Comment by monomania

They are also very difficult, and very expensive, to maintain as they age. There is a small hydro damn near where I live that predates WWII and the county/state cannot find anyone who wants to take it over because of the expense in repairing it. This is in part due to seismic retrofitting which needs to happen, but also a variety of other factors, including being built basically on silt, having a public road on top which people need to get to their homes, and simply updating the aged equipment which has been on its last leg for a while.

What's more, the dam installation changed the local water table level, and residential wells drilled since the 1940's only had to go down so far. If the dam is removed, all these wells will dry up, and the state is beginning to make it difficult to drill new wells, and they certainly don't want to shell out to install plumbing to unincorporated communities which don't vote for the majority party anyway.

Another dam nearby held a massive amount of water for one of the bigger cities in the state. This is on the west coast, and if we get the Big One, it'll flood a populated area with hundreds of thousands of people. So they've had to draw down on the water levels and it's basically useless now, but deconstructing the dam would probably be a project requiring federal money, and since it's an earthen dam in a seismic zone there isn't really any retrofitting that you can do to fix it.

Basically, dams will give you some cheap electric and a water supply for a while but after 50-70 years they're a huge liability.