Multipurpose Dam Projects » Environmental Impacts of Dams

What you'll learn this session

Study time: 30 minutes

The purpose and functions of multipurpose dam projects
Physical environmental impacts of dams (upstream and downstream)
Biological impacts on ecosystems and biodiversity
Social and economic impacts on local communities
Case studies of major dam projects and their environmental consequences
Mitigation strategies for reducing negative impacts

Introduction to Multipurpose Dam Projects

Dams are massive structures built across rivers to control water flow and serve multiple purposes. While they provide many benefits like hydroelectric power, irrigation and flood control, they also cause significant changes to the environment. Understanding these impacts is crucial for sustainable water resource management.

Key Definitions:

Multipurpose dam: A dam designed to serve several functions such as power generation, irrigation, flood control and water supply.
Reservoir: The artificial lake formed behind a dam.
Hydroelectric power: Electricity generated by the energy of flowing water.
Sediment load: The soil and rock particles carried by a river.
Riparian zone: The area of land alongside rivers and streams.

💡 Functions of Multipurpose Dams

Dams serve multiple functions that benefit human societies:

Generate hydroelectric power
Store water for irrigation
Control flooding
Provide drinking water
Create recreational opportunities
Improve navigation

⚠ Why Environmental Impacts Matter

Understanding dam impacts is important because:

Dams alter natural river systems that have evolved over millions of years
Changes affect both human communities and wildlife
Impacts can extend hundreds of kilometres upstream and downstream
Some effects are immediate while others develop over decades
Many impacts are irreversible or difficult to mitigate

Physical Environmental Impacts

Dams dramatically alter the physical environment by changing water flow patterns, sediment transport and river channel characteristics. These changes occur both upstream (in the reservoir) and downstream of the dam.

Upstream Physical Impacts

When a dam is built, the area behind it transforms from a flowing river to a standing reservoir, creating these physical changes:

Flooding of land: Large areas of land are permanently submerged, including forests, farmland and sometimes settlements.
Sediment trapping: The reservoir traps 90-95% of sediments that would normally flow downstream, causing the reservoir to gradually fill up.
Water temperature changes: Reservoirs develop temperature layers (stratification), with warm water at the surface and cold water at the bottom.
Microclimate alterations: The large water body can increase local humidity and change temperature patterns in the surrounding area.
Increased evaporation: The expanded water surface area leads to greater water loss through evaporation.
Potential for earthquakes: The weight of the reservoir can trigger small earthquakes in some geological settings (reservoir-induced seismicity).

Downstream Physical Impacts

Below the dam, the river environment changes dramatically due to altered water flow patterns:

Altered flow regime: Natural seasonal variations in river flow are replaced by regulated releases based on human needs.
Reduced sediment load: With sediments trapped in the reservoir, downstream water is "sediment-hungry" and erodes river banks and beds.
Channel incision: The riverbed can cut deeper without new sediment deposits, lowering the water table in surrounding areas.
Reduced flooding: Natural flood cycles that would deposit nutrients on floodplains are prevented.
Water temperature changes: If water is released from the bottom of the reservoir, downstream water is often colder than normal.
Water quality alterations: Oxygen levels, nutrient content and other chemical properties of the water change.

Biological Impacts on Ecosystems

The physical changes caused by dams create ripple effects throughout aquatic and terrestrial ecosystems, affecting plants, animals and entire ecological communities.

🌿 Aquatic Ecosystem Impacts

Fish migration blocked
Spawning grounds flooded
Changed water temperatures affect breeding
Altered food webs
Invasive species may thrive in reservoirs

🌲 Terrestrial Ecosystem Impacts

Habitat loss from flooding
Wildlife displacement
Forest fragmentation
Riparian zone destruction
Changed plant communities along rivers

🌊 Estuary and Coastal Impacts

Reduced nutrient flow to deltas
Coastal erosion increases
Saltwater intrusion upstream
Decreased productivity in coastal fisheries
Altered wetland ecosystems

Biodiversity Loss

Dams are a leading cause of freshwater biodiversity decline worldwide. Some key impacts include:

Fish population collapse: Migratory fish like salmon cannot reach breeding grounds, causing population crashes.
Habitat fragmentation: River ecosystems become disconnected, isolating populations.
Endemic species loss: Species found only in specific river sections may become extinct when their habitat is altered.
Riparian forest decline: Forests along rivers depend on natural flooding cycles that are disrupted by dams.
Wetland drainage: Downstream wetlands may dry up without regular flooding.

