Don't trees attract beavers and don't they hinder water movement?

Yes.

Trees do attract beavers but beavers can have a very positive influence on water movement. Most Iowa streams have become incised into a deep channel and no longer use their floodplain as a natural river does. In the process the water velocity of these streams has increased and with it the erosive potential of the water. This means that the steep bare banks are more subject to undercutting and collapse.

Grade control structures such as boulder weirs or low-head dams that are less than 1 m in height can be used to slow the water and reduce its velocity. In addition, these structures raise the base flow level of the water and reduce the height of the exposed stream bank which reduces bank collapse. Beaver dams can function in the same manner as these structures. However, beavers cannot be told where to build their dams and they may locate them so that their dam impoundments back water up drainage tiles. If, however, a buffer is between 100 and 150 ft in width it would be rare that beaver impoundments would actually back up water in the tiles as far back as the crop fields. If beaver become a nuisance, landowners can get permission to have them trapped outside of the regular trapping season.