Assessment and Synthesis of the Literature on Climate Change Impacts on Temperatures of the Columbia and Snake Rivers


Assessment and Synthesis of the Literature on
Climate Change Impacts on Temperatures of the
Columbia and Snake Rivers

EPA Region 10
Seattle, Washington

March 2020

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Acknowledgements

Tetra Tech, Inc. provided information, interpretation, and analysis for this assessment under
contract to EPA Region 10.

EPA appreciates contributions provided by the following organizations that provided review and
comment on a 2018 draft version of this assessment:

United States Army Corps of Engineers

United States Bureau of Reclamation

Bonneville Power Administration

Oregon Department of Environmental Quality

Washington Department of Ecology

Chelan County Public Utility District

National Council for Air and Stream Improvement

Contact Information
Ben Cope

Laboratory Services and Applied Science Division

EPA Region 10

1200 Sixth Avenue

Seattle, Washington 98101

cope.ben@epa.gov

Rochelle Labiosa
Water Division
EPA Region 10
1200 Sixth Avenue
Seattle, Washington 98101
labiosa.rochelle@epa.gov

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TABLE OF CONTENTS

1.0 INTRODUCTION	1

2.0 CURRENT IMPACT ASSESSMENT	2

2.1	Air Temperature Trends	3

2.2	Water Temperature Trends Based on Observations	5

2.3	Development of Climate Baseline Timeframe	7

2.4	Trends Estimated Using Models	9

2.5	Summary of Current Impacts	15

3.0 FUTURE IMPACT PROJECTIONS	15

3.1	Future Projections of Meteorological Changes	16

3.2	Future Projections of Water Temperature Increases	17

3.2.1	Regional Assessments	18

3.2.2	Columbia Mainstem	19

3.2.3	Columbia Tributaries	19

3.3	Summary of Historical and Future Impact Projections	24

3.4	Uncertainty in Future Impact Projections	24

4.0 REFERENCES	26

LIST OF TABLES

Table 2-1 Observed trends in annual average stream temperature in the Northwest	6

Table 2-2 Estimated baseline monthly mean air and water temperatures (1915- 1959) ...10
Table 2-3 Comparison of baseline and current air and water temperatures (1915-1959;

1997-2006)	11

Table 2-4 Mean monthly water temperatures and decadal changes predicted from trend

analysis of RBM10 model output	13

Table 2-5 Comparison of trend for mean monthly temperature increase for current and free-

flowing model scenarios using RBM10	14

Table 3-1 Projected stream temperature responses to future climate change scenarios in

the Northwest	23

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LIST OF FIGURES

Figure 2-1 Trend for annual average air temperature at Lewiston, Idaho	3

Figure 2-2 Trend for annual average air temperature at Yakima, Washington	4

Figure 2-3 Trend for annual average air temperature at Portland, Oregon	4

Figure 2-4 Factors influencing air temperature during the 20th century	8

Figure 2-5 Trend in monthly mean temperatures at Bonneville Dam	12

Figure 2-6 August average water temperature at Bonneville predicted by Mantua et al. 2010

regression model	14

Figure 3-1 Predicted changes in seasonal streamflow between the period 1976-2005 and

the 2030s	17

Figure 3-2 Difference in Columbia River tributary and mainstem August mean temperatures

at confluences and future projections	20

Figure 3-3 Estimated current August mean temperature in the Columbia River and

tributaries	21

Figure 3-4 Estimated 2080 August Mean temperature in the Columbia River and tributaries
	22

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Acronyms/Abbreviations

1 Acronyms/Abbreviations

Definition 1

°C

Degrees Celsius

AR

Assessment reports

BOR

U.S. Bureau of Reclamation

BPA

Bonneville Power Administration

CMIP

Coupled Model Intercomparison Project

CRSO

Columbia River System Operations

CWA

Clean Water Act

DART

(Columbia River) Data Access in Real Time

DM

Daily maximum

EIS

Environmental Impact Statement

EPA

U.S. Environmental Protection Agency

ESA

Endangered Species Act

FCRPS

Federal Columbia River Power System

kefs

Kilo cubic feet per second

MAE

Mean absolute error

MOA

Memorandum of Agreement

NHD

National Hydrography Database

NOAA

National Oceanic and Atmospheric Administration

PUD

Public Utility District

R2

Correlation coefficient

RCP

Representative concentration pathways

RM

River mile

RMSE

Root mean square error

USACE

U.S. Army Corps of Engineers

USFS

U.S. Forest Service

USGS

U.S. Geological Survey

VIC

Variable infiltration capacity

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1.0 INTRODUCTION

Across the Pacific Northwest, changing environmental dynamics including weather patterns and
air temperatures, river flow timing, flow source (snowpack or rainfed) and magnitude, and
wildfire prevalence are impacting river temperatures. As these trends continue into the future,
changing conditions are expected to have even more pronounced influences on water
temperature (May et al. 2018). These changes in river temperatures are expected to affect the
health, behavior, and survival of cold water fish at both the individual and population scale
(Crozier et al. 2008a). Where increased river temperatures result in exposure to temperatures
above the optimal range for Columbia River salmon, impacts can include increased heat stress
and migration delays, among other direct and indirect effects (Crozier et al. 2008b). In
downstream mainstem waters where large areas of contiguous cold water are absent, cold
water refuges may play an increasingly important role in mitigating the effects of exposure to
temperatures that exceed fish thermal tolerance thresholds. Because of the importance of cold
water availability to commercially, culturally, and recreationally important species, it is important
to evaluate where and when such deleterious increases in river temperature are expected to
occur across the Pacific Northwest and in the Columbia River basin.

Emerging research on climate change effects has shown regional variation in impacts on water
temperature (Kaushal et al. 2010; USEPA 2013; Johnson et al. 2015). The Pacific Northwest
and the Columbia River have a unique set of responses to climate factors (Beechie et al. 2013;
Mantua et al. 2010; Crozier et al. 2011; Isaak et al. 2018). In addition to climate change, water
temperatures are also affected by local watershed hydroclimatic and physiographic settings,
land use, water management infrastructure, and other factors (Webb et al. 2008; Kaushal et al.
2010; Isaak et al. 2012). This review presents a synthesis of available information on the
warming trend in stream temperatures across the Pacific Northwest, with a focus on the
Columbia and Snake Rivers, as well as information on projected future changes.

