Hmm, am I right in thinking you're in Vermont? If so you're in luck. It looks like the dam is in a culvert leading to the retention pond. It is very common for beavers to dam culverts (shortcut for them) and is the EASIEST problem to fix.
The "Beaver Deceiver" prevents damming of culverts/small water passages and was invented by Skip Lisle who lives in Grafton VT. You can read about the method here http://grandcanyontrust.org/documents/ut_workingBeaver2010.pdf and contact him here.
802-289-2899
The only way I know to "save beavers" is by getting attention and spreading the word so enough people complain about that the officials involved are willing to think for a nanosecond and try something besides trapping. The good news is that if you can get them to try it, it almost always works.
VT has more beaver lovers than most states. Try contacting Patti smith of the BonnyVale Environmental Education Center. She writes lovely columns about the beavers she's been following and she might help.
Good luck!
Heidi
