Weir
A weir is a barrier built across a river which changes the flow of the river. Depending on the design, some pose problems for fish, as the fish cannot get across the weir.
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 Many fish have the capability to jump over weirs. This shows a salmon Many fish have the capability to jump over weirs. This shows a salmon
 this kind of weir is difficult for fish to climb this kind of weir is difficult for fish to climb
 Weir designed to make it easy for fish to climb Weir designed to make it easy for fish to climb
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