Several of hydropower's disadvantages focus on fish. It is easy to forget how important fish and other aquatic life are, some of which reside at the bottom of the food chain. The environmental changes caused by hydroelectric projects may be obvious to the local biologist, but elude the average person. Most people will more readily notice a smoggy haze developing in an area where a coal plant is operating than a smaller population of a particular bird species where a hydropower facility functions. Such oversights lead people to believe that nothing is wrong.
Dams hold back silt, debris, and nutrients.
Eventually this renders the dam inoperable, leaving the mess for future generations, who will either have to remove the collected debris or live with a potentially catastrophic mudflow poised to inundate the area below the dam. There is also a debate between preserving rivers for their aesthetic value versus meeting the energy needs of thousands of people. The latter has prevailed. The only undammed river in the US that is longer than 600 miles is the Yellowstone. Hydropower may be better on the environment than fossil-fuel sources, but its future is so uncertain that we may need to focus on other alternatives.