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A dam is usually constructed across
a river to create a reservoir
in the valley behind by storing the water that flows into
it naturally. Sometimes, they are built across dry valleys,
or valleys with small streams, to create a storage area for
water that is transported from elsewhere.
  Streams
and rivers have to be diverted to create a dry area to construct
the dam. Small rivers and streams are usually diverted through
a tunnel, or a channel that is constructed around the side
of the dam. Soft soils and rocks are excavated to form the
route, while harder rocks have to be blasted with explosives.
Sometimes, dams are built across wide rivers
with large volumes of flowing water. It would be impracticable
and too expensive to construct a separate channel to divert
the water. Instead, a dry construction pit is formed on one
side of the river, leaving the other side open for the water
to flow through. The first portion of the dam is constructed
in the dry pit. When it is finished, another dry area is formed
on the other side of the river, and the remaining part of
the dam is built. Meanwhile, the river flows through openings
in the completed portion of the dam, and the reservoir can
start to fill behind it.
Find out about the construction
sequence for the Three Gorges Dam that is being built
across the River Yangtze in China.
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