pioneer and Flint Creek stocks are of lesser areal extent. These batholiths
and their associated contact metamorphic assemblages outcrop or underlie most
of this central portion and frequently are referred to as the "batholithic
province." The plutons are composed mainly of quartz monzonite and related
granitic rocks.
The availability of ground water from granite is rather limited. Water
availability from this type of crystalline rock is entirely dependent upon
secondary porosity because of insufficient primary porosity. Water from these
rocks must be obtained from secondary porosity which is produced by: horizontal
pressure release fractures that form as a result of sheet unloading; vertical
joint sets which were produced from tension release fracturing; and faults
that were a result of tectonic stresses. The combined interconnectedness of
these openings will provide space for ground-water storage and conduits for
movement. These features are often surficially expressed as lineaments and
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joint traces, and most of the higher yield wells are located where the fractures
intersect. The wells are generally between 100 and 200 feet deep because
permeability generally decreases with depth. Average well yields for granitic
rocks are approximately 2 to 5 gpm. There is a paucity of water quality data
from igneous rocks, but values of 300 mg/L of dissolved solids are common.
Proximal to these large batholiths and stocks are various Cretaceous and
Tertiary extrusive rocks. The Elkhorn Mountain and Lowland Creek volcanics
outcrop over considerable areas and are composed mainly of rhyolites and latites.
These rocks are a source of potable water for many rural families. Wells from
extrusive rocks generally have low yields because of their dependence on
fracture openings. This aquifer is mainly recharged from precipitation, and
wells and springs are the primary types of discharge.
Lying south and east of the batholithic province is an extensive area
that contains pre-Belt metamorphic rocks (2.7 to 1.7 billion years old). The
Beartooth and Ruby Ranges and part of the Tobacco Root Mountains are composed
of this metamorphic assemblage. They are mainly granitic gneisses and schists
that have been fractured enough to allow ground-water storage. Well yields
tend to be very small, but water quality is good (though often has some iron
concentrations). These rocks are therefore an important domestic source of
ground water in the southern portions of the Rocky Mountains region. Recharge
and discharge for this aquifer are also through precipitation and wells and
springs, respectively.
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III. GROUND-WATER USE
Information on water use in Montana prior to 1980 is extremely limited
because accurate data on withdrawal rates is practically non-existent. Com-
munities have the best opportunity to record water use, but in most instances
only new delivery systems are equipped to measure discharge. Similarly, rural,
agricultural and industrial water users often have no means of measurement, and
only estimates can be made for those values. Montana, however, is presently
quantifying its water use and consumption through a water-right adjudication
program. This program is being implemented through the Department of Natural
Resources and Conservation under Senate Bill No. 76. All water-use applications
are to be filed by April 30, 1982 and then will be reviewed and summarized.
Better estimates of ground-water and surface-water use will become available
after that date. The Department is also compiling a 1980 water-use and quanti-
fication survey which is yet to be released.
Major uses of ground water in the Rocky Mountains region are for irrigation,
municipalities, industry, rural-domestic and livestock. Table II-4 summarizes,
by county, the various well uses of this region. Most of these wells are com-
pleted in the Quaternary alluvial or glacial aquifers, although Tertiary and
bedrock aquifers are exploited locally. An estimate of the cumulative ground
water withdrawn from the Rocky Mountains region is approximately 146.14 million
gallons per day (mgd) or 448.65 acre-feet per day. This value for ground water
withdrawn represents about 3 percent of the total amount of water diverted
within the Rocky Mountains region, a figure that is believed to be a conservative
estimate. Even though current ground-water use is small, it is the only viable
source of potable water that can and will be further developed now that surface-
water supplies arc over-appropr i.nted in this regLon.