Water temperature reflects the balance between energy inputs, storage, and loss from a
waterbody. Weather conditions, which include air temperature, precipitation, solar (short-wave)
radiation, long-wave radiation, evaporation, convection, and wind are primarily responsible for
the heat exchange process at the air-water interface. Hydrologic processes that change the
volume of water, and thus the response to heat inputs, or the movement and mixing of water of
different heat contents can affect water temperature. These hydrologic processes, which include
snowmelt timing, streamflow, tributary flow, groundwater flow, and hyporheic flows among
others, are sensitive to climate changes (where climate represents long-term averages of
weather) that affect precipitation and evapotranspiration (i.e. water balance). For example, one
of the causes of increased summer water temperatures throughout the Norwest region has
been summer flow declines due to earlier snowmelt. Research shows that increases in cool
season temperatures (October through March) throughout the Northwest over the past 40-70
years can been linked to earlier snowmelt and a historical decline in summer flows (Karl et al.
2009). Karl et al. (2009) indicate that the April 1 snowpack in the Cascade Mountains, a key
indicator of natural storage in the Northwest, has decreased by approximately 25% over the
past 40-70 years primarily due to an increase of 2.5 °F in cool season temperatures during that
period. And summer flows are expected to further decline as April 1 snowpack in the Cascade
Mountains are projected to decline by as much as 40% by 2040 (Payne et al. 2004).

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Water temperature is influenced not only by climatic factors directly affecting heat flux and
hydrologic processes but also by channel characteristics, shading by riparian vegetation, and
thermal density stratification in large rivers and lakes (Poole and Berman 2001; Caissie 2006;
Webb et al. 2008; Hannah and Garner 2015).

To characterize the potential effects of climate change, it is a common practice to consider
system response, or sensitivity, to either historical climate variability or numerical model
simulations of system response to a range of plausible future climate change scenarios.
Assessment of the water temperature response to climate change can help identify the range of
potential impacts, identify vulnerabilities, and inform the development of adaptation strategies to
ameliorate current impacts and reduce future risks.

Due to current limits of knowledge, randomness of nature, and the uncertainty in future human
actions, exact forecasts of changes in streamflow and water quality endpoints decades into the
future are not possible. However, it is possible to project with some confidence a range of
possible future outcomes based on our current understanding of the earth-climate system and
use that information to put bounds on the science and policy questions related to future changes
in water quality. This document summarizes the estimated impacts of climate change on river
temperatures to-present as well as the future projected changes to river temperatures available
for the Pacific Northwest, including Columbia and Snake River temperatures.

Research in the Pacific Northwest shows that water temperatures are primarily driven by, and
can be modeled as a function of, air temperatures (Mantua et al. 2010, Mohseni et al. 1998).
Therefore, the analyses presented in Section 2.0 and Section 3.0 emphasize the historical and
projected impacts of increasing air temperatures on water temperatures on the Columbia River
and Snake River. Increased water temperatures, however, cannot be ascribed solely to
increased air temperatures due to climate change. Other processes such as dam impoundment,
flow regulation, land use changes, snowmelt, short term natural variability, and other factors can
also influence the historical and projected variability of water temperatures. These factors are
not explicitly evaluated in this study. Rather, the primary goal of this document is to identify long
term trends and future projections for air and water temperatures.

2.0 CURRENT IMPACT ASSESSMENT

Studies of system responses to historical climatic variability typically focus on trends and the
correlation between water temperature and climatic variables. In this context, climatic variability
refers to the inherent heterogeneity of observed data over time (e.g., temperature, precipitation,
environmental factors) (EPA 2011). Despite the presence of climate variability, numerous
research studies indicate warming trends in air temperature and water temperature in the
Pacific Northwest. Air temperature increases are especially important drivers of water
temperature increases, due to the physical linkage of increasing downwelling longwave
radiation and warmer groundwater (Isaak et al. 2018). The Columbia and Snake mainstem
water temperatures are strongly linked to air temperature as well, as shown by a sensitivity
analysis conducted with the RBM10 model (EPA 2019) and statistical regression (Isaak et al.
2017).

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2.1 AIR TEMPERATURE TRENDS

The U.S. Global Change Research Program (2017) performed an air temperature trend analysis
comparing the period of 1901 - 1960 with 1986 - 2016. This study reported that annual mean
Northwest air temperatures have increased by 0.86°C, with maximum air temperatures
increasing by 0.84°C, and minimum air temperatures increasing by 0.87°C. Isaak et al. (2018)
reported that across Pacific Northwest rivers, mean annual air temperatures increased at a rate
of 0.27°C per decade during the 40-year period of 1976 - 2015 (with a comparable rate of
increase over the last two decades of 0.23°C per decade between 1996 and 2015). The study
evaluated air temperature data collected at 168 sites in the Washington, Oregon, Idaho, eastern
Montana and Wyoming, and northeastern California. The highest monthly temperature
increases during the 1976 - 2015 period occurred in January (0.61°C), July (0.56°C), August
(0.38°C), and September (0.41°C) (Isaak et al. 2018).

To provide an additional line of evidence for air temperature trends in the Columbia basin, EPA
conducted an analysis of air temperature at select locations used for the RBM10 model of the
Columbia and Snake rivers (EPA 2019). Annual average air temperature was calculated for the
period spanning 1970 - 2016, and a linear regression performed to estimate magnitude. Trends
are shown for Lewiston, Idaho (Figure 2-1), Yakima, Washington (Figure 2-2), and Portland,
Oregon (Figure 2-3). The decadal changes estimated from the regression slopes are:

•	Lewiston, Idaho: 0.22°C per decade.

•	Yakima, Washington: 0.25°C per decade.

•	Portland, Oregon: 0.21 °C per decade.

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1970	1980	1990	2000	2010

Year

• Year Average 1970-2016		Linear (Year Average 1970-2016)

Figure 2-1 Trend for annual average air temperature at Lewiston, Idaho

3

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16
14

2
0

1970 1980 1990 2000 2010

Year

• Year Average 1970-2016 Linear (Year Average 1970-2016)

Figure 2-2 Trend for annual average air temperature at Yakima, Washington

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2.2 WATER TEMPERATURE TRENDS BASED ON OBSERVATIONS

The most comprehensive historical water temperature records come from USGS stream gage
locations, hydroelectric facilities on larger rivers, or drinking water treatment plant source water
intakes (Kaushal et al. 2010; Isaak et al. 2011). Many of these locations are influenced by local
thermal alteration from urbanization, power plant cooling, or dams and reservoirs (Webb et al.
2008; Kaushal et al. 2010; Isaak et al. 2011). This can make it difficult to identify climate-related
effects. However, there are several studies from various locations throughout the Pacific
Northwest characterizing water temperature trends from observed data. These studies are
discussed below and summarized in Table 2-1.

In a national-scale study, Kaushal et al. (2010) analyzed temperature trends from 1978 to 2007
in streams and rivers at 40 long-term river monitoring sites in the contiguous United States. Nine
of the sites were in Oregon watersheds with varying levels of human disturbance. Average
annual water temperature increased at eight sites, with rates ranging from 0.009°C to 0.030°C
per year (0.09°C to 0.30°C per decade). Increasing trends at five sites were statistically
significant. Water temperatures at one site, the Blue River, decreased at a rate of 0.038°C per
year (0.38°C per decade). The Blue River site is located downstream of a dam, and the dam
operating procedures may have influenced this temperature trend (Kaushal et al. 2010).

