I've heard a few people say that if we could translate the flow of a river into angular momentum (ie create a spin/vortex using copper riffling fins) gradually over a long distance of river then we can make equivalent hydro power to a large dam: but without the engineering and environmental cost of the dam. The idea that water has the same potential energy regardless of a dam/no dam situation I can grasp. Even the idea that angular momentum can replace pressure as the force that drives a turbine with great power. But (1) could a vortex of water really spin up to such speeds? and (2) why does the fins/riffling/tubing need to be made from copper (or as some say, aluminium)? and (3) if it really works, why hasn't someone developed such a hydro-electric plant? In other words, please provide me with some de-bunking! Thanks.