95
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TABLE 11-4
WELL USE BY COUNTY, ROCKY MOUNTAINS REGION
August 1981
COUNTY
COM
DOM
D+S
IRR
IND
PUB
STK
MU
OTH
NOT
RPT
TOTAL
Beaver head
11
663
168
42
4
14
258
53
30
6
1249
Broadwa ter
0
308
145
43
1
8
191
33
28
1
758
Deerlodge
3
532
59
16
6
4
23
22
14
2
681
Fla thead
19
2631
696
81
37
34
4 1
233
82
8
3862
Gallatin
15
2101
568
48
18
27
189
95
95
2
3158
Glacier
8
130
153
1
20
4
104
10
10
0
440
Granite
4
348
72
9
1
4
36
13
30
0
517
Jefferson
1
556
90
15
6
25
85
46
23
1
848
Lake
2
885
437
18
3
20
52
94
26
2
1539
Lewis and Clark
10
2121
297
67
15
36
149
135
142
4
2976
Lincoln
11
1091
89
7
6
15
14
31
39
2
1305
Madison
2
685
219
18
3
14
158
26
28
0
1153
Meagher
0
87
63
1
2
5
53
20
1 1
2
244
Minera1
5
210
25
6
3
8
6
12
18
2
295
Missoula
14
2354
292
47
6 5
60
47
153
103
48
3183
Park
1
564
183
25
9
1 1
65
57
36
10
961
Powell
1
363
100
15
3
6
71
28
8
2
597
Ravalli
10
3338
348
193
18
30
95
550
481
2
5065
Sand ers
5
479
193
61
2
8
43
80
27
1
899
Silver Bov
5
552
58
28
10
4
28
22
23
2
732
To tal
127
19998
4255
741
232
337
1708
1713
1254
97
30462
COM-Community; DOM-
Stockwater only;
-Domestic; D+S
MU-Multiuse;
-Domestic
OTH-Other:
and Stockwater; IRR-Irrig
; NOT RPT-Not Reported.
a t ion;
IND-Indu:
atrial; PUB-
-Public;
STK-
-------
AGRICULTURE
Agriculture, specifically crop irrigation and livestock watering, uses
the most ground water in the Rocky Mountains region. Tertiary uses which are
agriculturally oriented include fish farming and wildlife refuges.
Irrigation
There are approximately 1,244,000 acres of irrigated land in the Rocky
Mountains region. Most of the irrigated cropland in the region is hayland,
while small grains and potatoes account for a substantial portion of the
remainder. Other crops in the region dependent upon irrigation are sweet and
tart cherry orchards, tree farms and mint plantations. The percentage of this
acreage that is irrigated in any given year is uncertain. Roughly 4.89 billion
gallons per day (bgd) are diverted for this acreage, of which 1 percent is
withdrawn from ground-water sources. Almost all irrigation wells are completed
in the unconsolidated alluvial aquifer, but a few other large-capacity wells
obtain water from Tertiary aquifers.
Requirements for diversion are more than double the consumptive use,
resulting in a return flow 53 percent of the total diversion (DNRC, 1974).
Consumptive use varies with irrigation efficiency, rates of application and
other factors such as the crop, soil, precipitation, growing season and ambient
temperature. Nearly all irrigation is used for raising feed crops to support
the livestock industry.
Livestock
Stock consumptive use of ground water in the Rocky Mountains region is
estimated to be 8.5 mgd, of which 5 percent is withdrawn from ground-water
sources. Cattle and sheep account for most of the water consumed, with average
97
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daily consumption values of 15 and 3 gallons per head per day, respectively.
Pigs, horses and other livestock use the remainder of stockwater consumed.
About 1700 wells are used for stockwatering only and another 4300 rural
wells are used jointly for domestic and stockwater purposes. Most stockwater
wells derive ground water from the alluvial aquifers; Tertiary and bedrock
aquifers also offer viable sources for sufficient amounts of ground water.
Springs and seeps are another source for stockwater within the reigon, but
are not well identified because their source and discharge rate are often
unknown. Typical stock wells and springs usually have sustained yields of
10 to 15 gpm. Stockwater wells are an integral part of the livestock ranch-
ing industry within the Rocky Mountains region.
Aquaculture is a new and increasingly popular aspect of the agricultural
industry. Many privately owned fish farms have recently begun operations in
this region. Although these businesses use ground water non-consumptively,
they totally rely on springs and wells to maintain their livelihood. State-
owned fish hatcheries are also dependent upon ground-water sources in much
the same way.
MUNICIPAL AND DOMESTIC
A computer listing produced by the Montana Department of Health and
Environmental Services in 1980 showed that there were 65 communities in the
Rocky Mountains region of Montana that have a municipal water-supply system.
The total number of public-supply systems in this region is about 330 if
trailer courts, nursing homes and other institutional systems are included.
