weir / wi(ə)r/ • n. a low dam built across a river to raise the level of water upstream or regulate its flow. ∎  an enclosure of stakes set in a stream as a trap for fish.

weir An engineered structure extending across an open water channel. It is possible to calculate the volume of water flowing over a weir from a knowledge of its particular characteristics. Weirs are designed for flow measurement in a variety of shapes but the most common are the suppressed, contracted, V-notch, trapezoid, and broadcrested types. Other weirs may be used to control river water levels or divert flow into channels.

weir river-dam; enclosure for taking fish. OE. wer, corr. to OS. werr, MLG., MHG. wer(e) (LG. wēr(e), G. wehr), f. OE. werian defend = OS. werian, OHG. werian (G. wehren), ON. verja, Goth. warjan.