Renewable energy

The oldest machines for capturing the energy of moving water were waterwheels. Back in the day, the oldest machines for capturing the energy of moving water were waterwheels. In the days before electricity, it was common to use water wheels to provide the power for mills that ground grain or cut lumber. To start the mill, the miller simply opened a gate to let the water flow over the top of the wheel. The water wheel was connected to a big millstone or metal saw blade through a system of gears. Water for the wheel usually came from a small dam and reservoir, called the millpond.
It starts off as rain or snow falling on the highlands and mountains. Running water forms streams, which gather to form large rivers. Most rivers find their way to the edges of the continents, where they dump massive loads of fresh water and is sent into the oceans. Evaporation from the surface of rivers, lakes, and oceans brings the water back into the atmosphere as invisible water vapor. Under the right conditions, unseen water vapor condenses from the air to form clouds and possibly rain, snow, or hail. Seasonal rain and snowfalls bring fresh water back to the headwaters of streams, completing a system called the hydrologic cycle. By bringing fresh supplies of water to the highlands, the hydrologic cycle ensures that we always have energy available from flowing water.

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