Formation of Relief Features in Rwanda

Lessons

Senior 4 · Geography

Formation of Relief Features in Rwanda

5 lessons

Lesson 5 of 5

Rivers and Lake Shores, and What Relief Means for Rwanda

25 min

Rivers that carry the hills away

Look at almost any river in Rwanda after rain: it is brown. The colour is soil. REMA reports research showing that the Nyabarongo carried 51 kg of soil every second at Nyabarongo–Kigali, 44 kg/s at Kanzenze and 26 kg/s on the Akagera at Rusumo (REMA, 2009, citing Government of Rwanda, 2008). Rivers are both carving Rwanda's valleys and filling them in.

Worked example 1: a river's load in a year

Problem. If the Nyabarongo at Kigali carried 51 kg of soil per second all year, how many tonnes would pass in a year? Compare with REMA's estimate that Rwanda loses about 1.4 million tonnes of soil a year.

Solution. Seconds in a year: $365.25 \times 24 \times 3\,600 = 3.156 \times 10^7$ s.

51\ \mathrm{kg\,s^{-1}} \times 3.156 \times 10^7\ \text{s} = 1.61 \times 10^9\ \text{kg} = 1.61 \times 10^6\ \text{t}

About 1.6 million tonnes a year at one station is already more than the national estimate of 1.4 million tonnes. The two figures come from different studies and methods (a river may also erode its own banks and bed, and 51 kg/s may be a high-flow value), so a geographer quotes both with their sources and does not treat either as exact.

River erosion and its landforms

A river erodes by hydraulic action (the force of moving water), abrasion (load scraping the bed and banks), attrition (pebbles knocking into one another and wearing smaller) and solution (dissolving minerals). Where a river is steep and high above base level, as on the flanks of the Congo–Nile Ridge, it cuts downwards (vertical erosion), producing:

V-shaped valleys and gorges with interlocking spurs;
waterfalls and rapids where the river crosses hard rock or a fault step;
potholes drilled into the rock bed by swirling pebbles.

River deposition and its landforms

Where the gradient falls and the river slows, as in the broad valleys of the central plateau and the east, it erodes sideways (lateral erosion) and deposits its load. REMA's inventory counted 860 marshlands covering 278,536 ha, 10.6% of the country, and 861 rivers totalling 6,462 km (REMA, 2009, citing REMA, 2008). Many of those marshlands are river valley floors filled with alluvium.

Figure 7: a meandering river (plan) and a section through one bend. Fast water on the outside of the bend erodes; slow water on the inside deposits. When two bends meet, the river cuts through and leaves an ox-bow lake.

Landform	How it forms	Where in Rwanda
Meander	Erosion on the outer bend, deposition on the inner bend, so the bends grow and migrate	Nyabarongo and Akagera valleys
Ox-bow lake	A meander neck is cut through in a flood; the old loop is sealed off by deposits	Possible on wide floodplains such as the Akagera's; check a satellite image
Floodplain and marshland	Floods spread fine alluvium over the valley floor	Nyabarongo, Akanyaru, Akagera; the Bugarama plain
Levee	Coarse sediment is dropped first, on the banks, when a river floods	Along larger lowland rivers
Alluvial fan	A steep stream drops its load where it reaches flatter ground	At the foot of steep valley sides
Delta	A river drops its load where it enters a lake	Where streams enter lakes

Worked example 2: is the river meandering?

Problem. On a map, a river's channel between two bridges measures 9.6 cm, while the straight line between the bridges measures 6.0 cm. Find the sinuosity index.

Solution.

\text{sinuosity} = \frac{\text{channel length}}{\text{straight-line length}} = \frac{9.6}{6.0} = 1.6

A value of 1 means a straight channel; above about 1.5 the river is described as meandering. This reach meanders. (Illustrative measurements.)

Wave erosion and deposition on Lake Kivu

Lake Kivu is large and deep; REMA's table of major lakes gives 102,000 ha and a depth of 478–480 m (REMA, 2009). Wind blowing across such a long stretch of open water raises waves that work on the shore like small sea waves, and REMA notes erosion and waste accumulation by winds and waves on its shores (REMA, 2021). Waves erode by hydraulic action and abrasion, mostly on headlands, where they cut cliffs, notches and wave-cut platforms, and they deposit sand and gravel in the calmer bays as beaches and, occasionally, spits.

Figure 8: waves concentrate their energy on headlands, cutting cliffs and platforms, and drop sand in the calmer bays.