Of the 65 communities, 16 rely exclusively on surface water, another 16 use
both surface and ground water and the remaining 33 communities depend solely
on wells or springs for their water supply. Of the 393,625 people who reside
within the Rocky Mountains region of Montana, approximately 259,700 live in
municipalities. Of these, 93,070 depend exclusively upon ground water for
98
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their drinking and household needs; they withdraw a total of about 30.01
million gallons of water per day.
Quaternary alluvium is the primary aquifer used for municipal wells in
the Rocky Mountains region supplying, perhaps, as much as 70 percent of the
water withdrawn. Tertiary and glacial deposits provide most of the remainder.
The quality of water used by most of the communities in the Rocky
Mountains region of Montana is usually excellent, and all systems tested had
fewer dissolved solids than the maximum recommended by the EPA. Iron is
sometimes a problem in trailer court water supplies; as an example Wilsall had
a concentration of 2.9 mg/L of iron in its water supply. Several water systems
had measureable trace elements in the water they supplied. The highest lead
value was 0.18 mg/L in water from a trailer court near Big Sky. Arsenic
measured 0.6 mg/L in water supplied by Three Forks and mercury was highest near
West Glacier and Coram, 0.33 and 0.35 mg/L, respectively. Most community
water supplies had low nitrate values. Water from the supply system at Alberton,
however, had 4.4 mg/L; White Sulphur Springs had 4.8 mg/L and Wilsall had
11.8 mg/L. Many trailer courts had nitrates exceeding 5.0 mg/L in their water
supplies.
Domestic water is that ground water used by all persons not served by a
municipal or community water system. For the most part, domestic wells
primarily are used by rural residents, although many subdivision units also
have individual wells. The approximately 20,000 domestic wells comprise the
largest single category (65%) of permitted wells in the Rocky Mountains
region. It is estimated that an average withdrawal of 33.86 million gallons
is consumed daily in the Rocky Mountains region.
99
-------
Because most residential settlements are within valleys, the Quaternary
alluvial aquifer is the primary ground-water source. An example of this is
the Gallatin Valley where new subdividions with their accompanying wells are
being constructed throughout the valley. The Tertiary aquifer is also an
important source of ground water for many rural residents as in the Bitterroot
Valley; the Sunset Bench subdivision attains its potable supply from this
aquifer. As evidenced by the large percentage of domestic wells, ground water
is dominantly relied on for rural habitation.
INDUSTRY
The Montana Department of Natural Resources and Conservation defines
self-supplied industrial water as that which is obtained from a source of
supply by industry as opposed to that provided by a municipality. An industry
is also considered to be self-supplied if any of the water it uses is obtained
from its privately owned water supply facilities.
It is estimated that 28 mpg of ground water are withdrawn for industrial
use in the Rocky Mountains region, of which 30 percent is consumed. The water
that is not consumed is either discharged as surface-water flow, or treated and
recycled for re-use or disposed through injection wells.
Industrial water use in the Rocky Mountains region is dominated by the
minerals industry. The Anaconda Company operations at Butte and Columbia Falls
account for a major portion of the ground water withdrawn in the region. The
Butte operation withdraws roughly 7.5 mgd for mine dewatering and processing
the copper ore. Much of this water is recycled and the exact amount consumed
is unknown as it is dependent upon daily operations. The Anaconda Aluminum
Plant at Columbia Falls withdraws approximately A.63 mgd of which 0.18 mgd
are consumed for either refining or cooling uses. Another mineral industry
that withdraws large quantities of ground water is the Stauffer Chemical Company.
One million gallons per day are withdrawn for their phosphate-processing opera-
100
-------
tions of which one-fourth is consumed. The remaining minerals processing
industries use very small amounts of water.
Other large industrial water uses include the Horner-Waldorf pulp mill
and the White Pine Sash Company of Missoula. Horner-Waldorf has an intake
rate of 16 mgd of which 15 percent is consumed, whereas the White Pine Sash
Company is estimated to consume 0.14 mgd as steam.
Lesser amounts of ground water are withdrawn also for a variety of other
uses such as geothermal heating, sanitation and boiler feeding.
Of the total amount of water diverted for industry in the Rocky Mountains
region about 40 percent is ground water. Most industrial wells tap the
Quaternary alluvial aquifer, however, some obtain water from Pleistocene glacial
deposits. Both of these aquifers are a prolific source of good quality water
for industrial use.
