Mostovetsky, A., Phillips, J., Olis, M. and Veldhuisen, C., 2015. Skagit and Sauk Tributary Stream Temperature Monitoring – 2008-2013 Results and Interpretation. Skagit River System Cooperative, La Conner, WA. pp. 32.

Stream temperature is an important water quality component, known to strongly affect aquatic life and stream health. 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 for 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