Dams and weirs

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 over 40 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.

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.