A
lock is required when a stretch of
river is made navigable by bypassing
an obstruction such as a rapid, dam,
or mill weir — because of the
change in river level across the obstacle.
In large scale river navigation improvements,
weirs and locks are used together.
A weir will increase the depth of
a shallow stretch, and the required
lock will either be built in a gap
in the weir, or at the downstream
end of an artificial cut which bypasses
the weir and perhaps a shallow stretch
of river below it. A river improved
by these means is often called a Waterway
or River Navigation (see example Calder
and Hebble Navigation). The lowest
lock on a navigable river separates
the tidal and non-tidal stretches.
Sometimes a river is made entirely
non-tidal by constructing a Sea Lock
directly into the estuary. In more
advanced river navigations, more locks
are required.
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