Evaluation of juvenile salmon behavior to inform downstream fish passage development at a high head dam

Fish passage development is a priority at Tieton Dam, on the Tieton River in Washington state, because passage options were not included when the dam was constructed nearly 100 years ago. To inform downstream passage design, we conducted a study to evaluate migration and near-dam behavior of juvenile salmon. The primary goal of the study was to determine how fish approached the dam and to identify areas where they congregated. In its current configuration, water is passed through a deep intake located approximately 548 m upstream of the dam, so there was interest in determining if juvenile salmon spent more time near the intake or at the dam. We found that tagged fish primarily entered the dam forebay offshore and arrived at the dam near the spillway on the left bank of the reservoir. Tagged fish had extended residence times in the forebay and moved repeatedly between the intake and the dam. Using thermal profile data, we inferred that juvenile salmon likely occupied the upper 5 m of the water column during May and June before moving to deeper portions of the water column (10–27 m) during July–September when the reservoir became thermally stratified. We found that a small percentage of tagged fish passed the dam when the reservoir was rapidly drawn down during August–October, but additional research may be needed to fully assess entrainment risk through the intake. These findings suggest that downstream fish passage could be developed at Tieton Dam with a high probability of success, given that most fish survived reservoir passage and spent considerable time near the dam where they would be available to discover and utilize new passage routes.