Juvenile fish that are drawn into the turbine pits by the current can be killed or injured. Usually these mortalities or injuries are caused when the fish strike the spinning blades or the concrete walls. The intense water pressure also can kill the fish. Biologists estimate that if turbine passage is the only way past a dam, 10 to 15 percent of the fish that are drawn through the turbines will die. With that much mortality at least possible at each dam, fish that pass multiple dams, such as fish from central Washington or the Snake River, have a statistically high probability of dying before they pass the last dam, Bonneville. (source)
There are around 400 dams in the Columbia basin, some large like Bonneville and some much smaller. To mitigate the impact, it’s led to heroic efforts, building fish ladders, screens, fish hatcheries. They even truck fish around some dams and stop the turbines (or overflow them) during fish runs.
Here’s a simple hydroelectric system that creates an artificial whirlpool that powers a turbine, creating enough energy for 300 homes. Fish pass through unhurt.