Water power is a category that encompasses several methods of generating electricity from the movement of water. Hydroelectric power from dams is the most important method, but other sources include tides, waves, and miscellaneous less common sources. Water power is sustainable because it comes ultimately from the sun's energy driving the water cycle of rain and evaporation.
Before electricity, water wheels were used in Medieval times and before to power grinding mills and simple machines such as saws. A water wheel affixed with buckets will also lift water into a plumbing or irrigation system. Although the Roman aqueduct systems transported water over very long distances, mills that relied on water generally had to be located at or near a river.
Hydroelectric power comes from the energy of falling water turning a turbine, and thus a generator. Hydroelectric power is almost always produced by a dam and reservoir on a river. The reservoir raises the level of the river (increasing available power) and evens out annual flow. Gates and spillways in the dam regulate water outflow to meet electrical demands.
Although dams are built in part to control flooding and manage water supplies, the world's major dams – Grand Coolee on the Columbia, Three-Gorges on the Yangtze, and many others – all supply critical amounts of electricity. Hydroelectric power supplies about 20% of the world's energy.
On the downside, power from a dammed-up river is the antithesis of free-flowing falls. The energy discharged in a glorious waterfall is not being collected by humanity. River dams disrupt the natural flow of the river.
Tidal power is a much more modestly important source of energy. Oceanic tides follow an approximate 12 hour cycle based on the apparent movement of the moon around the earth. Strong tides are most promising for extracting power from places such as long thin bays and narrow straits between large bodies of water.
Tidal power is extracted most commonly by immersing water turbines in the tidal current to drive generators. This is conceptually much like using wind turbines, except that the much denser water produces more energy, and tides are more predictable than wind.
A much less common approach to exploiting tidal energy takes advantage of the change in ocean level from low to high tide. The incoming tide is allowed to fill a basin, either a natural lagoon that is dammed off or an artificial tidal basin constructed for the purpose. The basin drains again as the tide ebbs. Turbines drive generators during the filling, the emptying, or both.
Wave action can also generate power. This is mainly a theoretical source, being used in few locations on a small scale. A buoy on the ocean surface will bob up and down with the waves. This vertical motion can drive a generator. Wave motion is far from an ideal source, however. It is slow and irregular, usually small in amplitude, and diffused over a range of directions.
Although the various water power technologies are sustainable and nonpolluting, they are not without environmental impact. Hydroelectric dams in particular are big, expensive projects that change the landscape forever. The reservoir behind the dam floods large amounts of land. The building of the Aswan Dam on the Nile, for example, threatened archaeological sites that had to be moved.
Dams and turbines interrupt water flow, which will deposit suspended sediment in new locations. Thus, erosion and deposition patterns will change. River deltas may shrink when not renewed from upstream silt.
Turbines in the water flow can also kill fish and marine mammals. Larger mammals such as dolphins can be screened out. Also, water turbines turn more slowly than wind turbines. The actual mortality rate among fish is significant, but many make it through the system. Dams are also known to interfere with fish life cycles. Most famously, dams on the west coast of North America block salmon migration. This has been mitigated with water ramps that permit fish movement around the dams.