Set aside any potential environmental issues for a second, if that river were to flow thru my farm land then you would have no problem with me building a dam and using the backwater to irrigate my crops?
No need to do that. Just sow a few rocks across the river, the next kayaker comes down gets a hole in his boat, or has to do portage around it anywise. Eventually, they'll figure out that the stretch is a bad one and avoid it.