Effects of Dam Design and Operation on Downstream Fish Passage at Two High Head Dams

The design and operation of high-head dams create unique fish passage issues.  High head dams (those with a hydraulic head over about 30 meters), are commonly used for flood control and are often placed in areas with steep topography and small, but variable, river runoff.  Common conditions include a low discharge to volume ratio, seasonal variations in reservoir elevation, vertical gradients in water temperature, and fast-rotating turbines with small passageways.  We quantified reservoir migratory behaviors and downstream passage of acoustic-tagged juvenile Chinook salmon (Oncorhynchus tshawytscha) at two high-head dams in western Oregon over a two-year period.  We quantified the effects of reservoir elevation, dam discharge, fish size, and several other factors on rates of dam passage.  Dam passage rates of the tagged fish were high in summer and low in winter at one dam and the reverse at the other, and there were differences in migratory behaviors in the reservoirs.  Differences in dam operating rules and conditions were the primary causes of the seasonality of dam passage and likely affected differences in the migratory behaviors within the reservoirs.   These results indicate that dam operating conditions can affect downstream passage rates and could be part of downstream passage solutions.