A Seattle-based company called Hydrovolts has been winning awards for its miniature turbine that you can drop into streams, canals, and pretty much any place that has a steady flow of water.
The turbine, brand named Flipwing, will be produced in three sizes. The smallest will be about the size of a 2-drawer filing cabinet and the largest will be about 2 x 2 x 6 meters. It can be installed in the water stream vertically or horizontally.
The Flipwing's blades swing open on the reverse upstream stroke, presenting their edge to the current. Thus the kinetic energy of the water flow is used almost entirely in turning the blades in the downstream direction.
Hydrovolt's website is a bit opaque about the amount of electricity the Flipwing produces. Using an average market price of electricity of $0.11 per kWh, the site says the turbine can generate, in a 2m/sec current, $1,400 worth of electricity per year. By my calculations, that means it can make 12.7 MWh of electricity annually.
Here's a video of a small Flipwing unit in operation in a stream. Hydrovolt has several more videos on its YouTube channel: