World Multipurpose River Projects
Senior 6 · Geography
World Multipurpose River Projects
Lesson 2 of 4
40 min
On 9 September 2025 Ethiopia officially inaugurated the Grand Ethiopian Renaissance Dam (GERD) on the Blue Nile, close to the border with Sudan. Building began on 2 April 2011. With a capacity of 5,150 MW it is Africa's largest hydroelectric scheme, and its reservoir can hold 74 billion m³ of water (Ethiopian Electric Power; Webuild, 2025). Ethiopia sees it as the key to electricity for millions of homes and to power exports to its neighbours.
Downstream, Egypt and Sudan objected. Egypt depends on the Nile for almost all its water and feared that filling the reservoir, especially in dry years, would cut its supply; Sudan worried about the safety and operation of its own dams, although it also expected benefits from a more even flow. Years of talks produced no binding agreement before the inauguration. The same basin shows the wider argument: the Nile Basin Cooperative Framework Agreement, ratified by Burundi, Ethiopia, Rwanda (in 2013), Tanzania and Uganda and acceded to by South Sudan, entered into force on 13 October 2024, but Egypt and Sudan reject it (Nile Basin Initiative, 2024).
GERD shows one kind of problem, political. This lesson surveys them all, and the ways they can be reduced.
Definition
Sedimentation (siltation): the settling of silt and sand in a reservoir when the river slows down, which gradually reduces its storage capacity.
Riparian state: a country through which a river flows or which borders it. Upstream riparians (such as Ethiopia and Rwanda on the Nile) and downstream riparians (Egypt and Sudan) may want different things.
Resettlement: moving people out of the area to be flooded by a reservoir to new homes elsewhere.
Waterlogging and salinisation: the rise of the water table under irrigated land, and the build-up of salts in the soil when that water evaporates.
Figure 1. Effects of a dam from source to sea (not to scale). Drawn by Ganzaa.
| Letter | Effect | Example |
|---|---|---|
| A | The river slows in the reservoir and drops its load; silt builds up and storage shrinks | Sanmenxia Dam on the Huang He silted up within a few years of completion |
| B | Farmland, forest, homes and heritage sites are drowned; people must be resettled | About 57,000 Gwembe Tonga people moved for Lake Kariba |
| C | The dam blocks fish that migrate up and down the river | Fish catches fell below many large dams |
| D | Water leaving the dam carries little load, so it erodes the bed and banks below | Scour of the Nile bed below Aswan |
| E | Floodplain farms no longer receive fertile silt each year, so farmers buy fertiliser | The Nile valley in Egypt |
| F | Too little sediment reaches the coast, so the delta and beaches retreat and salt water moves inland | The Nile delta; the coast at Keta, Ghana, below the Akosombo Dam |
| Type | Problem | Possible solutions |
|---|---|---|
| Physical | Sedimentation of reservoirs | Soil conservation, terracing and tree planting in the catchment; check dams; flushing sediment through bottom outlets |
| High evaporation from large, shallow lakes in hot climates | Choosing deep, narrow sites; storing water underground or upstream in cooler highlands | |
| Drought and falling lake levels, which cut power output | Sharing water by agreed rules; mixing hydro with solar, wind and geothermal power | |
| Water weeds such as water hyacinth and Salvinia | Mechanical removal, biological control with weevils, cutting nutrient pollution | |
| Environmental | Loss of land, forest and wildlife habitat | Environmental impact assessment before building; rescuing wildlife (Operation Noah at Kariba, 1958–1964); creating protected areas |
| Water-borne diseases such as bilharzia and malaria spread along canals and lake shores | Lining canals, clearing vegetation, treating patients, providing clean water and latrines | |
| Downstream erosion, loss of fertile silt, delta retreat | Releasing managed floods; coastal defences; fertiliser support for farmers | |
| Social | Displacement of people and loss of cultural heritage | Fair compensation, consultation, good land and services in new settlements; moving monuments (Abu Simbel) |
| Unfair sharing of benefits: people near the dam may get no electricity | Local electrification, jobs and fishing rights for affected communities | |
| Economic | Very high cost, long construction, debt and dependence on foreign expertise | Joint financing by several countries (Rusumo), loans from development banks, training local engineers |
| Expensive maintenance, such as Kariba's eroded plunge pool and worn spillway gates | Regular inspection and international repair programmes | |
| Political | Disputes between upstream and downstream states | Basin-wide agreements and commissions (Nile Basin Initiative), data sharing and agreed rules for filling and drought |
Worked example 1: how long will a reservoir last?
Problem. An illustrative reservoir holds 3,000 million m³. The river brings 25 million m³ of sediment a year, and 80% of it is trapped behind the dam. (a) How many years until half the storage is lost? (b) A soil-conservation programme in the catchment cuts the sediment brought by the river by 40%. How many years until half the storage is lost now?
Solution.
Interpretation. Protecting the catchment adds 50 years to the useful life of the reservoir. Terraces, trees and grass strips on the hills upstream are therefore part of the project, not an extra.
Worked example 2: water lost to the sky
Problem. An illustrative reservoir in a hot, dry region covers 2,000 km² and loses 2.2 m of water a year to evaporation. Calculate the yearly loss in cubic kilometres.
Solution.
Interpretation. That is 4.4 billion m³ a year that can never be used for irrigation or drinking. It is one reason why Ethiopia argues that storing Nile water in its cooler highlands wastes less than storing it in the hot deserts of Egypt and Sudan, and why Egypt and Sudan reply that the timing of releases still matters.
Warning
Common mistakes
Al Jazeera English, Start Here: "What's behind the Egypt-Ethiopia Nile dispute?" (January 2020). As you watch, list the arguments of each side. The video was made before the reservoir was filled and the dam inaugurated in 2025, so check which concerns have since been tested.
Practice
Summary
Sources and credits
Photo: The Kariba Dam wall from the middle (JonGT, CC BY-SA 4.0), via Wikimedia Commons.
Video: "What's behind the Egypt-Ethiopia Nile dispute?", Start Here, Al Jazeera English, embedded from YouTube.
Figure 1, the worked examples and the exercises were drawn and written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
