I purchased a book recently which talks about reintroducing beavers to areas save the area.
I have not read it yet. BUT, It may have information about working with them.
I think lowering the water level is something the beaver may not cooperate with.
So perhaps get the book
I did research ' loving with Beavers' and it is very interesting
Here is one
living with Beavers
Walking over the dam seems impossible looking at the images, but I guess if you have, that great. I guess you could add planks etc??