Relief and development: benefits and problems

Relief feature	Benefits for development	Problems
Volcanoes	Fertile soils; tourism (gorillas, climbs); building stone	Eruption and gas hazards from active neighbours; landslides
Congo–Nile Ridge and steep hills	High rainfall and river sources; forests (Nyungwe, Gishwati); tea; scenery	Soil erosion and landslides; costly, winding roads; scattered settlement
Rift and Lake Kivu	Fishing, transport, tourism, methane gas, geothermal potential (hot springs)	Earthquakes; dissolved gas in the lake
Rivers and waterfalls	Hydroelectricity (Ntaruka between Burera and Ruhondo); water supply; irrigation	Floods; siltation of rivers, lakes and reservoirs
Valley marshlands and floodplains	Fertile alluvium for rice and vegetables (the rice cooperative of Bugarama is an example REMA mentions); water storage	Floods; malaria; pressure to drain wetlands
Eastern lowlands	Gentle land for large farms, ranching, roads and towns; Akagera National Park	Lower, less reliable rainfall and droughts (REMA)

Environmental problems from endogenic and exogenic processes

Process	Problem	Response
Endogenic: earthquakes	Collapsed houses, schools and hospitals in the west (2008)	Earthquake-resistant building rules; school drills
Endogenic: volcanic activity	Lava, ash and gas from Nyiragongo affecting Rubavu; refugees (2002)	Monitoring of Nyiragongo and of Lake Kivu's gases; evacuation plans
Exogenic: mass wasting	Landslides killing people and cutting roads (May 2023)	Risk mapping, relocation from high-risk zones, reforestation, drainage
Exogenic: soil erosion	Lost fertility; brown, silted rivers (51 kg/s at Kigali)	Terraces, agroforestry, protecting river banks
Exogenic: floods and deposition	Flooded valley farms and roads; silted reservoirs and lakes	Buffer zones along rivers and lakes; wetland protection

Warning

Endogenic and exogenic are about where the energy comes from. An earthquake or an eruption is endogenic even though we see it at the surface. A landslide is exogenic even if an earthquake triggered it: gravity moves the material.

Viking Geo: "Middle Course Landforms: Meanders, Ox Bow Lakes and Levees". How fast and slow water on a bend build river cliffs and slip-off slopes. Compare the diagrams with Figure 7 and the Nyabarongo photograph.

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Practice

Name and explain four processes of river erosion.
Explain why rivers on the flanks of the Congo–Nile Ridge erode vertically while the Nyabarongo in its middle course builds a floodplain.
With a diagram, explain how an ox-bow lake forms.
The Akagera at Rusumo carried 26 kg/s of soil (REMA, 2009). Convert this to tonnes per year.
A river channel measures 14.0 cm between two points 11.2 cm apart in a straight line. Is it meandering? Show your working.
Explain how waves shape the shores of Lake Kivu.
"Rwanda's relief is more an obstacle than an asset to its development." Discuss, with examples.

Show answersHide answers

Hydraulic action (force of water), abrasion (load scraping the bed and banks), attrition (load particles colliding and wearing down), solution (dissolving soluble minerals).
On the ridge the gradient is steep and the river is far above its base level, so its energy goes into cutting down; on the Nyabarongo the gradient is low and the river is slow, so it swings sideways and deposits alluvium on its valley floor.
Erosion on the outside of two bends narrows the neck between them; in a flood the river cuts through the neck, taking the shorter route; deposition seals the ends of the old bend, which becomes an ox-bow lake.
$26 \times 3.156 \times 10^7 = 8.2 \times 10^8$ kg $= 8.2 \times 10^5$ t, about 820,000 tonnes a year.
$\frac{14.0}{11.2} = 1.25$ ; below 1.5, so the reach is sinuous but not meandering.
Wind over the long, open lake raises waves; hydraulic action and abrasion cut cliffs, notches and platforms on headlands; sand is deposited as beaches in sheltered bays; the shore becomes a pattern of headlands and bays.
Obstacles: steep slopes cause erosion and landslides, make roads costly and limit mechanised farming. Assets: volcanic soils, high rainfall from relief, hydroelectric sites, scenery and tourism, lakes for fishing and transport, methane and geothermal resources. A good answer weighs both and shows that terracing, roads and planning can turn obstacles into assets.

Quick check

On a meander bend, where does the river erode most?

Key points

Steep rivers erode vertically (V-shaped valleys, gorges, waterfalls such as Rusumo); gentle rivers erode laterally and deposit (meanders, ox-bow lakes, floodplains, marshlands).
Rwanda's rivers carry heavy loads of eroded soil (51 kg/s on the Nyabarongo at Kigali, REMA 2009).
Waves on Lake Kivu cut cliffs and platforms on headlands and build beaches in bays.
Relief brings soils, water, energy and tourism, but also erosion, landslides, floods and tectonic hazards that planning must manage.

1Rivers that carry the hills away

2River erosion and its landforms

3River deposition and its landforms

4Wave erosion and deposition on Lake Kivu

5Relief and development: benefits and problems

6Environmental problems from endogenic and exogenic processes