Effect of hydroelectric dam operations on the freshwater productivity of a Columbia river fall Chinook salmon population

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, a series of operational constraints have been implemented at Priest Rapids Dam to reduce the effects of discharge fluctuations on the population of fall Chinook salmon that spawns and rears downstream from the dam. Initial protections that focused on preventing redd dewatering were subsequently increased to include postemergence life stages. 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 corresponded with 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.