Phillips, J., Olis, M., Veldhuisen, C., Morris, S., Couvelier, D., 2011. Skagit and Sauk River Basin Stream Temperature Monitoring – 2008-2009 Progress Report. Skagit River System Cooperative, La Conner, WA. pp. 54.

The Skagit River basin, consisting of the mainstem Skagit (including tributaries, sloughs and estuaries) and four secondary river basins (Baker, Cascade, Sauk, and Suiattle) contains essential habitat for anadromous salmonids, including several species that are listed as threatened under the Endangered Species Act (DOE, 2008). Five species of salmon (Chinook, coho, pink, chum and sockeye), two char species (Dolly Varden and bull trout) as well as steelhead and cutthroat trout exist in the basin (SRSC and WDFW, 2005). The Skagit has the largest run of Chinook and the second largest wild run of coho in the Puget Sound (DOE, 2008).
Salmon and trout face limitations in Skagit River tributaries during mid-summer, when habitat availability is reduced by low flows and high stream temperatures. Previous research, summarized in several comprehensive reviews, indicates that stream temperature is a significant factor that affects distribution and health of salmonids (Bjornn and Reisner, 1991; McCullough, 1999; and Hicks 2001). The direct effect of high temperatures on physiological functions of salmon is reasonably well understood and has been documented in laboratory settings. Water temperature is also important for regulating biological and physiological processes in other parts of the aquatic system that may indirectly affect salmon through loss of food supply, spread of disease and other factors. High temperatures may alter migration rates for spawning and rearing and promote growth of competing species (Beschta et al., 1987). Potentially lethal temperature-related limitations including reduced metabolic energy, reduced food supply, and competition from warm water species, can indirectly lead to fish mortality (Pollock et al., 2009). In general, the preferred temperature range salmon is 12° C to 14° C with most at risk of mortality when temperatures exceed 20° C, although the exact lethal limit temperature depends on species, life-stage of development and the temperature that the fish is acclimated to (Hicks, 2001). Table 1 contains the approximate temperature ranges for modes of thermally-induced mortality.