The British Dam Society
Caring for dams, people and the environment

About Dams

Back to buildingBuilding Dams - Diverting the river

A dam is usually constructed across a river to create a reservoir in the valley behind by storing the water that flows into it naturally. Sometimes, they are built across dry valleys, or valleys with small streams, to create a storage area for water that is transported from elsewhere.

Diversion tunnel

The diversion tunnel excavated through soft
rock for the stream at Baoshan 2 Reservior
in Tiwan

Streams and rivers have to be diverted to create a dry area to construct the dam. Small rivers and streams are usually diverted through a tunnel, or a channel that is constructed around the side of the dam. Soft soils and rocks are excavated to form the route, while harder rocks have to be blasted with explosives.

Sometimes, dams are built across wide rivers with large volumes of flowing water. It would be impracticable and too expensive to construct a separate channel to divert the water. Instead, a dry construction pit is formed on one side of the river, leaving the other side open for the water to flow through. The first portion of the dam is constructed in the dry pit. When it is finished, another dry area is formed on the other side of the river, and the remaining part of the dam is built. Meanwhile, the river flows through openings in the completed portion of the dam, and the reservoir can start to fill behind it.

Find out about the construction sequence for the Three Gorges Dam that is being built across the River Yangtze in China.