Weir

A weir is a barrier built across the width of a river or channel that alters water flow and raises the water level upstream, water spilling freely over its crest rather than flowing through it, which is the technical distinction civil engineers draw between a weir and a dam. Weirs range from low structures a few centimeters high used to measure stream discharge with a precisely shaped notch, such as the V-notch weir, to broad-crested structures many meters tall and hundreds of meters long used to control river levels, divert water to mills, or prevent flooding. The word descends from the Old English wer, related to a verb meaning to defend or dam, reflecting its long use in water management before it became a subject of formal hydraulic engineering.