For those people interested in micro hydrogeneration look up Banki turbines. These can be undershot wheels and if flow volume is sufficient even low head installations will provide some power generation. The Pelton wheel or Harris turbine require greater pressure (higher head) in order to deliver their best output.
Some areas have restrictions on what flowing water is designated fish habitat. Most juristictions will not allow dams or restrictive water confinement also called impoundment or ponding on fish habitat waters. But if you have a trickling runnel of water barely enough to be called a stream and practically free falling down a cliff, embankment or cliff you could build a catchment to build up volume and release water in bursts to drive the waterturbine at the required speed to produce real power.