WORKING PAPER NO. 24
COLUMBIA RIVER BASIN PROJECT
For Water Supply and Water Quality Management
SEDIMENT PRODUCTION RATING
CHEHALIS RIVER BASIN, WASHINGTON
Date: June 19, 1962
Prepared by WEB
Reviewed by
Approved by
DISTRIBUTION
Project Staff_
Cooperating Agencies_
General
U. S. DEPARTMENT OF HEALTH, EDUCATION AND WELFARE
Public Health Service
Region IX
Division of Water Supply and Pollution Control
570 Pittock Block
• Portland 5, Oregon
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This working paper contain* preliminary data and information
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SEDIMENT PRODUCTION RATING
CHEHALIS RIVER BASIN
WASHINGTON
INTRODUCTION
The sediment production rating is based on field inspection of representative
portions of the Chehalis Basin to determine extent of the sediment problem,
to locate sediment source areas, and to estimate total sediment movement within
the basin.
SUMMARY AND CONCLUSIONS
The sediment production rating given the Chehalis River Basin is "low", indi-
cating an average sediment yield between 50 and 150 parts per million of flow.
Physical features of the watershed are favorable, despite some soils with a
tendency to slump and slide. The forest cover comes back rapidly after the
timber harvest, and erosion and sedimentation following logging do not last
long. There are few or no problems on the cultivated lands. Bankcutting at
high water stages occurs along some of the channels, but does not appear to
be causing excessive sedimentation.
DESCRIPTION OF THE BASIN
The Chehalis River drains the southern end of the Olympic Mountains, the
Cascade Mountain foothills south of Puget Sound, and the northeast part of
the Willapa Hills in southwest Washington. It empties into the Pacific
Ocean through Grays Harbor. The basin occupies western Lewis County and
parts of Thurston and Mason and Pacific and Grays Harbor Counties, with a
total area of about 2,000 square miles. Highest elevations in the basin
are 3,100 feet on Bolstfort Peak In the Willapa Hills; 3,800 feet in the
Cascade foothills at the head of the Skookumchuck drainage; and 4,800 feet
in the Olympic Mountains on Discovery Peak at the head of the Wynoochee
drainage.
Geologic formations —' underlying the basin include recent alluvium along
the main river and the major tributaries; basalts and breccias of Eocene
to Miocene age interbedded with sandstones and conglomerates In the upper
Wynoochee drainage and along Elk Creek and in the Willapa Hills and Cascade
foothills; gravelly terrace deposits in the middle Wynoochee and Satsop
drainages; quaternary glacial drift and till in the Black River and Scatter
Creek drainages; nonmarine quaternary gravelly deposits in the Newaukum
Valley and along the main Chehalis River; marine sandstones and siltstones
of Miocene and Pliocene age in the lower Wynoochee area; and Eocene marine
sedimentaries in the Willapa Hills and central parts of the basin. The
I/ Geologic Map of Washington, Washington Dep't. of Conservation.
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sedimentary rocks are soft and readily weathered and subject to sliding and
slumping*
Soils developed on these rock formations range from clay loams to shot loams
in the mountains, from silty clay loams to peat and muck soils in the valleys*
The forest soils are generally deep and permeable, moderately erodible when
disturbed, and occasionally subject to slumping and sliding where subsoils
are clayey.
Cover is predominantly Douglas-fir forest in the hills and mountains, with
hardwood brush on nearestocked old cutovers and burned areas* tower hills
on the forest fringe are used for pasture; here the forest has been changed
to grass. The valleys are cultivated* Most of the forest has been cut over
once.
Climate is humid, with long wet winters and short dry summers. Temperatures
are moderated by the influence of the Pacific Ocean to the west. While some
mountain areas, as in the upper Wynoochee drainage, receive upwards of 160
Inches average annual precipitation, rainfall intensities rarely exceed an
inch an hour* At elevations above 2,000 feet much of the precipitation comes
as snow.
Streamflow follows the climatic pattern, high in winter and spring, low in
summer. Major floods have occurred from November to March* January and
February have the highest average flow in most streams, August the lowest.
With respect to erosion and sedimentation, the physiographic factors are
generally favorable. While much of the topography is steep, the soils are
stable, the cover vegetation dense, and the rainfall gentle. The densest
forest covers the higher elevations with the greatest precipitation.
RELATED STUDIES BY OTHER AGENCIES
In 1959 the Washington Pollution Control Cpmnission took sediment samples at
two-month intervals at various stations along the Chehalis River and some of
the major tributaries* Sediment loads i'observed ranged from less than 20
ppm most of the season to nearly 200 ppm at one station after a December
storm* Some of the loads over 100 ppm were noted as due to road construction.
The report of the Corps of Engineers on the Wynoochee River —' indicates that
4 acres of arable land are lost each year by stream changes and bankcutting
at times of high water.
I/ Unpublished data in files of the Washington Pollution Control Commission.
2/ Report on Survey of Wynoochee River, Washington, Corps of Engineers,
~" December, 1961.
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There are only small diversion dams on streams of the basin and no opportunity
for complete settling and trapping of sediments.
SURVEY METHODS
Visual inspection was made of the major stream channels and of as much of the
watershed area as could conveniently be reached by road. Channel conditions
with respect to bankcutting, sedimentation, and aquatic habitat were observed
and noted. Logged, burned, grazed, and farmed areas were inspected on dif-
ferent geology and topography to estimate relative sediment contributions.
