A more thorough explanation:

Definition: A weir is a barrier built across a river or stream to control the flow of water. It can be made of concrete, stone, or wood and is designed to raise the water level upstream.

Example: The city built a weir on the river to prevent flooding during heavy rains.

Explanation: In this example, the weir is used to control the flow of water during heavy rains. By raising the water level upstream, the weir helps prevent flooding downstream.