Isaak et al. (2012) conducted a regional-scale study (Pacific Northwest and Montana) to assess
stream temperature trends in unregulated (free-flowing) and regulated streams from 1980 -
2009. Seven sites located on unregulated rivers and streams with reconstructed temperature
trends averaged across the sites show statistically significant increasing trends in daily average
temperatures during the summer (+0.22°C per decade), fall (+0.09°C per decade), and winter
seasons (+0.04 °C per decade), as well as annually (+0.11°C per decade). Decreasing trends
were observed at these same sites during the spring (-0.07°C per decade). Seasonal and
annual stream temperature trends at unregulated sites were positively correlated with air
temperature and were associated with summer streamflow volumes.

In an assessment of stream temperature data using the NorWest statistical stream network
model from more than 20,000 sites in the western U.S., Isaak et al. (2017) found that Pacific
Northwest river and stream August mean temperatures have increased by an average of 0.17°C
per decade (standard deviation = 0.067°C per decade) from the reconstructed trend spanning
40 years, from 1976 - 2015. For larger northwestern U.S. rivers, including Pacific Northwest
rivers, estimated trends from time series at 391 sites revealed that warming trends are
ubiquitous in the summer and fall months, with July - September mean river temperature
increases of 0.18°C - 0.35°C per decade during 1996 - 2015 and 0.14°C - 0.27°C per decade
during 1976 - 2015 (Isaak et al. 2018). The average regional increase was linked to air
temperature increases; however, at a local to sub-regional scale, other drivers, such as changes
in flow, can be influential.

In the mid-Columbia River, results from the NorWest model show that August mean river and
stream temperatures have increased by approximately 0.20°C per decade from the
reconstructed trend over 40 years, from 1976 - 2015 (Isaak et al. 2017). At the Bonneville Dam
monitoring site, with the longest continuous river temperature record (since 1939), the increase
in river temperatures is most pronounced in summer. There is no complete time series that
reaches back to before dam construction on the Columbia River. From 1949 - 2010, mean July
water temperatures at Bonneville Dam increased by 2.6°C (Crozier et al. 2011). In the mid-
Columbia River basin, the increase in summer water temperatures is largely driven by air

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temperature increases rather than flow variation, based on the statistical underpinning of the
NorWest model (Isaak et al. 2017).

In addition to the chronic effects of increasing baseline river temperatures, acute exceedances
of thermal tolerance maxima occurred more frequently in recent years and are projected to be of
increasing magnitude and frequency (Isaak et al. 2018). One recent example of extreme
Columbia River basin temperatures occurred in 2015, when temperatures in early June reached
in excess of 21 °C weeks earlier than is typical and remained 2°C - 4°C above monthly average
temperatures for several weeks, contributing to a mass die-off of sockeye salmon in the
Columbia and Snake Rivers (Isaak et al. 2018, NMFS 2016). Approximately 14% of the sockeye
salmon that passed through the Bonneville Dam were detected upstream at McNary dam on the
Columbia River, while on average 68% were detected the previous five years (NMFS 2016). In
general, the first and last dates in each calendar year on which water temperatures exceed
20°C at Bonneville Dam are occurring earlier and later than they have historically (National
Research Council 2004).

Water temperature records spanning several decades are also available for the Columbia River
at the Data Access in Real Time (DART) website. These data were explored for trends, but
numerous issues were found with the data quality. Prior to 1984, measurements of water
temperature in the Columbia and Snake Rivers consisted of manual observations of
temperature from thermometers placed in the cooling water stream of dam turbines. These
observations, generally described as scroll case measurements, were made daily by dam
operations personnel. There were quality assurance issues in the instruments, location of the
instruments, and protocols for collecting and reporting data. Many of these deficiencies
appeared to be related to the original motivation for installing the thermometers, which was for
purposes of monitoring the operation of turbines rather than for analyzing temperature effects
on Pacific salmon. Temperature monitoring associated with the total dissolved gas program was
initiated in 1984 at many of the dams. In contrast to the scroll case temperature monitoring
program, the purpose of the total dissolved gas monitoring is to guide spill and discharge
management to minimize the production of excess dissolved gas. The resulting data reported
on the Columba River DART website show limited attention to temperature data quality control.
DART data from the mid-1990s to present tend to have fewer gaps and discrepancies, but
analysis of these data still requires quality assurance work to remove spurious data (Merz et al.
2018). As a result of the data quality issues before the 1990s, EPA has not conducted any
independent trend analyses with the DART data.

Table 2-1 Observed trends in annual average stream temperature in the Northwest

Waterbody

Location

Record of
observation

Temperature Change
( C/decade)

P value

Source

Fir Creek

Brightwood, OR

1978-2007

+0.21

< 0.05*

Kaushal
et al.
2010

North Santiam River

Niagara, OR

1979-2007

+0.21

< 0.05*

Rogue River

McLeod, OR

1979-2007

+0.3

< 0.05*

Bull Run River

Multnomah Falls, OR

1978-2007

+0.19

0.079

North Fork Bull Run
River

Multnomah Falls, OR

1979-2007

+0.09

0.34

South Fork Bull Run
River

Multnomah Falls, OR

1979-2007

+0.19

0.089

Rogue River at
Dodge Bridge

Eagle Point, OR

1979-2007

+0.21

< 0.05*

Blue River

Blue River, OR

1979-2007

-0.38

< 0.05*

South Santiam River

Foster, OR

1979-2007

0.000

0.977

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Waterbody

Location

Record of
observation

Temperature Change
( C/decade)

P value

Sou rcc

7 sites on
unregulated rivers
and streams

Washington, Oregon,
Idaho, Montana

1980-2009*

Spring: -0.07
Summer: +0.22

Fall: +0.09
Winter: +0.04
Annual: +0.11

<0.01*

Isaak et
al. 2011

Multiple sites in the
Pacific Northwest

Pacific Northwest

1976-2015

August: +0.17

Isaak et
al. 2017

391 sites on larger
rivers and streams

Northwestern U.S.

1976-2015

July: +0.27
August: +0.14
September: +0.15

Isaak et
al. 2018

Crozier

Columbia River

Bonneville Dam

1949-2010

July: +0.43

et al.,
2011

* Denotes significance at P < 0.05.

t Rates of change are based on reconstructed trend (multiple regression models were used to overcome potential bias from
missing years of observations and regional climate cycles).

2.3 DEVELOPMENT OF CLIMATE BASELINE TIMEFRAME

Analysis of the potential impacts of climate change on water temperatures requires definition of
a baseline condition prior to the onset of climate change. Water temperatures in the Columbia
and Snake River mainstems are strongly influenced by air temperature, with less influence from
other factors such as shade and tributary inputs (Isaak et al. 2018; EPA 2019). Both rivers are
wide, which minimizes the impact of shade on river temperatures, and many large
impoundments are present on each river, which result in pooling and flow retention, allowing for
enhanced heating due to atmospheric influences. Therefore, the baseline is first defined in
terms of air temperature. The air temperature baseline is then extrapolated to a water
temperature baseline for the same period.

