Weir

A weir is a barrier built across a river which changes the flow of the river, usualy for irrigation and smaller than a dam. Depending on the design, some pose problems for fish, as the fish cannot get across the weir.

Gallery

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  A weir in Mende, France
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  Many fish have the capability to jump over weirs. This shows a salmon
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  this kind of weir is difficult for fish to climb
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  Weir designed to make it easy for fish to climb