4.6 Multipurpose Dam Projects

---

What Is a Multipurpose Dam?

A dam is a large wall or barrier built across a river to hold back water. The water that builds up behind the dam forms an artificial lake called a reservoir.

A multipurpose dam is called "multipurpose" because it is designed to do several different jobs at once — not just one. Instead of building a dam only for electricity, for example, the same dam might also supply drinking water, control flooding, and support farming.

Well-known examples of multipurpose dams include:

• Three Gorges Dam — China (the world's largest dam by power output)
• Aswan High Dam — Egypt, on the River Nile
• Hoover Dam — USA, on the Colorado River
• Kariba Dam — on the Zambezi River, between Zambia and Zimbabwe

---

The Purposes (Uses) of a Multipurpose Dam

A multipurpose dam can serve many of these functions at the same time:

• Hydroelectric power (HEP): Water flowing through the dam turns turbines to generate electricity. This is a major use of most large dams.
• Irrigation: Water from the reservoir is channelled onto farmland to grow crops, especially in dry regions.
• Flood control: The dam holds back floodwater during heavy rain, stopping it from destroying towns and villages downstream.
• Water supply: The reservoir stores clean freshwater that can be piped to homes, factories, and cities.
• Navigation: The reservoir and controlled river levels allow boats and ships to travel more easily.
• Recreation and tourism: Reservoirs can be used for fishing, sailing, and tourism, bringing money to the local area.

---

Physical (Natural) Factors

• Narrow river valley with hard rock walls: The dam needs solid ground on either side to hold the weight of the water. A narrow gorge (a steep-sided valley) is ideal because less material is needed to build the dam wall.
• Hard, impermeable rock beneath: The ground under and around the dam must not let water leak through. Impermeable means water cannot pass through it easily.
• A large, deep valley behind the dam site: This allows a big reservoir to form. The larger the reservoir, the more water can be stored.
• High and reliable river flow: The river must carry enough water throughout the year, not just in one season. This makes it more reliable for generating electricity and supplying water.
• Low risk of earthquakes: Dams hold back enormous amounts of water. If an earthquake damages the dam, it could cause catastrophic flooding. Earthquake-prone areas are risky sites.
• Low risk of landslides: Unstable hillsides near the reservoir can collapse into the water, sending huge waves over the dam.

Human (Social and Economic) Factors

• Few people living in the flood zone: When a reservoir fills up, it submerges (covers with water) the land and villages behind the dam. Fewer people in that area means fewer people need to be moved.
• Low ecological value of the flooded land: If the land to be flooded has rare wildlife or important habitats, this makes the project more controversial.
• Proximity to where electricity is needed: If the dam is far from cities, it is expensive to build power lines to carry electricity there.
• Access for construction: Roads and transport links are needed to bring in heavy machinery and materials.