<P> 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). </P> <P> Sometimes a river is made entirely non-tidal by constructing a sea lock directly into the estuary . </P> <P> In more advanced river navigations, more locks are required . </P> <Ul> <Li> Where a longer cut bypasses a circuitous stretch of river, the upstream end of the cut will often be protected by a flood lock . </Li> <Li> The longer the cut, the greater the difference in river level between start and end of the cut, so that a very long cut will need additional locks along its length . At this point, the cut is, in effect, a canal . </Li> </Ul>

A strong wall build across a river to hold water is called