Simulations and retrospective analyses of fraser watershed flows and temperatures

In order to provide better estimates of the thermal‐induced stress encountered by salmon migrating to their spawning grounds, a model is used to hindcast temperatures throughout the mainstem Fraser and Thompson Rivers back to 1953. Tributary and headwater temperatures that are not available prior to 1993 are estimated with both linear regression and neural network techniques. The average root mean square difference between model temperatures and those observed at Hell's Gate, on the lower Fraser River, is computed to be 1.12°C. Historical flow and temperature observations are also used to establish patterns and trends for the Fraser River watershed. The Julian day numbers by which one‐third and one‐half of the integrated yearly discharge had occurred were computed and found to be progressing earlier at the rates of 0.11 and 0.09 days per year, respectively. Both values are significant at the 95% level. Average summer Hell's Gate temperatures from 1941 to 1998 are warming at the rate of 0.012°C per year, though the relatively large standard error of 0.008°C means that this result is not significantly different from zero at the 95% level. However, when the analysis is restricted to 1953 to 1998, the trend becomes 0.022°C per year and the significance level rises to 98%. An analysis of the 1953–98 atmosphere‐to‐river heat exchange at Kamloops and Prince George produces positive increases of 0.52 W m–2 per year and 0.90 W m–2 per year, respectively. When these trends are used to force the river temperature model, they explain approximately 35% of the 1953–98 temperature increase at Hell's Gate. An additional 20% is explained by related increases in the headwater and tributary temperatures. Though these estimates have considerable statistical uncertainty, they nevertheless suggest that most of the river warming can be attributed to climatic effects. Flow rates and river temperatures are also shown to exhibit significant differences in the summers following El Niño and La Niña winters. On average, during the summers following El Niño events, flows are approximately 800 m3 s–1 smaller and river temperatures are approximately 0.9°C higher on the lower Fraser River. Similar estimates are produced for the major tributaries and headwaters of the Fraser and Thompson Rivers. The implications for future salmon recruitment are briefly discussed.