It is important to note that the baseline temperature is not a single condition but rather a
temperature regime characterized by a distribution of temperatures. Air and water temperatures
in the Columbia River basin vary widely and are strongly influenced by decadal patterns in sea
surface temperature anomalies, such as the El Nino - Southern Oscillation. This inherent
variability makes it difficult to resolve a climate change signal. For example, some of the
warmest summer temperatures on record in the basin occurred during the 1930s (Abatzoglou et
al. 2014) and were attributed to coincident warming in both the Pacific and Atlantic Oceans
(Markus et al. 2016). To account for natural variation, the baseline temperature can be
characterized as a distribution with a mean and standard deviation.

To select a baseline timeframe for climate, it is important to parse the anthropogenic and natural
factors that affect air temperature in the Pacific Northwest. Abatzoglou et al. (2014) provides a
thorough analysis of the topic, allowing for discrimination of the anthropogenic influence from
other factors. The authors analyzed long-term air temperature records in the Pacific Northwest
dating back to the turn of the 20th century but focused on conditions since 1920 when the
density of observing stations increased dramatically. Using multiple linear regression
techniques, they attributed variation in the seasonal temperature records to the El Nino -
Southern Oscillation/Pacific-North American pattern, solar variability, volcanic aerosols, and
anthropogenic forcing (including climate change). Figure 2-4 from Abatzoglou et al. (2014)
provides the basis for selection of the baseline.

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MEI, PNAr Vol Solar Anthro

192519501975 2000 192519501975 2000 192519501975 2000 192519501975 2000

Figure 2-4 Factors influencing air temperature during the 20th century
(from Abatzoglou et al. 2014)

The columns in the figure correspond to time series plots showing the influence of the factors
affecting temperature: "MEI, PNAr" are the combined influence of the El Nino - Southern
Oscillation and the Pacific-North American pattern, "Vol" represents the influence of volcanic
aerosols, "Solar" represents effects of cyclical solar radiation (sunspot) variability, and "Anthro"
represents anthropogenic influence. The rows represent seasons, with "JJA" corresponding to
June, July, and August. A close examination of the "Anthro" plot for JJA shows a small upward
trend in temperature between 1900 and 1960, and a more dramatic upward trend beginning in
1960. Based on this analysis, 1960 was selected as the year in which climate change begins to
have a stronger effect on air temperature. Note that climate change does have a small effect
prior to 1960, but the influence of climate change was minimal to that point. The air temperature
baseline is therefore defined as the distribution of air temperatures prior to 1960. Given the
interdecadal oscillations and other factors influencing air temperature, the mean should be
taken from a relatively long monitoring period of record.

Given the linkage between air temperature and water temperature in the Columbia River, the air
temperature baseline can be used to identify a water temperature baseline distribution. It is
noted that some of the Columbia and Snake River dams were constructed after the 1960
baseline. Bonneville Dam and Grand Coulee Dam were completed and began operations in
1938 and 1942, respectively. Many of the other major dams in the Columbia River Basin were
completed in the 1950s and 1960s, including McNary (1954), Chief Joseph (1955), The Dalles
(1957), Ice Harbor (1961), Priest Rapids (1961), Wells (1967), and John Day (1968). In the

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1970s, the current dam configuration was completed with the addition of Dworshak (on the
North Fork Clearwater, 1974)), Lower Granite (1975), and an Ice Harbor expansion (1976).

Water temperature trends can be estimated using long-term air temperature records as inputs to
statistical models and mechanistic models. For example, EPA's RBM10 model (EPA 2019) has
an input database that allows for a simulation of daily temperatures from 1970-2016, which
captures most of the period from the 1960 baseline to the present. The next section describes
water temperature trend estimates from modeling assessments.

2.4 TRENDS ESTIMATED USING MODELS

A simple and direct approach to estimating the water temperature baseline is to use statistical
models of water temperature developed by Mantua et al. (2010). Mantua et al. used a logistic
regression approach developed by Mohseni et al. (1998) to predict weekly river temperatures
for weeks 15 to 42 of the year (encompassing the warmer months) as:

In this equation, Ts is the predicted weekly average stream water temperature in °C, Ta is the
average weekly air temperature. The variables ju (minimum stream temperature), a (maximum
stream temperature), y (steepest slope of the function), and /3 (air temperature at the inflection
point of the logistic curve) are all fitting parameters. Once the site-specific parameters are
determined, the prediction depends solely upon air temperature.

Mantua et al. developed and calibrated these models for 124 stations throughout Washington
state, 23 of which are along the Columbia River from the Canadian border to the mouth of the
Columbia River. Historic air temperature data were obtained from eight National Climate Data
Center monitoring sites and were supplemented using gridded 1/16° spatial resolution air
temperature estimates spanning 1915 - 2006 developed from Eisner et al. (20101). Weekly
results of model application for historical air temperature conditions at each site, commencing in
1915 and running through 2006, have been made publicly available. Note that the water
temperatures are estimates based on the regression from air temperatures, and do not
necessarily reflect observed historical water temperatures, which would have been influenced
by changing conditions as dams were installed at various dates. However, they do reflect a
reasonable approximation of the response of water temperature to air temperature using the
historical air temperature record. Using the Mantua et al. (2010) model results, EPA derived
monthly baseline water temperature distributions for the period 1915 - 1959 for five locations
(Table 2-2). Water temperatures in a given month vary across sites, with the warmest
temperatures occurring at Bonneville Dam, and the coldest temperatures at Wells Dam.

1 Eisner et al. (2010) developed the historic gridded estimates of air temperature from several sources of
observed station data, primarily from the National Climate Data Center Cooperative Observer network
and Environmental Canada daily data. Methods were used to account for varying periods of record, to
interpolate point station data to the 1/16° grid, and to adjust the estimates for topographic influences.

