weir

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

A weir , or low-head dam is a barrier across the width of a body of water that alters the flow characteristics of water and usually results in a change in the height of the water level. Weirs are used to control the flow of water for rivers, outlets of lakes, ponds, and reservoirs, industrial discharge, and drainage control structures. 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. Weirs pose a serious danger to boaters and have been involved in several fatal drownings.