Sediment problems and consequences during temporary drawdown of a large flood control reservoir for environmental retrofitting

Abstract

Retrofitting a large flood control dam on the South Fork McKenzie River, Oregon, USA with a temperature control structure required drawdown of Cougar Reservoir. The drawdown initiated incision of the reservoir delta that had developed in the 40 years since Cougar Dam was constructed. Remobilization of deltaic sediments resulted in a sustained release of turbid water from Cougar Reservoir, prompting concern that sediment contained within the turbidity plume might intrude into river gravels, with potentially negative effects for fish and other aquatic biota. We sampled gravels both upstream and downstream of Cougar Dam and on the mainstem McKenzie River both above and below the confluence with the South Fork to compare affected gravels to unaffected gravels. The results suggest that intrusion of very fine clays into gravel substrate can occur even when the clay is carried as wash load.