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Table 2-2 Estimated baseline monthly mean air and water temperatures (1915 - 1959)

(based on Mantua et al., 2010)

Month

Mean Air Temp
(°C)

Mean Water
Temp (°C)

Standard
Deviation Water
Temp (°C)

Grand Coulee Dam tailrace, Columbia River

July

22.1

18.1

0.43

August

20.9

17.9

0.53

September

17.0

16.4

1.46

Wells Dam tailrace, Columbia River

July

21.3

16.8

0.96

August

20.4

16.5

1.06

September

15.8

13.2

2.60

Priest Rapids Dam forebay, Columbia River

July

23.4

18.7

0.60

August

22.4

18.5

0.60

September

18.1

16.4

1.89

Bonneville Dam forebay, Columbia River

July

18.1

19.5

1.14

August

17.9

19.4

1.02

September

15.5

17.5

2.05

Ice Harbor Dam tailrace, Snake River

July

23.0

20.3

1.19

August

21.7

19.7

1.19

September

17.7

16.8

2.20

The Mantua et al. historical climate results were reported through 2006. EPA analyzed the last
ten years of the Mantua et al. data (1997 - 2006) to derive the estimated water temperature
change since 1960 (Table 2-3). The average change per decade was smallest in July at each
site (mean 0.09°C) and largest in September (mean 0.17°C). There was also variability in the
decadal changes between the sites, with the smallest changes at Grand Coulee Dam, and the
largest changes at Ice Harbor Dam. The Mantua et al. (2010) monthly mean predictions of water
temperature vary little between July and August across the sampled sites, while the analysis of
observed water temperature data shows consistently higher water temperatures in August than
July. This likely reflects a limitation of the Mantua et al. (2010) regression method.

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Table 2-3 Comparison of baseline and current air and water temperatures (1915-1959;

1997-2006)

(based on Mantua et al., 2010)

Month

1915 1959 Mean
Water Temp (°C)

1997 2006 Mean
Water Temp (°C)

Change per
Decade* (°C)

Grand Coulee Dam tailrace, Columbia River

July

18.1

18.2

0.02

August

17.9

18.2

0.06

September

16.4

16.7

0.07

Wells Dam tailrace, Columbia River

July

16.8

17.1

0.07

August

16.5

17.1

0.13

September

13.2

14.0

0.18

Priest Rapids Dam forebay, Columbia River

July

18.7

19.0

0.07

August

18.5

18.9

0.10

September

16.4

17.2

0.18

Bonneville Dam forebay, Columbia River

July

19.5

20.1

0.15

August

19.4

20.2

0.17

September

17.5

18.4

0.23

Ice Harbor Dam tailrace, Snake River

July

20.3

21.0

0.16

August

19.7

20.7

0.25

September

16.8

17.8

0.23

* Ending date assumed to be 2002 (midway between 1997 and 2006)

Yearsley (2009) used the RBM model, an earlier version of the RBM10 model EPA is currently
using for the Columbia and Snake Rivers (EPA 2019), to predict the cumulative increase in
Columbia River mainstem daily average temperatures between a baseline condition (1951 -
1978) and an early century (2020) climate scenario. The climate scenarios are discussed in
more detail in Section 3.0. The results showed a river temperature increase at Bonneville Dam
of approximately 1°C during the summer months.

For this assessment, EPA has applied the RBM 10 model to provide an additional line of
evidence for decadal change since the 1960 baseline (EPA 2019). Model output for the
simulation of current conditions along the Columbia and Snake Rivers from 1970 - 2016 was
used. The hydroelectric system and dams were constructed prior to 1970 except for Lower
Granite Dam (completed in 1975).

Trend analyses of the model outputs were conducted for the Columbia River at Bonneville Dam
tailwater, Priest Rapids Dam tailwater, Wells Dam tailwater, and for the Snake River at Ice
Harbor Dam tailwater. Monthly average river temperature and monthly 90th percentile
temperatures were calculated for each year for the months of July, August, September, and
October. Trends for both statistics were nearly identical; therefore, only monthly average river
temperatures are provided here. The non-parametric Mann-Kendall test for trend (Mann 1945;
Kendall 1975) forms the basis of the method that was used for the trend analyses. The method
was developed and popularized by USGS researchers throughout the 1980s (Hirsch et al.

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1991), and USGS published computer code supporting its use. A non-parametric test is
frequently used for trend analysis since time series data exhibit autocorrelation. The null
hypothesis Ho is there is no trend, while the alternative hypothesis Ha is either an upward or
downward trend (a two-tailed test). A rate of change or trend slope was calculated based on
Sen's non-parametric slope estimator (Sen 1968). A confidence interval (p value) was also
estimated. An example plot is provided in Figure 2-5.

24
22

V 20

3
•*—>

S. 18

« • • f • * I * • #¦" f • • • ft

T ....• m f * ~"'••• — • •# IX*

i • ;4 • •: • • •

S'V • • • .

• • •

= 16 • • . • ....* • ••¦•*

CD « • • * m •

£ . • • # # » • * • •

14 .

1970 1975 1980 1985 1990 1995 2000 2005 2010 2015

Year

• July • August • September • October
Linear (July) Linear (August) Linear (September) Linear (October)

Figure 2-5 Trend in monthly mean temperatures at Bonneville Dam

The results for all locations and timeframes are shown in Table 2-4. Estimated 1960 water
temperatures are based on a backwards projection of the water temperature trend predicted by
RBM10 for 1970 - 2016. All the trends were considered significant at a p-value of 0.05, with the
exception of mean temperatures in August and September at Ice Harbor Dam. Changes per
decade are highest at Bonneville Dam and lowest at Ice Harbor Dam. The lower trend at Ice
Harbor Dam is likely due to the influence of cold water releases from the Dworshak Dam.
Summer releases were increased beginning in the late 1990s to provide cooler water
temperatures in the lower Snake River.

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Table 2-4 Mean monthly water temperatures and decadal changes predicted from trend

analysis of RBM10 model output

Water Temperature (°C) I

Location

Month

1970

2016

Change

per

Decade

Wells Dam

July

14.3

16.4

0.38

Wells Dam

August

16.6

18.5

0.35

Wells Dam

September

17.0

18.9

0.33

Wells Dam

October

15.5

17.1

0.27

Priest Rapids Dam

July

15.7

18.0

0.41

Priest Rapids Dam

August

17.7

19.9

0.40

Priest Rapids Dam

September

16.7

19.2

0.41

Priest Rapids Dam

October

14.2

16.2

0.34

Bonneville Dam

July

17.3

19.9

0.48

Bonneville Dam

August

19.4

21.7

0.40

Bonneville Dam

September

17.6

20.3

0.45

Bonneville Dam

October

13.9

16.5

0.45

Ice Harbor Dam

July

17.8

20.1

0.41

Ice Harbor Dam

August

20.8

21.1

0.06

Ice Harbor Dam

September

18.8

19.3

0.09

Ice Harbor Dam

October

14.1

16.3

0.39

The Mantua et al. (2010) analyses and RBM10 analyses (EPA 2019) show different warming
rates per decade, with Mantua et al. (2010) reporting lower rates. During summer months such
as August, the Mantua model does not have a clear increasing trend until the early 1980s at
most locations, such as at the Bonneville Dam tailrace (Figure 2-6). The Mantua et al. (2010)
model 10-year moving average shows an increase of 0.4°C per decade from 1980 to present,
which is similar to the RBM10 model trend rate for 1970 - 2016 at Bonneville Dam. However,
the reported trend rate from the Mantua et al. (2010) analysis based on the average of 1997 -
2006 vs. the average of 1915 - 1959, represented as a 43-year span (1960 - 2002) is only
0.17°C per decade at Bonneville Dam tailrace in August. The RBM10 analysis is a linear
regression on results beginning in 1970, which is a period of fairly steady increases. Projecting
the RBM10 linear increase after 1970 back to 1960 is a rough estimation approach, because
there was not the same rate of consistent increase observed between 1960 and 1970.

