Weir

A weir on the Humber River near Raymore Park in Toronto, Ontario, Canada
A weir on the Yass River, New South Wales, Australia, directly upstream from a shared pedestrian-bicycle river crossing
A weir on the Tikkurilankoski rapids in Vantaa, Finland
Time-lapse video of a new tilting weir being installed in the Caldicot and Wentloog Levels

A weir /wɪər/ or low-head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. Weirs are also used to control the flow of water for outlets of lakes, ponds, and reservoirs. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. There is no single definition as to what constitutes a weir.

Weir can also refer to the skimmer found in most in-ground swimming pools, which controls the flow of water pulled into the filtering system.