Plant things that beavers like to eat, and they have a habit of showing up.
However, if the stream bed has cut down too low, they can't fix it, and I would ask officials if it would be permissible to put in an "artificial" beaver dam, to slow the
water, and help the stream recover enough that beavers can then take over the job.
The artificial version is usually something like
wood posts in the ground every few feet across the stream bed, then things like willow woven between the posts, then mud tossed at it if available, and other larger branches tied into it. No where near as good as a beaver would do, but it has the desired effect of starting the healing process.