The Skykomish, Snoqualmie, and Middle Fork Snoqualmie River Basins have historically provided critical spawning, rearing, and core habitat for several salmonid species. These salmonid species include natural populations of Chinook salmon (O. tshawytscha), steelhead trout (O. mykiss), and bull trout (Salvelinus confluentus)—listed as “Threatened” under the Endangered Species Act—as well as coho salmon (O. kisutch)—listed as a ”Species of concern”—pink salmon (O. gorbuscha), chum salmon (O. keta), and native char (S. malma) (Solomon and Boles, 2002; Stohr and others, 2011; Svrjcek and others, 2013; Snohomish County Surface Water Management and the Sustainable Lands Strategy Executive Committee [SWM], 2017; U.S. Fish and Wildlife Service, 2022). Because of the thermal constraints on salmonids and other aquatic species, the Washington Department of Ecology maintains temperature criteria for waters designated for aquatic life uses. These standards range between 12 degrees Celsius (°C) and 17.5 °C, referring to the highest permissible 7-day average of the daily maximum temperatures (7-DADMax), and vary depending on the habitat classification and time of year (Washington Department of Ecology, 2020). Over the past two decades, however, summer 7-DADMax water temperatures within the Skykomish, Snoqualmie, and Middle Fork Snoqualmie Rivers have frequently exceeded these temperature criteria, as well as the 23 °C threshold above which temperatures can be lethal to salmonids (Stohr and others, 2011; Svrjcek and others, 2013; Kubo and leDoux, 2016).
To inform salmonid restoration efforts within the Skykomish, Snoqualmie, and Middle Fork Snoqualmie Rivers, this study used high-resolution thermal infrared (TIR) and co-acquired true-color red, green, blue (RGB) imagery from airborne surveys conducted in August 2020 and 2021. The imagery mosaics from the airborne TIR and RGB surveys were used to measure the longitudinal stream temperature profiles (LTPs) of the Skykomish, Snoqualmie, and Middle Fork Snoqualmie Rivers and identify the location of significant thermal features (STFs) expressed at the water’s surface, including cold-water anomalies that could represent thermal refuges and serve as salmonid habitat. These surveys were done twice to evaluate the interannual persistence of STFs and the temporal variability of water temperature patterns in the LTPs, because the presence of STFs and the patterns in LTPs have been shown to vary over time in other studies (Dugdale and others, 2013).
The study area for the TIR and RGB surveys, from upstream to downstream, was (1) the Middle Fork Snoqualmie River from above the Goldmyer Hot Springs trailhead to the North Fork Snoqualmie River confluence (33 river miles), (2) the mainstem Snoqualmie River from the North Fork Snoqualmie River confluence to Chinook Bend Natural Area in Carnation, Washington (23 river miles), and (3) the Skykomish River from Gold Bar, Washington to its confluence with the Snoqualmie River in Monroe, Washington (26 river miles).
Those results are presented with the following items:
- TIR and RGB imagery mosaics (.tiff and .sid, respectively) of the Skykomish Snoqualmie, and Middle Fork Snoqualmie Rivers.
- LTPs (.shp) produced from the thermal infrared imagery mosaics.
- STFs (.shp) identified using the thermal infrared and true-color red, green, blue imagery mosaics.
In addition, water temperature float surveys were conducted on the Skykomish and Middle Fork Snoqualmie Rivers, August–September 2020, and a follow-up survey on the Middle Fork Snoqualmie River August 2021, to evaluate this less expensive and low-tech method of producing LTPs. Float survey data was collected by measuring near-surface and near-streambed (henceforth, “near-bottom”) water temperature, conductivity, and GPS position at three-second intervals from an inflatable kayak drifting downstream at ambient river velocity, following the method of Vaccaro and Maloy (2006). By moving downstream at ambient velocity, the change in water temperature within a theoretical parcel of water can be tracked, with deviations from the diurnal heating of the parcel presumably from groundwater discharge, surface water inflows or losses, and riparian shading. The resulting LTPs from the float surveys were adjusted by subtracting the estimated diurnal heating that occurred since the start of the survey to produce pseudo-synoptic LTPs for comparison with the near-synoptic LTPs from the airborne TIR surveys.