Effects of construction of an extensive system of mountain roads for the
timber harvest were also noted*
Ratings of sediment production were made in five classes which may be broadly
interpreted in terms of p.p.m. suspended sediment load in the average annual
total water yield as follows:
Rating Class Average Suspended Load, p.p.m.
Slight Under 50
Low 50 to 150
Moderate 150 to 500
High 500 to 1500
Very High 1500 plus
The estimated ratings are subjective, based on comparing physical conditions
of the Chehalis Basin with those of other basins previously surveyed.
SURVEY FINDINGS
The Skookumchuck and Newaukum tributaries draining from the Cascade Mountains
foothills have stable watersheds with few slide or slump areas and not much
sediment contribution from roads or logging. Channel beds are stable, popu-
lated with a variety of small aquatic fauna and flora, and appear to provide
excellent aquatic habitat. Popularity of these streams with fishermen bears
this out. Though visited shortly after a spring storm and therefore flowing
at above average rates, these streams did not show appreciable turbidity in
the upper reaches. Where they break out of the hills into the rolling to
flat main valley, there was some bankcutting and the waters were somewhat
turbid. In only one place, however, was there excessive bankcutting and
downstream sediment deposition noted. Through the agricultural area there
was occasional evidence of minor bankcutting, but it did not appear to be
creating any serious problems. These two watersheds are rated "low".
The Black River and Scatter Creek tributaries on the northeast corner of the
basin drain the Black Hills south of Olympia and the rolling glacial drift
terrain north of Centralia. Erosion and sedimentation present no problems.
While there has been erosion in the upper tributaries following logging and
some bankcutting, the eroded material has not moved out of the tributaries.
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Old beaver dons, stabilized by hardwood growth, have trapped it. The channel
of Black River is on a very flat gradient and is choked with cattails and
willows that serve to screen out floating debris and suspended sediment.
These two watersheds are rated "slight"; however, because of its swampy nature,
Black River is very dark with dissolved organic humic acids, etc.
The Satsop and Wynoochee and Wishkah Rivers drain the south side of the Olympic
Mountains on the north end of the basin. Topography in the headwaters is rough,
and the unstable sedimentary formations along the streams have produced many
slips and slumps directly into the streams. In its lower reaches the Wynoochee
cuts away its banks at nearly every high-water stage* Timber access road con-
struction and artificial channel changes have put considerable debris in the
Wynoochee* In the upper reaches, particularly along the channels, the Wynoochee
and Satsop watersheds are rated "high". Sixteen Creek, tributary to the Wyn-
oochee, showed heavy sediment movement as the result of road construction in
an unstable slide area, and is rated 'Very high". In the gentler rolling
topography of the middle and lower portions of their watersheds the Satsop and
Wynoochee are rated "low", and "slight" on the glacial drift areas. The
Wishkah Watershed was not visited, but is assumed to be similar to the lower
Satsop and Wynoochee Watersheds. Since the average annual water yield of the
three rivers well exceeds one million acre-feet, the high-flow bankcutting
erosion losses along the lower reaches do not significantly bulk up the total
stream sediment loads. The overall rating, therefore, is "low" to "moderate"*
The small drainages south of the Chehalis River in its lower course drain
relatively gentle and heavily forested topography underlain by sedimentary
rocks. Though this area was not visited, it is assumed that it would have
only insignificant erosion and sediment problems caused by an occasional
slump and limited bankcutting in the lower stream courses. This area is
assigned a "low" rating.
Bunker and Lincoln Creeks in the center of the basin also drain areas of gentle
rolling to flat topography and show no erosion and sediment problems. These
watersheds are rated "slight".
Elk Creek, the upper Chehalis River, and South Fork Chehalis River drain the
very rough topography of the Willapa Hills on the southern end of the basin.
Below the towns of Pe Ell and Boistfort the topography is relatively gentle
and erosion and sediment problems are limited. These areas rate "slight" to
"low". The higher and steeper areas still have some of the original forest
cover and where undisturbed also are rated "slight" to "low". Extensive
timber access road construction has put a lot of debris into stream channels,
and in many places the road fills still erode directly into the channels.
Some recently logged areas show considerable erosion in the skidtrails, and
some of the minor tributaries below are choked with sediment. Though these
areas - not extensive - would be rated "high", the sediments do not seem to
move far downstream. Recovery of the logged areas is fairly rapid, excellent
second-growth stands are developing on the slopes logged ten or twelve years
ago. Channel bed conditions in the major tributaries are very good, with
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plentiful aquatic life on the stones. In two or three places where the
streams were turbid and the beds somewhat silted by current road con-
struction, the condition did not carry very far downstream. Effects of
road building and logging are apparently only temporary* This part of
the basin is rated "low" to "moderate".
The main stem of the Chehalis River has a very flat gradient. Although it
does some bankcutting at flood stages, it is not a heavy producer of
sediment.
The Chehalis Basin over-all is rated "low" for sediment production.
Even before the beginning of summer* some of the tributaries are showing
discoloration from dissolved organic matter. This was quite noticeable
in the upper Chehalis River at Rainbow Falls. Below the falls in an eddy
there was a small accumulation of suds piled up, caused perhaps by deter-
gents from sewage disposal in the small towns and farms upstream, or
possibly by the dissolved natural organic matter.
RECOMMENDATIONS FOR IMPROVEMENT
Cleanup of floating drift that jams up and causes channel changes and bank-
cutting, less use of tractors in logging on steep slopes and wet soils, and
more care in timber access road location and construction weald help greatly
in reducing what erosion and sediment production now occurs.
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