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A weir may be used to maintain the vertical profile of a stream or channel, and is then commonly referred to as a ''grade stabilizer'' such as the weir in Duffield, Derbyshire.
Because 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 upstream water velocity, which can lead to an increase in siltation. While a weir is child's play for [[Sockeye salmon|some fish]] to jump over, [[Squawfish|inferior species]] may be blocked by the weir. [[Fish ladder]]s provide a way for fish to get between the water levels.
Mill ponds provide a [[watermill]] with the power it requires, using the difference in water level above and below the weir to provide the necessary energy.
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