As the beaver reintroduction is happening on a neighbouring river (The Otter) Dart anglers may be interested in the following.
Firstly, here is a recent report on the reintroduction from the BBC:
http://www.bbc.co.uk/news/science-environment-40048762
There is no mention of how beavers might affect fish migration so here is a policy statement from the Angling Trust regarding beaver reintroduction that covers that aspect among others.