External Landform Processes
Senior 2 · Geography and Environment
External Landform Processes
Lesson 2 of 3
18 min
After heavy rain the Nyabarongo and the Sebeya turn a thick brown. That colour is soil, washed off hillsides and carried downstream. It is lost to farmers, and it fills reservoirs and silts up rivers. This is erosion at work, and Rwanda, with steep slopes and two rainy seasons, feels it more than most countries.
Erosion is the wearing away of the land surface and the removal of the loosened material by moving agents: running water, wind, ice and sea waves. Erosion is always followed by transport (carrying the material) and deposition (dropping it when the agent slows down).
| Agent | Where it matters | Typical landforms |
|---|---|---|
| Running water (rain and rivers) | Everywhere with rainfall; the main agent in Rwanda | V-shaped valleys, gullies, waterfalls, floodplains, deltas |
| Wind | Deserts and dry, bare land | Sand dunes, rock pedestals, dust storms |
| Ice (glaciers) | High mountains and polar regions (in Africa only on Rwenzori, Kilimanjaro and Mount Kenya) | U-shaped valleys, cirques, moraines |
| Waves | Sea coasts and large lake shores | Cliffs, bays, headlands, beaches |
Explain why the Sebeya River in Rubavu District is brown after a heavy storm but clear in the dry season.
The Sebeya burst its banks during the storms of early May 2023, one of the events that killed more than 130 people in Rwanda's Western and Northern provinces.
Tip
Memory hook. For river erosion, remember HAAS: Hydraulic action, Abrasion, Attrition, Solution. And the sequence is always erode, transport, deposit.
Solution: running water, wind, ice and waves.
Solution: running water, because Rwanda has two rainy seasons and steep slopes, so runoff is fast.
Solution: attrition.
Solution: any two of: cutting forests, overgrazing, ploughing up and down slopes, mining, road building.
Summary
Sources and credits
The agents table, worked example and practice items are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
