There is little you can do to enhance a beaver dam. It takes a little while for a new dam to start to grow the plants that water fowl like but the waterfowl themselves take care of that by transporting the seeds on their bodies as they move from one place to the next. I would recommend if you want to erect nesting devices for wood ducks you use the wooden ones that are made so you can remove two screws and the front swivels down so you can replace the wood shavings every year before the nesting season. Old shavings can harbor lice that can harm ducklings. If you want to put up a device or two for mallards I would suggest the wire baskets on a stake out in the water filled with straw. For both wood ducks and mallards I strongly advise raccoon guards which are inverted funnels that go around a tree or post to prevent coons from climbing up and raiding the nest.