what is a weir

A weir is a small barrier built across a river or stream that raises the water level upstream and controls the flow of water over its top, typically by allowing water to flow over the crest and pass downstream at a controlled rate. It is used for purposes such as measuring discharge, reducing flood risk, maintaining navigable water levels, or creating a small upstream pond or head for various downstream uses. In some contexts, the term “weir” is also used for the overflow structures on large dams or for skimmers in swimming pools, though these are related concepts in engineering that share the basic principle of controlling water height and flow.