Electricity produced through the use of moving water to turn turbines and drive generators. On a small scale hep stations can be placed to take advantage of natural situations such as waterfalls, but usually they are built on a large scale by damming suitable valleys to create a reservoir. The water in the reservoir thus gains potential gravitational energy. When allowed to fall through the dam to the turbines this energy is converted to kinetic energy which drives the turbines. Require huge initial capital outlay but then produce highly efficient and cheap electricity. Also criticized for flooding farming land and villages thus displacing people and changing the environment and the river regime downstream of the dam. Nevertheless, a non-polluting and renewable form of energy. Provides a small percentage of world energy but highly important in some countries having ideal geological, meteorological and political conditions e.g. Norway and sweden.