Physical and ecological evaluation of a fish-friendly surface spillway

Spillway passage is one of the commonly accepted dam passage alternatives for downstream-migrating salmonids and other species. Fish passing in spill near the water surface may have improved chances of survival over fish that pass deeper in the water column near spillway structures. In this study, an autonomous sensor device (Sensor Fish) was deployed in 2005 to evaluate fish passage conditions through the Removable Spillway Weir (RSW) at Ice Harbor Dam on the Snake River in south-central Washington State. The Sensor Fish deployment was undertaken concurrently with a separate live fish injury and survival study. Conditions at the RSW-Spillway Chute Transition and Deflector region were found to be potentially detrimental to fish. As a result, the spillway slope and deflector radius were modified, and the efficacy of the modifications was evaluated in 2015 using Sensor Fish and a concurrent live fish study. The frequency of severe acceleration events (acceleration ≥ 95 G) during passage decreased significantly (from 51% to 35%; p-value = 0.049), and collisions with structures decreased from 47% to 27% (p-value = 0.015). Pressures observed in the Spillway-Deflector region and pressure rates of change decreased as well. Overall, the modifications resulted in hydraulic conditions that contributed to improved fish passage conditions and increased fish survival.