weir -
A fence placed across a river to catch fish.
Wikipedia
A weir, also called a lowhead dam, is a small overflow dam used to raise the level of a river or stream. Weirs have been used to create mill ponds in such places. Water flows over the top of a weir, although some weirs have sluice gates releasing water at a level below the top of the weir.
OmegaWiki Dictionary
Ω
weir An adjustable dam placed across a river to regulate the flow of water downstream.
