Major Rivers and Drainage Basins of Rwanda – Drainage System in Rwanda (Senior 4 Geography) | Ganzaa

Drainage System in Rwanda

Lessons

Senior 4 · Geography

Drainage System in Rwanda

3 lessons

Lesson 1 of 3

Major Rivers and Drainage Basins of Rwanda

25 min

Two raindrops, two oceans

Imagine two raindrops falling a few metres apart on the crest of the Congo–Nile Ridge in Nyungwe. One runs west, into Lake Kivu, down the Rusizi to Lake Tanganyika and along the Congo River to the Atlantic Ocean. The other runs east, into the Nyabarongo and the Akagera, through Lake Victoria and down the Nile to the Mediterranean Sea. The line of high ground that separates them is one of the great water partings of Africa.

Figure 1. Rwanda's nine level-1 catchments and the Congo–Nile divide. Drawn by Ganzaa from the catchment layers of the Rwanda Water Resources Board (RWB) IWRM GeoPortal, with rivers and lakes from OpenStreetMap and Natural Earth.

Use Figure 1 to answer: which two catchments drain to the Congo? Which river leaves Rwanda for Lake Victoria? Why does the divide run so close to the western border?

Definition

Drainage system: the network of rivers, lakes and wetlands that collects and carries water from an area.

Drainage basin (catchment): the area of land drained by a river and its tributaries. Example: the Upper Nyabarongo catchment, 3,348 km².

Watershed (divide): the boundary of high ground between two drainage basins. Example: the Congo–Nile divide along the Congo–Nile Ridge.

Tributary: a river that flows into a larger one. Confluence: the point where two rivers meet. Example: the Mwogo and the Mbirurume meet to form the Nyabarongo.

Drainage density: the total length of rivers divided by the area of the basin, in km per km².

Two basins, nine catchments

Rwanda's National Water Resources Master Plan divides the country into catchments at four levels. At the top are two basins: the Congo basin in the west, fed by the Kivu and Rusizi catchments, and the Nile basin in the east, fed by seven others. Below them are nine level-1 catchments, 20 level-2 sub-catchments, dozens at level 3 and hundreds at level 4 (RWB, Upper Nyabarongo Catchment Plan 2018–2024).

Basin	Level-1 catchment	Area (km²)	Main rivers
Congo	Kivu	3,466	Sebeya, Koko and other short streams into Lake Kivu
Congo	Rusizi	1,009	Rusizi, Ruhwa (to Lake Tanganyika)
Nile	Upper Nyabarongo	3,348	Mwogo, Mbirurume, Rukarara, upper Nyabarongo
	Mukungwa	1,828	Mukungwa, draining lakes Burera and Ruhondo
	Lower Nyabarongo	3,306	Lower Nyabarongo, Base, Nyabugogo
	Akanyaru	3,405	Akanyaru (on the Burundi border)
	Upper Akagera	3,056	Akagera, lakes of Bugesera
	Lower Akagera	4,294	Akagera, lakes of Akagera National Park
	Muvumba	1,568	Muvumba

Areas: RWB IWRM GeoPortal catchment layer (rounded). A further 39 km² in the north is mapped separately and belongs to the Nile basin. The total, about 25,300 km², is less than Rwanda's 26,338 km² because the catchment map leaves out most of the surface of Lake Kivu.

Worked example 1: how much of Rwanda drains to the Congo?

Problem. Use the RWB catchment areas to find the share of Rwanda's mapped catchments that drains to each basin. REMA's 2009 report gave the Congo basin 33% of the country, and the Wikipedia article "Geography of Rwanda" gives about 20%. Which is better supported?

Solution.

A_{\text{Congo}} = 3\,466 + 1\,009 = 4\,475\ \text{km}^2 \qquad A_{\text{total}} \approx 25\,318\ \text{km}^2

\frac{4\,475}{25\,318} \times 100 = 17.7\% \qquad \text{so the Nile basin drains } 100 - 17.7 = 82.3\%

Evaluation. The RWB figures come from the official catchment map used for water planning today, so they are the best evidence: about 18% Congo and 82% Nile. Wikipedia's "about 20%" is a fair rounding; REMA's older 33% does not match the current map. When sources disagree, prefer the one closest to the measurement, and say which you used.

