IB Geography A.4.2 Dam Building for Multipurpose Water Schemes Notes

Multipurpose Dams Redistribute Water, Energy, Risk and Sediment

A Reservoir Changes the Timing and Direction of River Flows

1. Impoundment: a dam slows river water and stores part of the wet-season discharge in a reservoir.
2. Regulation: operators release water according to electricity demand, irrigation schedules, flood forecasts and minimum downstream flow rules.
3. Hydropower: stored water gains gravitational potential energy, which turns turbines as it falls through the dam.
4. Flood control: empty storage space can capture part of an incoming flood peak, although a full reservoir or an extreme event reduces this protection.
5. Water supply: dry-season releases can support cities and farms, but evaporation and competing demands reduce the volume available.
6. Navigation: a deeper and more stable channel can carry larger vessels, while locks move them around the height difference created by the dam.

Benefits Depend on Storage Capacity, Operating Rules and Location

1. Reliable electricity: reservoir storage allows generation to continue beyond the moment when rain falls, so output can respond to changes in demand.
2. Irrigation and urban supply: controlled releases extend water availability into dry periods and can reduce exposure to short droughts.
3. Reduced flood peaks: temporary storage lowers downstream discharge when releases remain below the natural peak.
4. Economic connection: improved navigation and roads across a dam can reduce transport costs and link inland producers to markets.
5. Recreation and fisheries: reservoirs may create new activities, although these gains do not automatically replace river fisheries or flooded farmland.

Costs Spread Upstream, Downstream and Across Generations

1. Displacement: reservoir filling floods homes, farmland and cultural sites, so compensation must replace livelihoods as well as buildings.
2. Sediment trapping: slower reservoir water deposits its load, which reduces storage capacity and cuts the sediment reaching downstream floodplains and deltas.
   1. Less downstream sediment can reduce soil renewal, increase channel erosion and leave a delta more exposed to subsidence and sea-level rise.
3. Ecological fragmentation: a dam blocks fish migration and changes the seasonal flood pulse that supports wetlands, spawning and floodplain farming.
4. Water quality: deep reservoirs can stratify, so cold or oxygen-poor releases alter downstream habitats.
5. Greenhouse gases: decaying flooded vegetation can release methane, especially from warm, nutrient-rich reservoirs.
6. Risk concentration: a major scheme creates dependence on one structure, while landslides, extreme inflows, poor maintenance or conflict can disrupt several services at once.

Yangtze Basin Expansion Turns Individual Dams Into One Managed Cascade

A Dam Should Be Judged by Distribution as Well as Total Output

1. Counterfactual: compare the dam with realistic alternatives such as demand management, smaller reservoirs, solar or wind power and floodplain restoration.
2. Stakeholders: separate benefits to electricity users, irrigators and shipping firms from costs borne by displaced households, downstream farmers and ecosystems.
3. Time: construction benefits may appear quickly while sediment loss, maintenance costs and reservoir filling develop over decades.
4. Operating rules: the same structure can produce different outcomes when flood-storage targets, environmental flows and compensation rules change.
5. Judgement: a high national benefit does not prove equitable management when people who lose land or downstream resources receive little of the gain.