Rowski,
Yep, the home owners association that I used to own a lot in use a similar device to lower the pond level. Once this was done the beavers left. The pond leveler that was used was something like a 6 inch PVC pipe pushed through the dam. Six to ten feet of the pipe was on the pond side of the dam. At that end of the pipe quite a few holes where drilled. I think we left the downstream end alone. I saw plans with a 90 degree bend put on the downstream end with a veritcal piece installed. By changing the length of the vertical pipe the level of the pond could be controlled.
The key piece is the horizontal pipe pushed through the dam. It has to go a bit into the pond to work. The beavers know that the the water level is going down. And they know the pipe ain't supposed to be there but with the long pipe with lots of holes they can't figure out how to plug up the leak. It works.
I think Clemson had a website years ago on the pond levelers. They where the first or one of the first to come up with this idea as far as I know. We did this to a pond back in the early/mid 90s and it worked. Sold the lot in 1999 I think and the pond was still lowered and the beavers where gone.
Later,
Dan McCarty