weir
WEER
A low dam built across a river to raise the water level, regulate flow, or divert water into a channel. Unlike a dam, a weir is designed to be overtopped — water flows over it continuously. Weirs create a smooth, glassy lip of water that spills into turbulence below. They are ancient structures — some of the oldest human modifications of rivers.
Etymology
Old English wer, a dam or enclosure for catching fish. The word is as old as the practice of managing rivers.
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