The fall and flow of water is part of a continuous natural cycle. The sun draws moisture up from the oceans and rivers, and the moisture then condenses into clouds in the atmosphere. The moisture falls as rain or snow, replenishing the oceans and rivers.
Gravity drives the water, moving it from high ground to low ground.
The force of moving water can be extremely great. Hydropower is called a renewable energy source because it is replenished by snow and rainfall. As long as the rain falls, we won't run out of this energy source. History of Hydropower Hydropower has been used for centuries. The Greeks used water wheels to grind wheat into flour more than 2000 years ago.