101
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IV. WATER QUALITY
Data Sources
More than 3,000 water-quality analyses contained in the computer files at
the Montana Bureau of Mines and Geology (MBMG) were reviewed for the UIC Project:
approximately 375 of these analyses are from the Rocky Mountains region. Addi-
tional analyses are contained in MBMG and U.S. Geological Survey (USGS) bulletins,
memoirs, open-file reports, professional papers and unpublished reports.
The MBMG water-quality file contains water-quality analyses generated by
the MBMG Analytical Division. Primary customers of this Division are the MBMG
Hydrology Division, the USGS Water Resources Division in Helena, Montana, and
the U.S. Forest Service (USFS). The USGS and the MBMG Hydrology Division
furnish water samples taken from ground-water sources within the state of
Montana to the MBMG laboratory for analysis; the results of these analyses have
become part of an integrated data bank.
Previously published geologic and hydrologic reports for the intermontane
basins of the Rocky Mountains region contain water-quality analyses. The most
recent USGS publication, Open-File Report 80-1102 by Moreland and Leonard (1980),
discusses ground-water characteristics of the Helena Valley. The water-quality data
contained in this report were processed at the MBMG laboratory during 1979 and
1980 and are contained in the listing of water quality data in Appendix E.
Older reports including those written by Coffin and others (1971) on the Tobacco
and Upper Stillwater Valleys; Hackett and others (1960) on the Gallatin Valley;
Konizeski and others (1968) on the Kalispcll Valley; McMurtrey and others (1972)
on the Bitterroot Valley; and McMurtrey and others (1965) on the Missoula Basin
all contain water-quality analyses and descriptions of ground water for their
respective areas. An ongoing project at the MBMG is to assimilate these previously
published analyses in the data-management system.
102
-------
Appendix E contains a tabulation of those analyses in the water-quality
system selected for this project. These analyses have been sorted by formation
or aquifer and by township, range and section; many have been plotted on the
"Dissolved Solids Map series" included with this report. Occasionally, points
will appear on the listings that have not been plotted on the maps and, con-
versely, points will appear on the maps which are not contained in the tabulation.
This has occurred because much previously published data are not computerized
and because the listings may include data created since the compilation of the
maps.
General Water Quality
Ground-water quality data for three aquifers or aquifer groups in the
Rocky Mountains region were extracted. These aquifers include:
]) Cenozoic basin fill deposits
2) Early Tertiary through Precambrian
consolidated sedimentary rocks
3) Igneous and Metamorphic rocks
Table 11-5 compares selected elements and ions to drinking water quality
standards published by the U.S. Environmental Protection. Agency (EPA). However,
since no standard has been established for sodium plus potassium, an arbitrary
value of 250 mg/L has been selected as a reference point.
Based on these data, ground water in the Rocky Mountains region is
generally of better quality than that recommended by the EPA's standards. In the
three aquifer groups, 97 percent of the samples had dissolved solids concen-
trations of less than 500 mg/L; less than one percent of the samples had nitrate
(as N) concentrations greater than 10 mg/L; approximately 99 percent of the anal-
yses reported sulfate concentrations of .Less than 250 mg/L; and there were no
chloride concentrations greater than 250 mg/L. Manganese and iron are the two
103
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TABLE II-5
COMPARISON OF SELECTED ELEMENTS
AND IONS IN WATERS OF
THE ROCKY
MOUNTAINS REGION,
MONTANA TO DRINKING WATER QUALITY
STANDARDS4
AQUIFER
CONSTITUENTS
NUMBER OF
% GREATER
% LESS THAN
AVERAGE
AND
VALUES
THAN STANDARD
STANDARD
CONCENTRATION
STANDARDS
REPORTED
IN MG/L
Cenozoic Basin Fill
Na+K(250)1
305
< 1
> 99
33.
Deposits
Fe(.3)2
304
12
88
.3
Mn(.05)2
305
23
77
.09
CI(250)2
305
< 1
> 99
11.
SO4(250)2
305
3
97
.44
N(10)3
304
< 1
> 99
1.
Ds(500)2
305
7
93
264.
Early Tertiary through
Na+K(250)1
27
4
96
83.
Pre-Cambrian
Fe(.3)2
27
11
89
. 1
Consolidated
Mn (.05)2
27
7
93
.017
Sediments
CI(250)2
27
0
100
13.
