Dams and weirs

What is a dam?

A dam is a wall-like structure that’s built across a creek or a river to block the flow of water through the landscape. When it rains, water builds up behind the dam, creating an artificial lake or reservoir.

A dam allows water flow to be regulated and controlled by gates or spillways. A dam usually doesn’t allow water to go over the crest (which is the top).

At WaterNSW, we operate and manage 41 major dams across NSW.

Dams can be built from a combination of earthfill, rockfill, concrete or stone. Dams need to be strong enough to hold back the force of the water that pushes against them, which means they’re very strong!

What are the purposes of a dam?

We all need water to survive which means dams are a vital resource.

Dams supply water for:

  • irrigation
  • hydropower
  • flood mitigation
  • agriculture
  • town water supply.

What is a weir?

A weir is a small barrier built across a stream or river to control and raise the water level slightly on the upstream side; it’s like a small-scale dam. A weir allows water to flow over the crest or sometimes underneath some sections.

Weirs are usually made of concrete or steel. Some older weirs were also made from natural rock and compacted soil-fill to create a barrier to hold water.

Fact: The word ‘weir’ comes from the old English word ‘wer’ which means ‘to defend or dam’.

What are the purposes of a weir?

A weir has a very important purpose in the water supply system. This is because a weir is used to prevent flooding, measure water flow and hold water. This water can be used in:

  • towns
  • homes
  • industry
  • irrigation.

There are over 3000 weirs in NSW! One of the oldest weirs in NSW are the ancient stone fish traps made by the Ngaemba People of Brewarrina on the Barwon River.

Weir Safety

While weirs are commonly used for recreational activities like fishing or kayaking, they can be dangerous. Water coming out of the weir can cause a circulation of water known as a hydraulic jump, which can hold swimmers and canoeists under the water.


Both weirs and dams require regular maintenance to operate efficiently and safely. At WaterNSW, we continuously monitor and check our dams and weirs.

Dam maintenance is a complex process. As water naturally seeps through the dam foundation, this can create “uplift” pressure on the dam which must be able to escape so that it doesn’t cause instability.

Additionally, the area around an earthfill or rockfill dam must be kept clear of trees. This is important as when a tree dies, its roots rot and can form a “pipe” or seepage pathway. Water seeping through a pipe has the potential to lead to dam failure.

Go to top

WaterNSW acknowledges the traditional custodians of the lands and waters on which we work and pay our respects to all elders past, present and emerging. Learn more