For example, after the construction of the Tucuruí Dam in Brazil, at least 11 fish species disappeared from the reservoir area and downstream fisheries collapsed by over 60%.

Case Study Focus: Three Gorges Dam, China

The Three Gorges Dam on the Yangtze River is the world's largest hydroelectric project. Completed in 2012, it demonstrates many typical environmental impacts:

Flooded 632 km² of land, displacing 1.3 million people
Submerged over 1,300 archaeological sites
Threatens the endangered Yangtze River dolphin and Chinese sturgeon
Traps approximately 40 million tonnes of sediment annually
Has been linked to landslides along reservoir banks
Altered water quality, with increased algal blooms
Changed the flow regime of the Yangtze River for 600 km downstream

While generating clean electricity (equivalent to burning 50 million tonnes of coal annually), the environmental costs have been substantial and some were underestimated during planning.

Social and Economic Impacts

Dam projects don't just affect the natural environment they transform human communities and livelihoods, particularly for those living near the river.

Community Displacement

One of the most direct human impacts is the forced relocation of communities from reservoir areas:

The World Commission on Dams estimates 40-80 million people have been displaced by dams worldwide
Relocation often leads to loss of livelihoods, especially for farming and fishing communities
Cultural heritage sites and ancestral lands may be submerged
Indigenous communities are disproportionately affected
Compensation is often inadequate for true economic and cultural losses

Changes to Livelihoods

Dam projects transform traditional economies both positively and negatively:

👍 Potential Benefits

Improved irrigation for agriculture
Flood protection for downstream communities
Electricity access for rural areas
New fishing opportunities in reservoirs
Tourism and recreation development

👎 Potential Harms

Loss of floodplain agriculture
Decline in river fisheries
Reduced access to river transport
Changes in water quality affecting health
Increased waterborne diseases in reservoirs

Case Study Focus: Aswan High Dam, Egypt

Built in the 1960s on the Nile River, the Aswan High Dam illustrates the complex trade-offs of dam projects:

Benefits: Prevented catastrophic floods, expanded irrigated agriculture, generated hydropower and created Lake Nasser for fishing
Environmental costs: Trapped nutrient-rich sediment that previously fertilized the Nile Delta, causing coastal erosion and declining soil fertility
Social impacts: Displaced over 100,000 Nubian people, submerged archaeological treasures (some were rescued) and increased schistosomiasis (a parasitic disease) in irrigation canals
Long-term issues: The reservoir is gradually filling with sediment, reducing its storage capacity and lifespan

The Aswan Dam shows how benefits and costs are often unevenly distributed, with farmers gaining irrigation while fishing communities in the Nile Delta suffered from reduced nutrient flow.

Mitigation Strategies

Modern dam projects increasingly incorporate measures to reduce environmental impacts. These include:

Environmental flow releases: Scheduled water releases that mimic natural flow patterns to support downstream ecosystems
Fish ladders and passages: Structures that help fish bypass dams during migration
Sediment flushing: Techniques to periodically move sediment past the dam
Reservoir drawdowns: Temporarily lowering reservoir levels to allow sediment movement
Habitat restoration: Creating or enhancing habitats for affected species
Improved resettlement programs: Better planning and compensation for displaced communities
Comprehensive impact assessments: More thorough evaluation of potential impacts before construction
Alternative designs: Run-of-river dams that allow more natural water flow

While these measures can reduce impacts, they cannot eliminate all environmental consequences. The decision to build a dam must carefully weigh benefits against environmental and social costs.

Summary: Balancing Benefits and Impacts

Multipurpose dam projects present complex trade-offs between development needs and environmental protection. Key points to remember:

Dams provide crucial benefits including clean energy, irrigation and flood control
Environmental impacts occur both upstream and downstream, affecting physical, biological and social systems
Some impacts are immediate while others develop over decades
Mitigation measures can reduce but not eliminate environmental consequences
Each dam project has unique impacts based on its location, size and design
Careful planning and impact assessment are essential before construction
Alternative energy and water management solutions should always be considered

As you continue studying environmental management, consider how the principles of sustainable development can be applied to water resource projects like dams to minimize harm while meeting human needs.

🧠 Test Your Knowledge!