Where upstream fish passage facilities have been provided migration delays and increased mortality of adults persist. Fishway design and flow are very important to attract and guide adult salmon into passage facilities. At dams, injury and mortality to juveniles occurs as a result of passage through turbines, sluiceway, juvenile bypass systems, and adult fish ladders. Impact with turbine blades, rough surfaces, or solid objects can cause death or injury. Changes in pressure within turbines or over spillways also can result in death or injury.
Dams also result in changes in concentrations of dissolved oxygen and nitrogen.
Below hydroelectric facilities, nitrogen supersaturation may also negatively affect migrating salmon, by causing gas-bubble disease. Mortalities from gas bubble disease increase in years of high flow and high spill. Losses of juvenile fish due to passage mortalities are estimated at 11 to 15percent per dam for those passing through turbines, compared to one to two percent or more for those going over spillways or through bypasses. This mortality is cumulative along the course of a river on which a number of dams are built. On the Columbia River, where some stocks have as many as eight or nine dams to pass, cumulative juvenile passage mortality can routinely exceed 75percent.