A weir is a small barrier built across a stream or river to raise the water level slightly on the upstream side; essentially a small-scale dam. Weirs allow water to pool behind them, while allowing water to flow steadily over top of the weir. Additionally, the term weir can be used to refer to the crest of a spillway on a large embankment dam.
In a weir, the surface over which the water flows is known as the crest. The flow of water that moves overtop of this crest is known as the nappe, which is simply the water that makes it overtop the weir. This nappe does not exist with dams, as dams permit no flow of water over the structure. If this nappe falls a significant distance through the air - meaning that the weir increases the elevation of the water prior to the weir - the weir is said to have free discharge. However, if water flows partially underwater as a result of little elevation increase from the weir it is said to be submerged or drowned.
Weirs differ from dams in one major way. First and foremost, in a weir the water from upstream runs over top of the weir itself, constantly overflowing. In a dam, water flows through spillways and penstocks instead of over top of the dam, which could actually cause damage and is known as overtopping.
Weirs can be built to measure water flow rate, to alter the flow of rivers, or to prevent flooding. Additionally, small-size weirs can be used in large hydropower developments as a way to mitigate potential harm that may come about as a result of dam development. Small weirs can help improve fish numbers, stabilize water levels, and stabilize effects of river alteration.
Weirs do have drawbacks, as they can increase sedimentation and pose potential migration barriers to fish. To aid in migration, fish ladders are sometimes incorporated into weirs to aid in fish passage. Sedimentation can occur as the water slows as it goes over the weir, dropping sediment as it slows. As well, as water passes over the top of the weir, the motion can alter the dissolved oxygen levels. If the water is over or under-oxygenated (called hypoxia or anoxia), this can harm the local ecosystem.
Weirs are particularly dangerous to humans - and are thus commonly called "drowning machines". Particularly, the circulating "backwash" of water behind the weir is easy to get trapped within, and if a swimmer or boater is caught within this cycle escape and rescue is incredibly difficult. Additionally, debris that can be trapped within this cycle of water can pose harm to anyone trapped behind the weir.
In the video below Grady Hillhouse discusses how weirs work. He's built a small scale model of a weir (skip ahead to 5:24 to see the scale model), as well as other versions of weirs that exist.
For further information please see the related pages below: