Well there’s that and also the fact that this is an in-line flow meter. You’d have to use some sort of rather complicated and hugely variable calculation to convert the flow in the pipe (which would be directly related to the velocity of the stream) to the flow in the river. I think it would be better to look for existing structures, do some bathymetry and then install a level gauge. You can either use a pressure sensor or an ultrasonic sensor for that. Then use Manning’s equation on the cross-section of the river/stream/canal to get your flow from the river level.