We can. One presumes it is less efficient than just removing the dam. It is more work versus less work - fish ladders and the like.

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jaggederest 42 minutes ago | parent [-]

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.

	▲	RajT88 2 minutes ago \| parent [-]
		Fisherman like them because fish bunch up beneath them. Despite the overall populations being higher once all the fans are gone. There 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.)