Micro-hydro power is practical for small farms with streams, according to an article in Small Farm Canada. All you need is about $9,000 for materials, and a creek with a fall of 5 meters per 100 meters. Micro-hydro is one of the most environmentally benign energy conversion options because it doesn not interfere significantly with river flows.
The article cites a New Zealand couple who have been able to produce 7 kW hours per day, just enough to provide power in their cottage and barn, from a small creek on their land. They estimate they can pay back the cost of the materials in fewer than five years. They say that an additional turbine–just a washing machine pump–will make them self-sufficent.
Installation included setting the intake box into the upstream bed of the creek, laying the pipe to the turbine, and cables connecting the generator to the batteries and power outlets. It took three people about three weekends.
For more information on micro-hydro power: http://microhydropower.net and www.energyalternatives.ca.