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The bridge and weir mechanism at Sturminster Newton on the River Stour, Dorset.

A weir is a small overflow-type dam commonly used to raise the level of a river or stream. Weirs have traditionally been used to create mill ponds in such places. 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.

The weir at Coburg lake in Victoria (Australia) after heavy rainfall.

Weirs are used in conjunction with locks to render a river navigable. In this case, the weir is made significantly longer than the width of the river by forming it in a 'U' shape or running it diagonally. This is done in order to minimize fluctuation in the depth of the river upstream with changes in the flow rate of the river. Doing so avoids unnecessary complication in designing and using the lock.

Weirs also give hydrologists and engineers a simple method of measuring the rate of fluid flow 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 depth of water flowing over the weir will be an indication of the flow. There are different types of weir. It 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 upstream water velocity which can lead to an increase in siltation. The weir may pose a barrier to migrating fish. Fish ladders provide a way for fish to get between the water levels.

A weir in Warkworth, New Zealand

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.

A manually operated needle dam type weir near Revin on the Meuse River, France

A walkway over the weir is likely to be useful for the removal of floating debris trapped by the weir or for working staunches and sluices on it as the rate of flow changes. This is sometimes used as a convenient pedestrian crossing point for the river.

Types of weir:

• Sharp Crest Weir
• Broad Crest Weir
• Cramp Weir
• Needle dam

A weir is a low dam built across a stream to divert its water or to control its flow, or a fence-like trap put on a stream for catching fish.

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Category:Weirs