A large part of the reasoning behind building many of the dams in the U.S. was flood control. In fact, I think the Hoover Dam was built for flood control reaso

A large part of the reasoning behind building many of the dams in the U.S. was flood control. In fact, I think the Hoover Dam was built for flood control reasons, and the hydroelectric part was the means to pay for it. Enabling consistent river flows, which enables human settlement and farming in otherwise desert areas is definitely an environmental impact, but it's not clear that it is a bad impact. And not burning fuel is certainly a positive in the eyes of global warming. All that fuel burning could result in rather permanent environmental changes of its own as sea levels rise, etc. If global warming is caused by CO2, then if we got all our power from dams instead of burning fuel, that seems like a place we'd like to get to right about now.