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Bonneville Tailrace, August Average

Weekly 10 per. Mov. Avg. (Weekly)

Lna^cor^tHinaicor^t-Hmchror^rHLnGnmr--

rH
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2.5 SUMMARY OF CURRENT IMPACTS

In summary, predicted trends in water temperatures (present compared to a baseline of 1960)
vary between sites as shown by the analysis of the Mantua et al. (2010) historical data, and
there is considerable variation at the sites themselves as indicated by the standard deviations
(which is expected given the influence of the El Nino - Southern Oscillation and the Pacific-
North American pattern). Amidst this variation, there is strong evidence of a warming trend in
Pacific Northwest waters and in the Columbia River mainstem since 1960, as indicated by
literature and the analyses conducted herein by EPA. Based on available information (Table
2-1), the estimated increase in river temperatures since the 1960 baseline ranges from 0.2°C to
0.4°C per decade, for a total temperature increase to date of 1.5°C ± 0.5°C. The trend analysis
of the RBM10 model output shows decadal changes at the upper end of that range, while the
analysis of historic Mantua et al. regression estimates of water temperature shows decadal
changes at the lower end of the range. The RBM10 model results also indicate that Columbia
River dam impoundments exacerbated the climate-related warming in mid-summer and early
fall, compared to predicted trends in a free-flowing river (Table 2-5).

3.0 FUTURE IMPACT PROJECTIONS

Modeling studies use statistical or numerical simulation approaches to estimate water
temperature responses to scenarios of potential future climate change. Scenarios typically
include a range of potential futures based on different climate models, different assumptions
about future greenhouse gas emissions, and different future time periods. Climate model output
can be used as inputs to hydrologic and water quality models, providing a capability to evaluate
a wide range of potential climate futures, including interactions with changes in land use and
other factors affecting water quality. Simulated results are commonly presented as average
(annual or seasonal) changes in water temperature (median or average) relative to a historical
baseline period. The results of all modeling studies are directly conditional on the specific
methods, models and climate change scenarios evaluated.

Studies of climate change in the Columbia River basin have used downscaled output from two
rounds of the Coupled Model Intercomparison Project (CMIP) global climate modeling
experiments conducted for the Intergovernmental Panel on Climate Change Assessment
Reports (ARs): CMIP3, associated with AR4 (2005/2006); and CMIP5, associated with AR5
(released in 2012). CMIP5 included a variety of process improvements to global climate models,
but simulated responses to a given degree of radiative forcing are generally similar between
these two rounds. One significant way in which the two phases differ is that CMIP3 uses future
scenarios from the Special Report on Emission Scenarios that are based on scenarios of future
emissions that rely on various assumptions about economic growth and human responses2.

2 CMIP3 emissions scenarios referenced in this document include:

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CMIP5 scenarios, termed Representative Concentration Pathways (RCPs) take a different
approach in which explicit but highly uncertain socioeconomic projections are not made; rather,
each RCP is based on an assumption of a specific level of radiative forcing at the end of the 21st
century. For instance, RCP 8.5 is an upper-bound scenario that assumes radiative forcing
increases to 8.5 W/m2 by 2100. The middle-of-the-road Special Report on Emission Scenarios
A1B scenario from CMIP3 is similar to the RCP 6 scenario from CMIP5 (radiative forcing
increases to 6.0 W/m2 by 2100), with both reaching an approximate CO2 equivalent
concentration of 850 ppm by 2100.

3.1 FUTURE PROJECTIONS OF METEOROLOGICAL CHANGES

Across the Pacific Northwest, an increase in average annual air temperatures of 1.8°C to 5.5°C
is projected by the end of the century (compared to the period 1970 - 1999), depending on the
scenario, with the greatest increases projected for the summer (Mote et al. 2014; Rupp et al.
2017). Rupp et al. (2017) estimates that air temperatures at the end of the century will increase
by 3.0 and 5.5 °C above the 1970-1999 baseline average temperatures under the RCP 4.5 and
RCP 8.5 scenarios respectively. The estimated temperature differences between the RCP 4.5
and RCP 8.5 projections are 0.5 °C by 2050, and 2.5 °C by 2099.

The River Management Joint Operating Committee authored a report characterizing projected
climate change impacts in the Columbia River basin and other basins in western Oregon and
Washington (2018). The focus is on projected temperature, precipitation, snowpack, and
streamflow changes through the rest of the 21st Century. The study found that, on average, air
temperatures have already increased by 0.8°C in the region since the 1970s, and projected
future increases by 2070 range from 1.7°C to 3.3°C (based on CMIP5 RCP 4.5 emissions
pathways). Changes in future precipitation are more uncertain, but are generally predicted to
increase, notably during winter. Summers are expected to become drier. Winter snowpacks are
expected to decline since more winter precipitation will fall as rain rather than snow. As a result,
average summer flows are expected to be lower and/or there will be a longer period of low
summer flows, which will likely cause a tighter coupling of water temperature to air temperature.
Predicted seasonal flow changes between recent historic and 2030s conditions are shown
graphically in Figure 3-1. Flows are expected to increase during winter and spring and
decrease during summer and fall.

A1B. This future climate emissions scenario assumes very rapid economic growth, a global population
peaking mid-century followed by a decline, rapid development of new and more efficient technologies,
and a balance between fossil intensive and non-fossil energy sources.

B1. This future climate emissions scenario assumes the same population pattern as A1B, but with rapid
change in economic structures toward a service and information economy, with reductions in material
intensity and the introduction of clean and resource-efficient technologies.

A2. This future climate emissions scenario assumes a continuously increasing population, with per capita
economic growth and technological change more fragmented and slower than other scenarios

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>-4—

E
03

i—

03
d

03
CD
C/)

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3.2.1 Regional Assessments

In the Methow River near Winthrop, Washington, average annual stream temperatures are
projected to increase by 0.4°C to 0.8°C (early century to late century, respectively) in response
to moderate emissions (CMIP3 A1B) (Caldwell et al. 2013). Similar increases ranging from
0.6°C to 1.7°C (early century to late century, respectively) are projected in response to
moderate emissions (A1B) for watersheds in Washington, Oregon, Idaho, western Montana,
and portions of British Columbia (Wu et al. 2012). Simulations by Mantua et al. (2010) for over
100 sites predict increases in stream temperatures in Washington ranging from approximately
1°C to 2°C for early century, 1°C to 4°C for mid-century, and 1°C to 5°C for late century. Results
varied across sites and by CMIP3 emission scenario (low [B1] to moderate emissions [A1B]).

