Executive Summary

Prior to operational changes in 2007, Detroit Dam on the North Santiam River in western Oregon had a well-documented effect on downstream water temperature that was problematic for endangered salmonid fish species. In this U.S. Geological Survey study, done in cooperation with the U.S. Army Corps of Engineers, an existing calibrated CE-QUAL-W2 model of Detroit Lake (the impounded waterbody behind Detroit Dam) was used to determine how changes in dam operation or changes to the structural release points of Detroit Dam might affect downstream water temperatures under a range of historical hydrologic and meteorological conditions.

Many combinations of environmental, operational, and structural options were explored with the model. Two downstream temperature targets were used along with three sets of environmental forcing conditions representing normal, hot/dry, and cool/wet conditions. Three structural options were modeled, including the use of existing outlets, one hypothetical variable-elevation outlet such as a sliding gate, and a hypothetical combination of a floating outlet and a fixed-elevation outlet. Finally, four sets of operational guidelines were explored to gain an understanding of the effects of imposing different downstream minimum streamflows or managing the level of the lake with different timelines in autumn.

Several conclusions can be made from these interim model scenarios:

• Temperature targets just downstream of Detroit Dam can be met through a combination of new dam outlets or a delayed drawdown of the lake in autumn.
• Spring and summer dam operations greatly affect the available release temperatures and operational flexibility later in the autumn. Releasing warm water during mid-summer tends to keep more cool water available for release in autumn.
• The ability to meet downstream temperature targets during spring depends on the characteristics of the available outlets. Under existing conditions, for example, although warm water sometimes is present at the lake surface, such water may not be available for release if the lake level is either well below or well above the spillway crest in spring and early summer.
• Managing lake releases to meet downstream temperature targets depends on having outlet structures that can access both (warm) lake surface water and (cold) deeper lake water throughout the year. The existing outlets at Detroit Dam do not allow near-surface waters to be released during times when the lake surface level is below the spillway (spring and autumn).
• Model simulations indicate that delayed drawdown of Detroit Lake in autumn would result in better control over release temperatures.
• Compared to the existing outlets at Detroit Dam, floating or sliding-gate outlet structures can provide greater control over release temperatures because they provide better access to warm water at the lake surface and cooler water at depth.

This report provides interim study results to the U.S. Army Corps of Engineers. The full study will be completed in 2012.

First posted October 4, 2011