SO^(250)2
27
0
100
59.
N(10)3
27
0
100
.4
Ds(500)2
27
0
100
316.
Igneous and
Na+K(250)1
42
0
100
21.
Metamorphic Rocks
Fe(.3)2
42
5
95
. 1
Mn(.05)3
42
12
88
.032
CI(250)2
42
0
100
7 .
SO,(250)2
42
0
100
30.
N(10)3
42
0
100
.5
Ds(500)2
42
2
98
195.
^ No standard has been set. A concentration of 250 Mg/L has been selected as a point of reference.
2
Secondary drinking water standard in Mg/L
3
Primary drinking water standard in Mg/L
4
Source: U.S. Environmental Protection Agency, 1976
-------
most likely elements to exceed the water quality standards with 13 percent
and 9 percent of the reported values, respectively, being greater than the
standards. In some areas, such as unconsolidated sediments in the Bitterroot
Valley, dissolved iron concentrations commonly exceed .3 mg/L and often are
several mg/L. In areas such as these, iron staining of household fixtures and
clothing are common.
Cenoiioic Basin-Fill Deposits
Most of the 305 analyses in this group are for ground water from uncon-
solidated to semiconsolidated, Holocene to Tertiary sediments deposited in the
intermontane basins. The geographic distribution of these data is uneven as
certain locales, such as the Helena Valley or the Little Bitterroot Valley
have been sampled extensively during MBMG or USGS projects and other areas, such
as the Beaverhead Valley, have had only a few water samples collected.
Dissolved solids in the Cenozoic basin-fill and alluvial deposits range
from a high of 1,273 mg/L for water from a 1,498 feet deep geothermal test well
producing water from Tertiary sediments at the Warm Springs State Hospital in
T. 5 N., R. 10 W., sec. 13DCC in Deer Lodge County to a low of 27 mg/L for water
from a USGS research well completed in glacial deposits in T. 28 N., R. 33 W.,
sec. 9BDDB in Sanders County near Libby. The average dissolved solids for all
analyses from this group is 260 mg/L.
Figure 11-25 is a plot of dissolved solids versus the number of occurrences
for the samples in this group. The majority of the analyses plotted represent
calcium bicarbonate type but some sodium bicarbonate type waters occur in almost
every dissolved solids range. The lowest dissolved solids value in the group
is for a calcuim bicarbonate water while the highest reported dissolved solids
value is for a calcuim sulfate type water.
Numerous older analyses exist for basin-fill and unconsolidated alluvial
-------
200 250 300 350 400
DISSOLVED SOI.IDS IN MC/I,
40_L
A 50
106
-------
deposits in the Rocky Mountains region. Coffin and others (1971) report that
waters in the Tobacco and Upper Stillwater Valleys of northwestern Mountana range
in dissolved solids from 80 to 1,500 mg/L. The waters are generally calcium
bicarbonate type and iron concentrations range from below the detection limit
to .07 mg/L and occasionally cause problems. Based on 58 analyses, Hackett and
others (1960) report that waters in the Gallatin Valley are predominately
calcuim bicarbonate type and range from 154 to 597 mg/L dissolved solids.
Konizeski and others (1968) report that Quaternary aquifers in the Kalispell
Valley produce water ranging in dissolved solids from 132 to 788 mg/L. Water
types are generally calcium bicarbonate, but iron concentrations as high as
14.1 mg/L are reported. McMurtrey and others (1965) in discussing the hydrology
of the Missoula Basin report dissolved solids concentrations range from 94
to 326 mg/L. Iron concentrations range from .04 to 6.9 mg/L. Calcium and
bicarbonate are generally the most common cation and anion present. McMurtrey
and others (1972) report dissolved solids concentrations for the Bitterroot
Valley south of Missoula range from 42 to 748 mg/L. Iron concentrations range
from .01 to 4.1 mg/L and can cause problems by staining clothing and household
fixtures. Dissolved solids concentrations along the west side of the valley
often are lower than those for the east side because the granitic rocks to the
west contain fewer soluble minerals than the sedimentary and igneous rocks to
the east. Moreland and Leonard (1980) describe the qualities and types of ground
water for the Helena Valley. These waters are generally calcium bicarbonate type
and range in dissolved solids from 111 to 936 mg/L with an approximate average
of 330 mg/L. Anomalous samples from T. 10 N., R. 3 W., sections 16, 17, and 18
show evidence of poor sewage disposal practices and/or leachate from a landfill.
