FIGURE 3-8 Maximum and average August water temperatures in the Columbia River at Bonneville Dam (straight lines reflect trends for maximum and average values). SOURCE: USACE, 2000. Data provided to Stuart McKenzie for a report prepared for the Northwest Power Planning Council.

of great concern with regard to the survival of Columbia River salmon. For example, August temperatures at Bonneville Dam exceed temperatures preferred by cold water fish like salmonids (~10° to 15°C; Kling et al., 2003). More importantly, it means that Columbia River water temperatures are approaching the upper limits of thermal tolerance for cold water fishes (~20° to 24°C; Mohseni et al., 2003) such as salmonids.

These temperature changes appear to have been driven by (1) construction of the dam and reservoir system (the large surface areas of Columbia River reservoirs and the increased residence time of water in these reservoirs both contribute to higher water temperatures) and (2) increased temperatures of inflows from tributaries from watersheds that have lost riparian cover that provided shade for those streams.