Beavers and Water Quality?

I didn't realise it until today but it was reported on BBC's Countryfile programme that South-West Water is one of the main enthusiasts for the reintroduction of the beaver into Devon and Cornwall. Their logic is that beavers, via their dam creation activities, would slow the flow of rivers, resulting in nitrates and phosphates in run-off from agricultural activities having the opportunity to fall to river bottoms with silt. They feel that this would discourage the problematic blooming of toxic blue-green algae that has negative consequences for recreational water use etc. Other people, concerned with creating environments for spawning salmon and trout (fast rivers with clean gravel substrates) do not appear so keen on beavers. One thing was very evident, in following the one male beaver that has escaped quarantine in that area, is that it is very difficult to predict where the animals will go and the precise nature of their activities (they can certainly take down substantial trees!). I feel that we need some seriously controlled studies in a variety of locations before any question of systematic release.