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  The
construction of a dam across a river forms a reservoir
that raises the water level upstream,
stores the water, and slows down its rate of flow. This improves
the navigation conditions upstream of the dam for ships and
boats. Dangerous areas of rocks and sandbanks, previously
in shallow water, become well covered, and rapids in the river
disappear. Also, water from the reservoir can be released
into the river downstream
during the drier seasons of the year to make sure that it
is deep enough for navigation all the year round.
A dam forms a barrier to any
shipping, so navigation locks (similar to the canal locks
in Britain) can be constructed to one side of the dam, or
special lifts can be built into the dam for raising and lowering
vessels.
Model of the Three Gorges Dam with ship lift and ship lock
A ship lift and a navigation
lock are being provided in the Three
Gorges Dam which is currently being constructed across
the Yangtze River in China. The ship lock will have two channels
each 1.6 kilometres long. There will be five steps in each
channel to raise and lower vessels over a maximum height of
113 metres.
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