Environmental Impacts from Weirs

A weir is any control or barrier placed in an open channel to permit measurement of water discharge. Weirs provide hydrologists and engineers a simple way of measuring the rate of flow in small to medium-sized streams, or in industrial discharge locations. Since the geometry of the weir is known, and all water flows over the weir, the depth of water behind the weir can be converted to a rate of flow.

A weir typically increases the oxygen content of the water as it passes over the crest and it can effect the local ecology of a river system. A weir will artificially reduce the upstream water velocity, which can lead to increased siltation. The weir may pose a barrier to migrating fish. For these reasons, an environmental impact assessement is usaully necessary before construction.