In the northern forested part of Wisconsin, the dams flood the forest kill the surrounding trees that haven't been taken down by the beaver. Then the beaver ponds warms up and there is downstream thermal pollution. The dams block the upstream migration of spawning trout. Beaver are bad for trout streams.


http://fyi.uwex.edu/beaver/files/201...ster-20111.pdf


"WDNR and USFS Fisheries Managers in Wisconsin generally agree upon the adverse affects which beaver dams create to the trout fishery. Many of Wisconsin's 2,674 trout streams are located along low gradients and can be easily dammed. The average trout stream is 5 miles (Kmiotek 1980). Beaver dams on small trout streams usually produce effects which follow a definite pattern. First, the vegetation flooded by a new pond will decay, fertilizing the water and increasing the food supply. The trout then grow rapidly, and good fishing may result for a period of 1 to 3 years. If the pond area is shallow and exposed to the sun, it becomes warmer than the stream thereby favoring a great increase in minnow abundance. The minnows then eat much of the available food, reducing the production of trout. After a few years, the beaver pond may become quite shallow and warm because of silting, while decomposing organic deposits increase acidity of the water. Thus the pond and its outlet are likely to deteriorate in suitability for trout. Also, good spawning areas may be smothered by deposits of silt or physical barrier to upstream migration. In the long run, the damage to the trout and habitat may far outweigh the initial benefits, and it may take years before conditions improve, even if the beaver leave the area or are removed (Pasko 1969."

http://digitalcommons.unl.edu/cgi/vi...context=ewdcc4