There's actually several problems with most hydro dams that sadly is often completely ignored due to it being "green energy." For example:
- Disrupting Fish Migrations. The dams can act like walls, stopping annual fish migrations. Though this is sometimes minimized using fish-ladders, often many other aquatic species that can't use the ladders are ignored.
- Displacing people and wildlife. For example, Lake Minnewanka in Alberta, Canada, has an underwater town that was flooded for a hydro-dam. The spot was also of importance to the indigenous people of the area, as it was seen to be connected to the spirit world.
- Polluting the Water. By causing higher levels of sediment and algae in the reservoirs. This is called “eutrophication.”
- Hurting or Killing Wildlife. The released water is often cold and low in oxygen, which can shock and even kill wildlife downstream.
- Water-Depletion. Reservoirs can cause water-depletion for an area, since still water evaporates faster than the water in a moving river, and the plants consume water to work.
That's why I don't always classify hydro as actually being green energy. There's hope in small-scale turbines making a comeback in a fish-friendly style https://youtu.be/KEsrAmM07fs , as well as updated takes on tidal energy, such as the wave swing. https://youtu.be/mxesgXdw0Zw