Is flooding what you are worried about? If it is - then explosives are the way to go. Usually you can get your county to either approve you for it, or they will actually do it themselves if it is deemed the water will do damage.
The houses are made of sticks and mud, they look like a big pile of sticks that go down into the water, the entrance is actually underwater.
The houses are not usually next to the dams, they are usually in a spot that has low water flow, and good amounts of shore line on either side away from rivers coming in and out so seasonal fluctuations don't affect the house. Do a google search on beaver houses - they are very easy to spot - like I said, big ass pile of sticks that goes down into the water on the shore, a round top to the mound, then gently sloping down from there.
The dam is what you have to worry about, it has to be destroyed. It gives the beavers more habitat, and if they have built it up to decent size, chances are the water is going to get even higher than it is now without any more work. Explosives work great for this. If you can get explosives on it, getting a rope and a winch, loop the rope through a few feet of the damn, winch it open. Repeat repeat repeat. It is important you do this ASAP. Brand new beaver dams are easy to take out, but old ones, once they are full of mud and leaves binds it all up like clay, then you HAVE to use explosives.