The study area for the 2020 float surveys, from upstream to downstream, was (1) the Middle Fork Snoqualmie River from the Taylor River confluence to the Three Forks Natural Area in Snoqualmie, Washington (21 river miles), and (2) the Skykomish River from Big Eddy River Access in Gold Bar, Washington to its confluence with the Snoqualmie River in Monroe, Washington (24 river miles). The follow-up float survey in 2021 was done on the Middle Fork Snoqualmie River from the Granite Creek confluence to the intersection of Tanner Road and North Bend Way in historical Tanner, Washington (about 8 river miles).
Prior to the airborne TIR and float surveys, U.S. Geological Survey, National Oceanic and Atmospheric Administration, and King County staff deployed 37 fixed instream thermistors spaced about every 1 to 5 river miles throughout the study area to record water temperature during the data acquisition time frame. Data from these thermistors was used to radiometrically calibrate the TIR imagery, perform quality-assurance checks on the float survey data, and compute adjustments to the float survey data to subtract the estimated amount of temperature increase during the survey from diurnal heating.
Water temperature records from the fixed instream thermistors are available for download through the Washington Department of Ecology's Environmental Information Management System (Washington Department of Ecology, 2023).
References Cited:
Dugdale, S.J., Bergeron, N.E., and St-Hilaire, A., 2013, Temporal variability of thermal refuges and water temperature patterns in an Atlantic salmon river: Remote Sensing of Environment, v. 136, p. 358–373, accessed December 15, 2022, at https://doi.org/10.1016/j.rse.2013.05.018.
Kubo, J., and leDoux, B., 2016, Hot water and low flow—The summer of 2015 in the Snoqualmie River watershed: Snoqualmie Watershed Forum & King County’s Department of Natural Resources and Parks, 96 p., accessed December 13, 2022, at https://your.kingcounty.gov/dnrp/library/2016/kcr2750/kcr2750-txt.pdf.
Snohomish County Surface Water Management and the SLS Executive Committee [SWM], 2017, Lower Skykomish River Reach-scale Plan: Snohomish County Department of Public Works and the Executive Committee of the Sustainable Lands Strategy, 119 p., accessed January 25, 2023, at https://salishsearestoration.org/images/c/cf/SnoCo_2017_lower_skykomish_reach_scale_plan.pdf.
Solomon, F., and Boles, M., 2002, Snoqualmie Watershed Aquatic Habitat Conditions Report—Summary of 1999-2001 Data: King County Department of Natural Resources and Parks, 80 p., accessed December 15, 2022, at https://your.kingcounty.gov/dnrp/library/2002/kcr1212.pdf.
Stohr, A., Kardouni, J., and Svrjcek, R., 2011, Snoqualmie River Basin temperature total maximum daily load—Water quality improvement report and implementation plan: Washington Department of Ecology, 210 p., accessed December 12, 2022, https://apps.ecology.wa.gov/publications/documents/1110041.pdf.
Svrjcek, R., King, A., Steg, R., Sievers, M., and O’Donnell, J., 2013, Skykomish River Temperature Total Maximum Daily Load Development—Water Quality Study Design (Quality Assurance Project Plan): Washington Department of Ecology, Publication No. 13-03-104, 87 p., accessed December 15, 2022, at https://fortress.wa.gov/ecy/publications/documents/1303104.pdf.
U.S. Fish and Wildlife Service, 2022, Environmental conservation online system—FWS-listed U.S. Species by taxonomic group—All animals; U.S. Fish and Wildlife Service database, accessed December 15, 2022, https://ecos.fws.gov/ecp/report/species-listings-by-tax-group?statusCategory=Listed&groupName=All%20Animals.
Vaccaro, J.J., and Maloy, K.J., 2006, A method to thermally profile long river reaches to identify potential areas of ground-water discharge and preferred salmonid habitat: U.S. Geological Survey Scientific Investigations Report 2006–5136, 16 p. [Also available online at https://pubs.usgs.gov/sir/2006/5136/]
Washington Department of Ecology, 2020, Fresh water designated uses and criteria, sec. 200 of Water Quality Standards for Surface Waters in the State of Washington: Washington Administrative Code, title 173, pt. II, accessed December 15, 2022, at https://app.leg.wa.gov/WAC/default.aspx?cite=173-201A.
Washington Department of Ecology, 2023, Water temperature mapping in the Snoqualmie and Skykomish River Basins (Study ID WQC-2020-00164): Environmental Information Management System, accessed December 8, 2023, at https://apps.ecology.wa.gov/eim/search/Detail/Detail.aspx?DetailType=Study&SystemProjectId=99972583.
