Hydro means water. Hydro-electric means making electricity from water power.
Hydroelectric power uses the kinetic energy of moving water to make electricity. Dams can be built to stop the flow of a river. Water behind a dam often forms a reservoir Like the picture of Shasta Dam in Northern California pictured on the right. Dams are also built across larger rivers but no reservoir is made. The river is simply sent through a hydroelectric power plant or powerhouse. You can see this in the picture of The Dalles Dam on the Columbia River along the border of Oregon and Washington State.
The world's ocean may eventually provide us with energy to power our homes and businesses. Right now, there are very few ocean energy power plants and most are fairly small. But how can we get energy from the ocean?
There are three basic ways to tap the ocean for its energy. We can use the ocean's waves, we can use the ocean's high and low tides, or we can use temperature differences in the water.