A weir is a small dam built across a river either to reduce the speed of the river, or to provide a mill pond. Weirs are often fitted with sluice gatess to regulate the flow, and on navigable rivers lock gates allow boats to pass.
Mill ponds provide a water mill with the power it requires, using the difference in water level above and below the weir to provide the necessary energy.
Weirs often double as convenient pedestrian crossing points for the river.
Weirs are also fences made of nets in rivers or along sea coasts used for catching fish.
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