Early Tertiary Through Precambrian Consolidated Sedimentary Rocks
There are only 20 analyses recorded for ground water from Early Tertiary
107
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through Precambrian consolidated rocks in the MBMG data system. The average
value of dissolved solids for these samples is 316 mg/L. A major reason for
the small number of analyses is that relatively few people live in areas under-
lain by these materials resulting in few wells. Most of the ground-water
development in the Rocky Mountains region has occurred in the intermontane
basins and stream valleys where water is obtained from basin fill deposits or
alluvium. Dissolved solids concentrations range from 481 mg/L for water
from McMenomy warm springs in T. 9 S., R. 10 W., sec. 29AAAC in Beaverhead
County to 106 mg/L for water from a spring in T. 4 S., R. 13 E., sec. 5ADCB in
Sweetgrass County. Both of these springs represent discharge from Madison
Group rocks but illustrate differing circulation regimes. The higher dissolved
solids waters from the McMenomy spring represent waters from a deep circulation
system accounting for its calcium sulfate character and relatively warm temp-
erature (19 degrees C). The lower dissolved solids water from the spring in
Sweetgrass County is a calcuim bicarbonate type and is from a shallow circula-
tion system in the Madison group.
Figure 11-26 is a plot of dissolved solids versus the number of occurrences
for ground water from Late Tertiary through Precambrian consolidated sedimentary
rocks. As can be seen from the patterns representing water types, calcum bi-
carbonate waters are most common followed by sodium bicarbonate waters.
Igneous and Metamorphic Rocks
There are 42 analyses of water from igneous and metamorphic rocks which
also represent portions of the Rocky Mountains region where relatively little
ground-water development has taken place. Dissolved solids concentrations
average 195 mg/L and range from a high of 672 mg/L in water from a 6,970 feet
deep geothermal test well in T. 12 N., R. 6 W., sec. 32ABD near Marysville in
108
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1 5
U
I 3
11
1 I
10
9
8
7
f>
5
/«
'3
1
0
Fi CURE — 11-26 FREQUENCY OF OCCURANCF VERS IS
DISSOLVED SOLIDS FOR WATERS FROM EARLY TERT I.ARY
THROUGH PRECAMRRLAN CONSOLIDATED ROCKS, ROCKY
MOUNTAIN REGION, MONTANA.
-I
-14
-1 3
CallCO.
3
-1 2
NallCO
3
-10
OTHERS
NaSO.
NaCO
3
CaSO
-9
u
z
<
u
u
o
-7
-fi
UJ
03
500
DISSOLVE.!) SOLIDS IN MG/l,
109
-------
Lewis and Clark County to a low of 28 mg/L in water from a spring in T. 13 S.,
R. 2 E., sec. 31CADD in the Centennial Valley in Beaverhead County. The higher
dissolved solids water from the geothermal test is a sodium bicarbonate sulfate
type produced by plutonic rocks and represents a deep, warm-water circulation
system. The lower dissolved solids water is a calcium bicarbonate type and is
discharged from metamorphic rocks.
Figure 11-27 is a plot of dissolved solids versus the number of occurrences
for ground water from igneous and metamorphic sources. As can be seen from the
patterns representing the various water types, the majority of the analyses are
for calcium bicarbonate type waters.
110
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FIGURE — I.I.-27 FREQUENCY OF OCCURANCE VERSLS
DISSOLVED SOLIDS FOR WATERS FROM IGNEOUS AND
METAMORPHIC ROCKS, ROCKY MOUNTAIN' REGION, MONTANA.
CallCO
NalICO
OTHERS
NaSO
4
MgHCO^
NnCO
:ij> i
40b
40 l_ 1 t\ 5 1 '>501
4 50 500
DISSOLVED SOLIDS IN MC/l.
Ill
-------
V. SUMMARY MD CONCLUSIONS
(In Progress)
VI. REFERENCES
Appendix A: System of Geographical Locations
Appendix B. Glossary
Appendix C: Montana Water Law
Appendix D: Printout of Injection Wells
Appendix E: Printout of Water Quality Analyses
(In Progress)
112
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