There are two main types of hydro generation systems available in the home power market - the pelton wheel and the in stream propeller.
Pelton wheel systems are highly efficient at utilising the available resource, but usually require considerable modification to the stream, including damming and/or re routing of the water flow. Check with your local government to find out what permits may be required. Pelton wheels usually require that the stream have at least a 6 foot drop from the top of the inlet dam to the outlet water level, and produce from 40 to several thousand watts.Pelton wheel use is not practical at all sites, especially where the lay of the land is generally flat.
In stream propellers can be operated in water as shallow as 1 foot in depth, moving at least as fast as a brisk walk. They require only very minimal modification to the water source, and can often be held in place with poles without nodifying the resource at all. The only commercially available example that we know of produces 100 watts at 12 volts in water moving at 7 mph.