Trends in Stream Temperature in the Snake River
Identification
1.	Description
This feature examines water temperatures in the Snake River, which winds through Washington,
Oregon, and Idaho. Water temperature is particularly important in the Snake River and other rivers of
the Columbia Basin because they are home to many species of salmonids (salmon and related fish),
which require relatively cold water to migrate, spawn, and thrive. Migrating salmon are an important
part of the ecology of the Pacific Northwest, and they also play vital cultural, spiritual, and economic
roles for tribal nations in the region. Rising air temperatures associated with climate change can also
raise water temperatures, which could make some watersheds less hospitable to salmon. This feature
focuses on temperatures in August, which is when rivers and streams in the region typically register
their annual maximum temperatures.
2.	Revision History
August 2016: Feature published.
April 2021:	Updated feature with data through 2020.
Data Sources
3.	Data Sources
This feature is derived from stream temperature measurements collected by the U.S. Geological
Survey's (USGS's) long-term stream gauge on the Snake River near Anatone, Washington (site #
13334300). The site is located in eastern Washington, near Nez Perce tribal lands. Data from this site
have been collected since October 1959.
4.	Data Availability
Daily temperature data, including maximum, minimum, and mean temperatures, are publicly available
through the USGS National Water Information System (NWIS) database at:
https://waterdata.usgs.gov/nwis. Records for this particular location can be found at:
https://waterdata.usgs.gov/nwis/inventorv/Psite no=13334300&agencv cd=USGS. Site information
available at this link includes location, streamflow, temperature, partial pressure of dissolved gases, and
gauge height.
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Methodology
5.	Data Collection
Temperature data at the sampling location near Anatone are collected hourly by automated USGS
monitoring equipment. Daily temperature data are available beginning in 1959, so this feature starts
with data from 1960. August data were not recorded for the years 1975, 1984, and 1985, so these years
are noted as gaps in Figure 1.
6.	Derivation
Many aspects of water temperature (e.g., maximum, average, variability) are relevant to salmonid
physiology and therefore relevant to an assessment of habitat quality. This feature focuses on average
August water temperatures, based on an arithmetic mean of all daily mean temperatures during the
month. During early years of the temperature record, only daily maximum and minimum temperatures
are available. For these years, mean daily stream temperatures were calculated as the average of the
daily maxima and minima (Isaak et al., 2012).
7.	Quality Assurance and Quality Control
Temperature data are included in the NWIS database only after extensive quality assurance procedures
have been met. Sensors are regularly inspected, and data are labeled as provisional until they are
reviewed and receive approval. Approved final data were used through 2020.
Analysis	
8.	Comparability Over Time and Space
This feature includes data from one study site, precluding any issues of comparability over space. NWIS
quality assurance requirements help to ensure comparability over time.
9.	Data Limitations
Factors that may impact the confidence, application, or conclusions drawn from this feature are as
follows:
1. Many factors other than climate change can lead to warming of rivers. Specific anthropogenic
effects that may result in variable heat loading include:
•	Removal of streamside vegetation through urban development, agriculture, grazing, and
forestry.
•	Changes in stream shape due to bank erosion. Wider and shallower streams increase the
surface area that is subject to solar radiation and atmospheric heat exchange.
•	Water withdrawals for industrial, municipal, and agricultural uses.
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•	Heated water discharges from industrial facilities, wastewater treatment plants, and
irrigation return canals.
•	Reduced groundwater flow due to river channeling, straightening, or diking. Increased
prevalence of impervious surfaces due to urban development also reduces groundwater
flow.
•	Dams and associated reservoirs. Reservoirs may increase maximum temperatures by
limiting water movement in shallow areas, and because of their increased heat capacity,
they may also reduce diurnal temperature variability and seasonal change.
10.	Sources of Uncertainty
Uncertainty has not been quantified for this feature. The uncertainty associated with temperature
measurements taken at an individual site is thought to be minimal, as the data are collected at regular
intervals by electronic instruments that do not depend on human interpretation.
11.	Sources of Variability
Water temperatures can vary substantially from year to year, due in part to the variable nature of
annual weather patterns. Air temperature, rainfall, and snowpack can all vary naturally from one year to
the next, and all of these variables can influence stream temperature. Additionally, climate in the Pacific
Northwest can reflect periodicity in ocean conditions (known as the Pacific Decadal Oscillation, or PDO)
that may have strong effects on temporal variability in stream temperatures (Mantua and Hare, 2002).
Because the length of the dataset includes opposing PDO cycles (Luce and Holden, 2009), however, the
potential magnitude of this source of variability is minimized.
The Snake River at Anatone is downstream of the Brownlee Reservoir, which could alter thermal trends,
but the likelihood of meaningful changes to the river's thermal regime due to the reservoir's presence is
minimal. The reservoir is located more than 160 kilometers upstream of the measurement site. Over this
distance, river temperatures should equilibrate to local climatic conditions, as spatial lags in the
correlation of stream temperatures are typically much shorter than 160 kilometers (Isaak et al., 2010).
Additionally, two large, unregulated tributaries enter the Snake River downstream of the reservoir and
upstream of the measurement site. These tributaries, the Salmon and Grande Ronde rivers, double the
Snake River's volume and further dilute any remaining reservoir effects.
12.	Statistical/Trend Analysis
The long-term rate of change in average August water temperatures was computed using the Sen's
slope method, which finds the median of all possible pair-wise slopes in a temporal data set (Theil, 1950;
Sen, 1968; Helsel et al., 2020). This calculation resulted in a slope of +0.0177°C (+0.0319°F) per year,
with a Mann-Kendall p-value of 0.001, which makes the trend significant to a 95 percent level (p < 0.05).
The annual rate was multiplied by 60 years to derive an estimate of total change over the period from
1960 to 2020.
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References
Helsel, D.R., R.M. Hirsch, K.R. Ryberg, S.A. Archfield, and E.J. Gilroy. 2020. Statistical methods in water
resources. Techniques and methods, Book 4. Chapter A3. U.S. Geological Survey.
https://pubs.er.usgs.gov/publication/tm4A3.
Isaak, D.J., C.H. Luce, B.E. Rieman, D.E. Nagel, E.E. Peterson, D.L. Horan, S. Parkes, and G.L. Chandler.
2010. Effects of climate change and wildfire on stream temperatures and salmonid thermal habitat in a
mountain river network. Ecol. Appl. 20(5):1350-1371.
Isaak, D.J., S. Wollrab, D. Horan, and G. Chandler. 2012. Climate change effects on stream and river
temperatures across the Northwest U.S. from 1980-2009 and implications for salmonid fishes. Climatic
Change 113:499-524.
Luce, C.H., and Z.A. Holden. 2009. Declining annual streamflow distributions in the Pacific Northwest
United States, 1948-2006. Geophys. Res. Lett. 36(16):L16401.
Mantua, N.J., and S.R. Hare. 2002. The Pacific Decadal Oscillation. J. Oceanogr. 58(l):35-44.
Sen, P.K. 1968. Estimates of regression coefficient based on Kendall's tau. J. Am. Stat. Assoc.
63(324): 1379-1389.
Theil, H. 1950. A rank invariant method of linear and polynomial regression analysis, I, II, III. P. K. Ned.
Akad. A Math. 53:386-395, 521-525, 1397-1412.
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