Effects of water velocity on the survival of downstream‐migrating juvenile salmon and steelhead: A review with emphasis on the Columbia river basin

Restoration of salmon and steelhead stocks in the Columbia River Basin depends in large part on the adequacy of streamflows needed to transport juveniles safely downstream to the ocean through a series of dams and reservoirs. Compared with preimpoundment conditions, lower spring and summer river flows and decreased water velocities are believed to increase juvenile salmonid travel times to the ocean, and potentially to reduce survival. Because of continuing disagreement about the quantities of flow releases needed to increase smolt survival, we reviewed literature from within and outside of the Columbia River Basin relating to the influence of water velocity on the survival of juvenile salmon and steelhead. Most of the studies reviewed found a positive relationship between outmigration flows and survival, but there is substantial uncertainty about many of the estimates. Early survival estimates made in the basin did not quantify variance and contain biases, errors, or reflect interactions with factors other than water velocity. Other influential factors that were examined in the review included predation, water quality, and physiological state of the smolts at the time of migration. Despite the limitations of existing data, the general relationship of increasing survival with increasing flow in the Columbia River Basin still appears to be reasonable.