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Aqueduct
Aqueduct
A canal, tunnel, or pipe designed to carry water. Aqueducts have been in use since ancient times. Some of the largest modern aqueducts are
in California, where there is a great need to supply irrigation water
to farmers and drinking water to cities such as Los Angeles. The California Aqueduct, for example, is over 700km long.
Aquiclude
A rock layer that does not allow water to flow through it. Rocks that form aquicludes include granite, clay, and shale.
Because most rocks are in
fact aquicludes, the word is not commonly used. Instead, people use the word aquifer to describe the less common kinds of rock that allow water to flow through them.
Springs often occur at the junction of an aquifer and an aquiclude. The springs appear at the top of the aquiclude and at the bottom of the aquifer.
 Aqueduct—Aqueducts are essential for carrying water from places of surplus to places of demand. In the past stone aqueducts were often used, as shown in the picture on the left from France. Modern high- capacity aqueducts (as shown by the California Aqueduct in the lower picture) are concrete lined channels.
Aquifer
A water-bearing rock. Chalk, limestone, and sandstone are typical water-bearing rocks. If a large part of an aquifer is trapped between two layers of watertight rocks (aquicludes), the trapped water is called artesian water. Aquifers are vital sources of drinking water in most countries of the world. However, they can
be overused if the amount taken out is greater than the amount added by rainfall. The water in
the rock helps keep the rock from becoming compacted, so when too much water is removed from the aquifer, through water “mining,” the land often collapses. The land surface in the San Joaquin Valley in California has sunk several meters
as a result of water mining. Once the rock has compacted, it will never expand again, so the amount of water that can be stored is reduced. (See also: Groundwater; Spring; Well.)
Artesian water
Groundwater that comes from an aquifer trapped between
two confining watertight beds (aquicludes). The water may seep into an aquifer where the rock is exposed in hills; but because it is confined between watertight layers, it has no means of escape. As a result, the water in the artesian aquifer is often under considerable pressure.
When a well is drilled to an artesian aquifer, the water often
gushes out onto the surface, sometimes with enough force to make a fountain. For example, for a long time the fountains in London’s Trafalgar Square were run using the natural pressure of the artesian aquifer below London rather than with pumps.
Artesian pressure is reduced over time as the water flows to
the surface. As a result, bore
holes that begin flowing without assistance often have to be pumped after a few years. The world’s largest artesian basin lies under much of northern central Australia. One of the largest artesian basins in the United States is the Olgallala aquifer, which lies to the east of the Rocky Mountains, under much of the Great Plains.
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