Drainage System in Rwanda

Lessons

Senior 4 · Geography

Drainage System in Rwanda

3 lessons

Lesson 2 of 3

Major Lakes, Their Formation, and Wetlands

25 min

One giant and a crowd of small lakes

A national inventory in 2008 counted 101 lakes in Rwanda covering 149,487 ha, and 860 marshlands covering 278,536 ha, 10.6% of the country (REMA, 2009). Figure 2 compares the main lakes.

Figure 2. Area of the main lakes and lake groups. Data: REMA (2009), Table 1, adapted from national studies.

Notice two things: Kivu alone is larger than all the others put together, and most of the other lakes lie in groups in the east (Bugesera, Gisaka, Nasho, Akagera). Their different sizes, depths and shapes are clues to how they formed.

Definition

Lake: a body of standing water filling a hollow in the land. Wetland (marshland, swamp): land that is waterlogged or shallowly flooded for much or all of the year, with plants adapted to wet soil, such as papyrus.

Shoreline development index $D_{L}$ : how indented a lake's shore is, compared with a circle of the same area: $D_L = \frac{L}{2\sqrt{\pi A}}$ , where $L$ is the shoreline length and $A$ the area. A circle has $D_{L} = 1$ .

How Rwanda's lakes formed

Figure 3. Four ways lakes formed in Rwanda. Drawn by Ganzaa (schematic, not to scale).

Type	Process	Examples and evidence
Rift (tectonic) lake	Faulting lowered a long block of crust between the faults of the western rift; water filled the trough	Kivu: long, straight-sided and very deep, 478–480 m (REMA, 2009)
Lava-dammed lake	Lava from the Virunga volcanoes flowed across river valleys and solidified, damming them	Burera (5,500 ha) and Ruhondo (2,800 ha), high-altitude lakes near the volcanoes
Crater lake	Water collects in the crater on top of a volcano	The lake on the summit of Bisoke
Drowned (flooded) valley lake	Warping tilted the land east of the rift; river valleys and their tributaries were flooded	Muhazi, "a flooded valley lake" whose arms "were formerly the location of tributaries" (Wikipedia)
Floodplain (riverine) lake	In wide, gently sloping valleys, river deposits and swamps cut hollows off from the main channel	Ihema and the other lakes of the Akagera valley, which lie in the river's marshland and drain to it by short channels (Wikipedia)
Artificial lake (reservoir)	People build dams across valleys	Valley dams for cattle and irrigation in Nyagatare (42), Kayonza (23) and Gatsibo (12) (RWB, in REMA, 2021)

The eastern lakes are shallow: average depths of about 3–5 m, with high turbidity (REMA, 2009). Shallow lakes in wide valleys are what you expect from flooding and deposition, not from faulting.

Lake Muhazi from a hillside, 8 June 2020: the water winds between steep spurs, and arms of the lake reach back into side valleys. This branching outline is the shape of a drowned river valley. Photo: Reneplee, CC BY-SA 4.0, via Wikimedia Commons.

Worked example 1: what does the shape of a lake say about its origin?

Data (measured by Ganzaa on OpenStreetMap outlines). Muhazi: area $39\ \text{km}^2$ , shoreline $162\ \text{km}$ . Ihema: area $94.5\ \text{km}^2$ , shoreline $72\ \text{km}$ .

D_L(\text{Muhazi}) = \frac{162}{2\sqrt{\pi \times 39}} = \frac{162}{2 \times 11.07} = 7.3

D_L(\text{Ihema}) = \frac{72}{2\sqrt{\pi \times 94.5}} = \frac{72}{2 \times 17.23} = 2.1

Interpretation. Muhazi's shore is over seven times longer than a circle of the same area would have: a long, branching lake, as expected when a valley system is drowned. Ihema is much more compact, as expected of a hollow on a floodplain. (Mapped outlines are simplified, so both shorelines are a little too short; the comparison still holds.)

Wetlands and swamps

The 2008 inventory found that "the biggest marshlands are associated with and clustered around the rivers" (REMA, 2009). The main ones are:

Rugezi (Burera District): a highland peat wetland at about 2,005 m that feeds Lake Burera; a Ramsar site (REMA, 2021).
Kamiranzovu (Nyungwe): a high-altitude swamp at the source of streams.
Nyabarongo and Akanyaru: long valley-bottom swamps of papyrus along the two rivers.
Akagera wetland complex and Rweru–Mugesera: swamps around the eastern lakes; Rweru–Mugesera alone supplies more than 48,000 households (about 194,000 people) with water, papyrus, fish and crops (REMA, 2021).
Kigali's urban wetlands, such as Nyabugogo and Nyandungu, in the valleys between the city's hills.

Figure 4. Use of Rwanda's marshlands in 2008. Drawn by Ganzaa from REMA (2009).

Worked example 2: how much marshland was still natural?

