As the sun evaporates water from the oceans and drops it back onto the earth, gravity takes it back to the ocean through streams and rivers. We have dammed some of these rivers, and this water does produce some of the world's energy. This is a renewable resource, but there are ecological considerations as well. We have also already put dams across many of the sites that are the most promising. China is currently tackling and enormous engineering project with the worlds biggest dam across one of the worlds biggest rivers, primarily for power.

Hydro power is currently the world's largest renewable source of electricity, accounting for 6% of worldwide energy supply or about 15% of the world's electricity. In Canada, hydroelectric power is abundant and supplies 60% of our electrical needs. Traditionally thought of as a cheap and clean source of electricity, most large hydro-electric schemes being planned today are coming up against a great deal of opposition from environmental groups and native people.