Not only that, but a lot of these dams are not even in use any more, so doing additional work doesn't make much sense. Remove the dam, remove the problem, instead of duct tape solutions.
replyFisherman like them because fish bunch up beneath them. Despite the overall populations being higher once all the fans are gone.
replyThere is an aspect of property value as well - dams maintain a certain river level and removal may impact how nice people's river houses are. (Source: Dam near me partly removed, turned into a kayak course.)