None. The dams retain enough water to aquify the surrounding land but do not completely restrict the flow. The beavers thin out the older trees making new growt

None. The dams retain enough water to aquify the surrounding land but do not completely restrict the flow. The beavers thin out the older trees making new growth vibrant. The only downside I can imagine is having large swathes of property re-classified as wetlands, so that they fall under the purview of an increasingly militant and militarized EPA.