A weir is a small overflow type dam commonly used to raise the level of a small river or stream. Weirs have traditionally been used to create mill ponds. Water flows over the top of a weir, although some weirs have sluice gates which release water at a level below the top of the weir. The crest of an overflow spillway on a large dam is often called a weir.
Weirs also give hydrologists a simple method of measuring flow rate in small to medium sized streams. Since the geometry of the top of the weir is known, and all water flows over the weir, the height of water flowing over the weir will be an indication of the flow.
There are different types of weir, they may be a simple metal plate with a V notch cut into it or it may be a concrete and steel structure across the bed of a river. A v-notch weir will give a more accurate indication of low flow rates.
While a weir will typically increase the oxygen content of the water as it passes over the crest, a weir can have a detrimental effect on the local ecology of a river system. A weir will artificially reduce the up-stream water velocity which can lead to an increase in siltation. The weir may pose a barrier to migrating fish.
Mill ponds provide a water mill with the power it requires, using the difference in water level above and below the weir to provide the necessary energy.
Weirs often double as convenient pedestrian crossing points for the river.