Water Temperature Mapping of the Skykomish, Snoqualmie, and Middle Fork Snoqualmie Rivers, Washington—Longitudinal Stream Temperature Profiles, Significant Thermal Features, and Airborne Thermal Infrared and RGB Imagery Mosaics

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).