They can be switched off, depending on how they were built. During the building process you have to create a diversion tunnel, as the water has to keep flowing while you are building them. They should also have ways of draining the reservoir. Also in my state (Idaho, which is largely powered by dams) we have removed a some of the older ones. I don't have any source materials for you, but I had to go thru a ton of dam tours as a kid to learn how the dams worked. We have also had a dam burst (Idaho Falls with the Teton Dam). But every dam is unique, you certainly cannot account for all of them.

Also, dealing with the flood plain, which the dams protect against, that is another story.