Effects of Priest Rapids Dam Operations on the Productivity of Juvenile Fall Chinook Salmon in the Hanford Reach of the Columbia River

Although the Hanford Reach (HR) is the most undeveloped section of the Columbia River in the United States, it is not free of flow regulation.  A seven dam hydroelectric complex on the mid-Columbia River is operated under a load following mode to meet electrical demand in the Pacific Northwest and California.  Therefore, stream flow in the HR is largely governed by hydropower generation, which can result in substantial hourly and daily fluctuations in discharge.  The Vernita Bar Settlement Agreement (1988) and the HR Fall Chinook Protection Program Agreement (2004) provided constraints on Priest Rapids Dam flow fluctuations from spawning through early rearing.  Although these constraints have reduced the magnitude of flow fluctuations, large hourly and daily fluctuations still occur.  Despite these fluctuations, the HR fall Chinook population is one of the most productive fall Chinook populations in the Pacific Northwest.  To determine the dam operations that have affected the productivity of the HR fall Chinook population, we conducted a productivity analysis using data collected over a 35-year period (1975-2009), which was incorporated into a stock-recruit model.  Productivity was defined as the number of pre-smolts (recruits) produced per female spawner (stock), which ensured that only the life stages directly affected by dam operations in the HR were considered.  Adult escapement was estimated from dam counts and the proportion of females was estimated from creel survey data.  Numbers of pre-smolts produced from each brood year were estimated using CWT-based run reconstruction and a Ricker stock-recruit model was fit to the data.  Residuals from the model, which represented above- or below-average recruitment from each year, were regressed against dam operation-affected flow variables.  Flow-related variables during each distinct life history stage of fall Chinook salmon while in the HR (i.e., spawning, egg incubation, and rearing) were included in the analysis to determine not only the variables that have affected the productivity of the HR population during the last 35 years, but also to identify the life stage affected.  Results from this analysis will provide valuable information regarding (1) flow conditions that may have limited, or are currently limiting, the productivity of fall Chinook in the HR, (2) whether or not current flow constraints are sufficient or whether additional constraints are needed, and (3) whether dam operations may be partially responsible for the high productivity of the HR fall Chinook population, which may be used to guide dam operations elsewhere.