Climate change could alter seasonal and shorter-duration summer maximum stream
temperature, which is a critical period for aquatic ecosystems and could impact cold water biota
(Ficke et al. 2007). Effects on water temperature are more evident in the snow-dominant
watersheds, where reduced winter snowfall and earlier snowmelt is expected to result in lower
summer stream flows and higher summer stream temperature. For example, Wu et al. (2012)
predict streamflow decreases of about 19% to 30% in mid- and late century, respectively.
Several modeling studies in Washington watersheds suggest the greatest increases in stream
temperature will occur during the summer months (Caldwell et al. 2013; Wu et al. 2012; Cristea
and Burges 2010; Mantua et al. 2010). Cristea and Burges (2010) projected increases in future
average summer stream temperatures in Wenatchee, Icicle, and Nason Creeks in Washington.
The magnitudes of projected changes in water temperatures in these waters are strongly
influenced by riparian shading.

In the South Fork Nooksack River, Washington, late century water temperature increases
ranging from 3.5°C to 6°C are projected during critical summer low-flow conditions (simulations
based on moderate emissions, CMIP3 A1B) (Butcher et al. 2016). However, restoration of full
system riparian shading was predicted to mitigate potential future water temperature increases
by 30% to 60% during critical conditions. Maximum 7-day average stream water temperatures
were projected to increase by 1.1 °C to 3.6°C by late century even with system potential shade.
Model simulations also suggested that critical condition water temperatures could exceed
thermal tolerances for salmon in 60% to 94% of the Nooksack River by late century.

Using the CMIP3 A1B (medium emissions) scenario and the NorWest spatial statistical network
model, the Pacific Northwest regional August mean river temperatures are projected to increase
by 0.7°C in 2040 and 1.4°C in 2080, compared to a baseline period of 1993-2011 (Isaak et al.
2017).

Beechie et al. (2013) used a coupled model, called the dominant river tracing-based stream flow
and temperature model, to estimate flow and stream temperature in the Columbia River basin
for future climate conditions (CMIP3 A1B). They predicted an increase in the maximum weekly
mean temperature across the watershed of 1°C to 4°C for 2030 - 2069 and 2°C to 6°C for 2070
- 2099. They also predict that the number of stations where water temperatures are projected to
exceed 21 °C for more than nine weeks per year will increase dramatically by the end of the
century.

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3.2.2 Columbia Mainstem

Isaak et al. (2017) estimate that August mean river temperatures across the mid-Columbia
basin, calculated using the CMIP3 A1B (medium emissions) scenario and the NorWest spatial
statistical network model, will increase by approximately 1.0°C by 2040 and 2.0°C by 2080,
compared to a baseline period of 1993-2011.

Using the RBM model, Yearsley (2009) projected an increase in the daily average temperature
in the Columbia and Snake River mainstems. At Bonneville, from 1951 - 1978 until 2040, the
predicted increase was approximately 1°C during the summer months, with some variation over
the year. The projected increase depended upon location, with higher maximum temperature
increases predicted for certain times of year at Ice Harbor Dam, for example, compared to the
Bonneville Dam. The predictions used the A1B scenario with a composite of four downscaled
GCMs driving the VIC model for hydrology.

3.2.3 Columbia Tributaries

Simulations from a global-scale study by van Vliet et al. (2013) under CMIP3 A2 (high) and B1
(low) emissions scenarios suggest that average annual water temperatures in the Columbia
River basin could increase by an average of 1.6°C by late century.

Using the NorWest statistical stream network model (Isaak et al. 2018), EPA analyzed
estimates of current and future tributary temperatures (EPA 2018). Tributary temperatures are
predicted to increase between 0.6°C and 0.7°C relative to Columbia River temperatures (Figure
3-2). In addition, many tributaries in the future, despite being relatively cooler than the Columbia
River, could become warmer than water temperature thresholds for fish habitat and therefore
would be less functional as cold water refuges. Figure 3-3 and Figure 3-4 show current
conditions and 2080 projections, respectively.

EPA also conducted enhanced temperature modeling to assess Columbia River tributary
riparian shade restoration potential under current (1993 - 2011) and projected future (2040s
and 2080s) conditions (Fuller et al. 2018). The goal was to provide insight into potential changes
in cold water refuge for Pacific Salmon. Like the analysis in EPA 2018, this project used
geospatial representations of covariates that affect stream temperature and spatial stream
network models (Isaak et al. 2018). Current and future climate scenarios were paired with three
shade assumptions: no vegetation, current vegetation, and potential restored vegetation.

Overall, for the tributaries analyzed, mean August temperatures across the tributaries in the
basin decreased by 0.5°C when shade was restored (with a range of 0°C -1.3°C). The
improvement in temperature depended heavily on current extent of vegetation and stream
width. Results indicated that with current vegetation, August monthly mean stream
temperatures would increase on average by 1.1 °C for the 2040s scenario and 2.0°C for the
2080s scenario. For the restored vegetation scenario, the relative August mean increases
across all tributaries were about the same (1.0°C for 2040s and 2.0 for 2080s, compared to
current restored conditions).

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6 r

-6

Positive Values Indicate
Tributaries Colder than
the Columbia River at their
Respective Confluence

0.3

" !|| I |

-1.7

15 -1.6-1.6

Negative Values Indicate

"3'1 -3.3

Tributaries Warmer than

the Columbia River at their

¦ Current Future - 2040 ¦ Future - 2080

Respective Confluence

All 202 Tributaries Snake River
Flow Weighted 45% of Total
Average Tributary Flow
Volume

Willamette River Deschutes River Cowlitz River

15%ofTotal 8%ofTotal 6%ofTotal

Tributary Flow Tributary Flow Tributary Flow

Volume Volume Volume

Okanogan River Other 197

3%of Total Tributaries Flow
Tributary Flow Weighted Average
Volume (23% of Total

Tributary Flows)

Figure 3-2 Difference in Columbia River tributary and mainstem August mean temperatures

at confluences and future projections

(from EPA 2018)

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3
+->

«•

•^Columbia River - Interpolated From Measured Data
O Observed Columbia River Temperatures (*C) (2011-2015 Average)

• Tributary temperature >4*C cooler than the Columbia River

• Tributary temp, between 2*C and 4*C cooler than the Columbia

• Tributary temp, between 0*C and 2*C cooler than the Columbia

• Tributary temperature warmer than the Columbia River

100 200 300 400 500 600 700 800 900 1000 1100 1200
Columbia River (River Kilometer)

Figure 3-3 Estimated current August mean temperature in the Columbia River and

tributaries

(from EPA 2018)

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Columbia River - 2080

• Tributary temperature >4*C cooler than the Columbia River

• Tributary temp, between 2*C and 4*C cooler than the Columbia

• Tributary temp, between 0*C and 2*C cooler than the Columbia

• Tributary temperature warmer than the Columbia River

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Columbia River (River Kilometer)

