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The gorge at Niagara Falls is 11km and, if allowed to flow naturally, would continue to retreat at about the rate of 1m a year.
Waterfalls are mainly new features on the landscape, and
they do not last long. Older ones gradually erode themselves away, while new ones form. At the moment, because we are still quite close to the end of the last Ice
Age, the world has more waterfalls than normal, many of them falling downs the sides of glacially deepened valleys. Yosemite and Niagara Falls, for example, are both less than 12,000 years old. The world’s oldest waterfalls are in Africa and South America. Some of them may be several million years old. Angel Falls, in Venezuela, is one example of an old waterfall. (See also: Fjord.)
Water life
Water is vital to life. It has a number of special properties that have made it useful to all living things: It is a liquid at normal temperatures; also, it holds heat well (it has a good thermal or heat capacity) which means it doesn’t change temperature quickly, thus helping minimize changes within, for example, the human body.
No major group of living things originated in fresh water. This
is because rivers and lakes are much harsher environments than the oceans, where temperatures are very constant. This also means that life in inland waters is much less diverse than in oceans.
In both oceans and fresh water there are three main kinds of
life: those that convert the Sun’s energy into tissue (the plants); those that use the plants as food (either directly or by eating grazing animals); and those that eat the bodies of plants and animals once
they die (decomposers). In this way the nourishment (nutrients) in the water is recycled for use by new generations.
Estuaries are places where rivers meet the sea. They are where plant nourishment becomes trapped, so they support more life than an equal volume of river or ocean water.
The great mangrove forests
of tropical estuaries and estuary marshes of cooler regions depend on this nourishment, as do the free- living plants and the sea grasses that form dense mats on the muddy estuary floor. Plankton also grow rapidly in estuaries because of the amount of trapped nutrients.
The high level of plant matter means that many fish and invertebrates (such as shrimps, clams, and crabs) can thrive.
Many estuaries are now important sites for aquaculture (for example, shrimp and salmon farming). However, the same system that traps nourishment can also trap pollution, and so only
estuaries away from settlements can be used for aquaculture.
(See also: Ocean life.)
Water power
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Water power
Any form of water that produces useful energy.
Waterwheels use water power directly, although most water power is now converted into electricity by hydroelectric power plants.
Water power is only useful
if the flow of water can be kept reasonably steady throughout the year. In many cases the only way to do this is to build dams and store water in reservoirs behind the dam. The water can then be released steadily from the reservoir. This balances out the natural flow of
the river, which would otherwise rise after a storm and fall during a drought.
 Water power—Water power can be produced by either a large volume of water falling from a small height or a smaller volume falling from a large height. This barrage (below) uses the first method.
Turbine
Barrage Lock