Problem. Using Figure 4, find the area of marshland under natural vegetation and under crops in 2008.

0.41 \times 278\,536 = 114\,200\ \text{ha (natural)} \qquad 0.53 \times 278\,536 = 147\,624\ \text{ha (crops)}

REMA quotes "about 148,344 ha" under crops: the small difference shows that 53% is a rounded percentage ( $\frac{148\,344}{278\,536} = 53.3\%$ ). Conclusion: by 2008 more than half of Rwanda's marshland had been converted to farmland, which is why wetland protection became a national priority.

Warning

Don't explain every lake with one process. "Rwanda's lakes were formed by volcanoes" is wrong for Kivu (faulting), Muhazi (a drowned valley) and Ihema (a floodplain lake). In an exam, name the process and match it to the lake's evidence: depth, shape and position.

DW (Global 3000): "Rwanda's Rugezi Marsh: a source of life". How the Rugezi wetland stores water for Lakes Burera and Ruhondo and supports the people around it. Note the uses of the marsh and the rules that protect it.

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Practice

Name one lake of each type in Rwanda: rift, lava-dammed, drowned valley and floodplain.
Explain how Lake Kivu was formed and why it is so much deeper than the lakes of the east.
Describe two features of Lake Muhazi that support the idea that it is a drowned valley.
A lake has an area of 16 km² and a shoreline of 40 km. Calculate its shoreline development index and say what it suggests.
What percentage of the lake area counted in 2008 was Lake Kivu (Rwandan part)?
Give three uses of wetlands to people in Rwanda.
"Draining wetlands for farming is good for Rwanda." Evaluate this view.

Show answersHide answers

Rift: Kivu; lava-dammed: Burera or Ruhondo; drowned valley: Muhazi; floodplain: Ihema (or other Akagera lakes).
Faulting along the western branch of the rift lowered a long block between faults, and water filled the trough. Its depth (478–480 m) reflects the size of the fault movement; the eastern lakes fill shallow valley floors (3–5 m).
It is long and narrow, following a valley; it has many arms reaching into side valleys (former tributaries), giving a very indented shoreline ( $D_L \approx 7.3$ ).
$D_L = \frac{40}{2\sqrt{\pi \times 16}} = \frac{40}{2 \times 7.09} = 2.8$ : fairly indented, more than a floodplain lake like Ihema (2.1) but far less than a drowned valley like Muhazi (7.3).
$\frac{102\,000}{149\,487} \times 100 = 68.2\%$ .
Any three: water supply and storage; papyrus for mats and roofing; crops such as rice and vegetables in dry seasons; fish; regulating floods; filtering water; habitat for birds; tourism (Nyandungu eco-park).
Draining gives land for food (rice, vegetables), jobs and income. But wetlands store water, reduce floods, clean water and hold wildlife; draining Rugezi nearly stopped hydroelectricity in 2003–2004. A balanced conclusion: farm some wetlands with controls, but protect key wetlands that regulate water.

Quick check

Match each lake to the way it formed.

Tap one on the left, then its partner on the right.

Key points

2008 inventory: 101 lakes (149,487 ha) and 860 marshlands (278,536 ha, 10.6% of Rwanda); Kivu is about two-thirds of the lake area.
Formation: rift (Kivu, 478–480 m deep), lava-dammed (Burera, Ruhondo), crater (Bisoke), drowned valley (Muhazi), floodplain (Ihema and the Akagera lakes), artificial valley dams.
Shape and depth are evidence: Muhazi's shoreline index is about 7.3, Ihema's about 2.1.
Main wetlands: Rugezi (Ramsar), Kamiranzovu, Nyabarongo, Akanyaru, Akagera and Rweru–Mugesera, Kigali's urban wetlands; by 2008, 53% of marshland was cultivated.

Sources and credits

Sources: Rwanda State of Environment and Outlook Report, REMA, 2009 (2008 inventory of lakes, rivers and marshlands; table of major lakes; marshland use; wetland types); Rwanda State of Environment and Outlook Report 2021, REMA (Rugezi, Rweru–Mugesera, valley dams from RWB data); Lake Muhazi and Lake Ihema, Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa.

Figures 2–4 and the worked examples were drawn and written by Ganzaa; lake outlines for worked example 1 measured on © OpenStreetMap contributors (ODbL).

Photos: Muhazi Lake, Reneplee, CC BY-SA 4.0; Lake Ihema, John Cooke, CC BY 2.0; Downtown Kigali and papyrus marsh October 2012, Lemurbaby, CC BY-SA 3.0; via Wikimedia Commons.

Video: "Rwanda's Rugezi Marsh - a source of life | Global 3000", DW News, embedded from YouTube. The rest of the lesson and the exercises were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1One giant and a crowd of small lakes

2How Rwanda's lakes formed

3Wetlands and swamps

One giant and a crowd of small lakes

How Rwanda's lakes formed

Wetlands and swamps