Figure 3-4 Estimated 2080 August Mean temperature in the Columbia River and tributaries

(from EPA 2018)

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Table 3-1 Projected stream temperature responses to future climate change scenarios in

the Northwest

Watershed/
Region

Future
Period

Water
Model(s)

Scenario(s) Used to
Drive Water Models 31

Projected Water Temp. Changes (°C)

a , . Seasonal
Annual (Range) (Rgnge)

Citation

Regional Assessments

Methow River,
WA

Early
Century

Coupled
GLM-VIC
statistical
modeling

Climate

Number. 10
Emissions'. A1B

+0.4
(±1.6)

July:
+1.4

Caldwell et
al. 2013

Mid
Century

+0.7
(±1.8)

July:
+2.4

Late
Century

+0.8
(±1.9)

July:
+2.8

Unspecified
watersheds,
OR/WA/ID/MT

Early
Century

DRTT

Climate

Number: 20
Emissions'. A1B

+0.55
(+0.01 to +1.09)

Summer.
+0.92
(-0.27 to +2.66)

Wu et al.
2012

Mid
Century

+0.93
(+0.03 to +1.80)

Summer.
+ 1.37
(-0.47 to +4.08)

Late
Century

+ 1.68
(+0.08 to +3.17)

Summer.
+2.10
(-0.46 to+5.81)

124 sites,
WA

Early
Century

Statistical
modeling
approach d/

Climate

Number. 19
Emissions'. A1B, B1

Summer:c/
< +1 to +2

Mantua et
al. 2010

Mid
Century

Summer:c/
< +1 to +4

Late
Century

Summer: d
< +1 to +5

Wenatchee,
Icicle and
Nason Creeks,
WA

Early
Century

QUAL2Kw

Climate

Number. 39
Emissions'. A1B, B1

Summer.
A1B: +0.61 to+1.30
B1: +0.53 to+1.09

Cristea and

Burges

2010

Mid
Century

Summer
A1B: +0.95 to +2.08
B1: +0.74 to+1.66

Late
Century

Summer.
A1B: +1.35 to +2.86
B1: +1.05 to +2.30

South Fork
Nooksack
River, WA

Early
Century

VIC
Qual2Kw

Climate

Number: 3
Emissions: A1B

7-day changes for
7Q10 streamflow:
+1.4 to +1.9

Butcher et
al. 2016

Mid
Century

+2.2 to +3.1

Late
Century

+2.9 to +5.1

Pacific
Northwest
region

Mid
Century

NorWest
spatial
statistical
network
model

Climate

Number: 1
Emissions: A1B

August:
+0.73

Isaak et al.
2017

Late
Century

August:
+ 1.42

Columbia River
basin and
coastal
drainages of
OR and WA

Mid
Century

Dominant
river-tracing
streamflow
model

Climate

Number: 1
Emissions: A1B

Max weekly mean:
+ 1 to +4

Beechie et
al. 2013

Late
Century

Max weekly mean:
+2 to +6

Columbia River Mainstem

Mid-Columbia
River basin

Mid
Century

NorWest
spatial
statistical
network
model

Climate

Number: 1
Emissions: A1B

August:
+1.0

Isaak et al.
2017

Late
Century

August:
+2.0

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Watershed/
Region

Future
Period

Water
Model(s)

Scenario(s) Used to
Drive Water Models 31

Projected Water Temp. Changes ( C)

. . . Seasonal
Annual (Range) (Rgnge)

Citation

Columbia River
at Bonneville

Early
Century

VIC

Climate

Number: 4
Emissions: A1B

Summer:
+1.0

Yearsley
2009

Mid
Century

Summer:
+1.7

Columbia River Tributaries

Columbia River,

WA/ OR/ID
(258,000 km2)

Late
Century

VIC-RBM

Climate

Number: 6
Emissions: A2, B1

+1.6°C
(95th percentile:
+2.6°C)

van Vliet et
al. 2013

Columbia River
tributaries

Mid
Century

Spatial
stream
network
models

Climate

Number: 1
Emissions: A1B

August mean:
+ 1.1

Fuller et al,
2018

Late
Century

August mean:
+2.0

Columbia River
tributaries

Mid
Century

NorWest
spatial
statistical
network
model

Climate

Number: 1
Emissions: A1B

August mean:
+0.8

EPA 2018

Late
Century

August mean:
+2.0

DRTT= Dominant river-tracing-based temperature model; GLM= Generalized linear model; VIC= Variable Infiltration
Capacity model; QUAL2Kw= water quality model; VIC-RBM= Variable Infiltration Capacity model, combined with a
one-dimensional stream-temperature model referred to as the RBM. Early Century: approximately 2010 - 2030; Mid
Century: approximately 2030 - 2070; Late Century: approximately 2070 - 2100.

3.3 SUMMARY OF HISTORICAL AND FUTURE IMPACT PROJECTIONS

Climate change has already and is projected to continue to influence river temperatures across
the Northwest, including the temperatures of the Columbia and Snake Rivers, and will influence
multiple aspects of river hydrographs, including timing and magnitude of river flow. Based on the
synthesis herein, climate change has increased temperatures in the Columbia and Snake River
mainstems by 1.5°C ± 0.5°C since 1960. From the present-day baseline, the warming trend is
expected to continue in the coming decades. The two studies that specifically analyzed the
Columbia River (Isaak et al. 2018 and Yearsley 2009) predict an increase in summer mainstem
river temperatures of 1,7°C to 2.0°C by the end of the century. Similar increases are projected
for Columbia River tributaries. These estimates are provided in a context of regionwide
projections of river temperature increases in summer generally ranging from 1°C to 5°C by the
end of the century.

3.4 UNCERTAINTY IN FUTURE IMPACT PROJECTIONS

The available projections of future air and water temperatures in the Northwest region and the
Columbia and Snake Rivers are uncertain, because these projections are developed using
models that represent atmospheric, hydrologic and heat transfer processes based on our
current scientific understanding of those complex processes. In addition, the datasets used as
the basis for the future climate projections including historical climate information and future
estimates of human activities are imperfect and based on assumptions that may not hold in the
future. Despite the limitations and uncertainties, the scientific projections point towards a
distinctly warmer climate at the end of the 21st century (Rupp et al. 2017). The available
research is unanimous in projecting warmer temperatures, but there is uncertainty in the slope
of the trend at different periods of the 21st century. For example, Rupp et al. (2017) estimated
that air temperatures at the end of the century will increase by 3.0 and 5.5 °C above the 1970-

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1999 baseline average under the RCP4.5 and RCP8.5 scenarios respectively. The estimated
temperature differences between the RCP4.5 and RCP8.5 projections were 0.5 °C by 2050, and
2.5 °C by 2099. Future air and water temperature warming rates will ultimately be dictated by
the actual levels of greenhouse gas emissions and the evolution of the complex global energy
system (Isaak et al. 2018).

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