PNNL

Abstract

Altering the timing and magnitude of discharge fluctuations can minimize the adverse effects of operating hydroelectric dams on the productivity of downstream salmon populations. Hydroelectric operations at Priest Rapids Dam during the mid-1970s resulted in dewatering of fall Chinook salmon Oncorhynchus tshawytscha redds, causing mortality of intragravel life stages. Since then, the operational regime of Priest Rapids Dam has been altered several times to reduce the effects of discharge fluctuations on the population of fall Chinook salmon that spawns and rears downstream from the dam. We used stock-recruit analyses to identify changes to the population’s freshwater productivity that occurred over a 30 year period and coincided with changes to dam operations. We observed a 217% increase in productivity that coincided with discharge constraints enacted to prevent redd dewatering and an additional 130% increase that coincided with enactment of constraints to limit stranding and entrapment of juveniles. The information gained from this study may be used to guide efforts elsewhere to mitigate the effects of hydroelectric dam operations on downstream fish populations.