I follow on YouTube the channel of Thang010146, a Chinese mechanical engineer with thousands of short animations explaining mechanisms of all sorts. One video about flap-based propulsion made me wonder if you could use this system in reverse to capture energy from a river.

The main differences are several hydrofoils per vertical slide and a bottom plate to protect them in case of a low water level, allowing the system to work even if the river is nearly dry. There is some similarities to the floating boat mills used from the middle ages to the end of the industrial revolution.