The major rivers

Nyabarongo. Formed where the Mwogo and the Mbirurume meet, it flows north as the Upper Nyabarongo to its confluence with the Mukungwa, then turns south-east as the Lower Nyabarongo past Kigali towards the Akagera (RWB). It is Rwanda's longest river, about 351 km (Wikipedia). Its headwaters in Nyungwe are among the most distant sources of the Nile; RWB calls the Upper Nyabarongo catchment Rwanda's "water tower".
Akagera (Kagera). Continues the Nyabarongo from Lake Rweru on the Burundi border and flows north along the border with Tanzania, over the Rusumo Falls, to Lake Victoria. Its broad valley is full of lakes and swamps.
Akanyaru. Forms much of the border with Burundi and joins the Nyabarongo.
Mukungwa. Drains Lakes Burera and Ruhondo, whose water also turns the turbines at Ntaruka.
Muvumba. Drains the north-east (Nyagatare) towards the Akagera; RWB has a separate catchment plan for it.
Rusizi. Leaves the south end of Lake Kivu and flows south to Lake Tanganyika, forming part of the border with the DR Congo.
Sebeya, Koko and other short rivers fall steeply from the Congo–Nile Ridge into Lake Kivu.

A national inventory in 2008 counted 861 rivers with a total length of 6,462 km (REMA, 2009).

Worked example 2: how dense is Rwanda's river network?

Problem. Using the 2008 inventory, calculate Rwanda's drainage density and explain what it suggests.

D_d = \frac{\text{total length of rivers}}{\text{area}} = \frac{6\,462\ \text{km}}{26\,338\ \text{km}^2} = 0.245\ \text{km per km}^2

Interpretation. On average there is about a quarter of a kilometre of mapped river in every square kilometre. The real density is higher, because an inventory at national scale misses the many small streams. Density is greatest in the wet, steep west (high rainfall, impermeable rocks, steep slopes) and lowest in the drier, flatter east, where water collects in lakes and swamps instead of in many channels.

How the rivers behave

Feature	Western highlands (upper courses)	Central plateau and east (lower courses)
Gradient	Steep; rapids and waterfalls	Gentle; slow, winding channels
Main work	Vertical erosion, deep V-shaped valleys	Deposition, floodplains, swamps, lakes
Flow through the year	Two high-water periods that follow the two rainy seasons (about March–May and September–December), with low flows in the dry seasons
Load	Heavy loads of soil after rain, which turn the rivers brown

Warning

Basin, catchment and divide are not the same thing. A basin (or catchment) is an area; the divide (watershed) is the line around it. "The Congo–Nile divide" is a line on the ridge; "the Nile basin" is the whole area east of it. Also note that the Akagera leaves Rwanda for Lake Victoria, not for Lake Kivu.

From the documentary "The Nile Quest" (Terje Tvedt): the search for the most distant source of the White Nile in Rwanda's Nyungwe forest. As you watch, trace the route of the water on Figure 1 from Nyungwe to Lake Victoria.

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Practice

Define a drainage basin and a watershed, using Rwandan examples.
Name the two basins and the nine level-1 catchments of Rwanda.
Describe the course of the Nyabarongo from its source to the Akagera.
The Muvumba catchment covers 1,568 km². What percentage of the mapped total of 25,318 km² is this?
A sub-catchment of 420 km² contains 147 km of rivers. Calculate its drainage density and compare it with the national figure.
Explain why rivers on the flanks of the Congo–Nile Ridge are short and steep, while those of the east are long and slow.
"Rwanda is a source of water for its neighbours." Assess this statement.

Show answersHide answers

A drainage basin is the area drained by a river and its tributaries (e.g. the Upper Nyabarongo catchment); a watershed is the high-ground boundary between basins (e.g. the Congo–Nile divide).
Congo: Kivu, Rusizi. Nile: Upper Nyabarongo, Lower Nyabarongo, Mukungwa, Akanyaru, Upper Akagera, Lower Akagera, Muvumba.
Formed by the confluence of the Mwogo and Mbirurume, it flows north (Upper Nyabarongo) to the Mukungwa confluence, then south-east past Kigali (Lower Nyabarongo) to join the Akagera system, which flows to Lake Victoria.
$\frac{1\,568}{25\,318} \times 100 = 6.2\%$ .
$\frac{147}{420} = 0.35$ km per km², higher than the national 0.245, so its river network is denser (likely a wetter, steeper area).
The divide lies close to the western border, so western rivers have only a short distance to fall a long way to Lake Kivu (steep, short, erosive); eastern rivers cross the wide plateau and lowlands on gentle gradients (long, slow, depositing, with swamps and lakes).
Largely true: Rwanda lies at the top of both basins, as RWB notes, so most of its rivers rise inside the country and flow out to neighbours (Akagera to Tanzania and Lake Victoria, Rusizi to Lake Tanganyika). This gives Rwanda a responsibility to protect water quality and to cooperate with downstream countries; the Akagera and Rusizi are shared border rivers.

Quick check

Which two level-1 catchments of Rwanda drain to the Congo basin?

Key points

The Congo–Nile divide runs north–south along the western highlands. On RWB's map the Congo basin (Kivu, Rusizi) covers about 18% and the Nile basin (seven catchments) about 82%.
Nine level-1 catchments, 20 level-2 sub-catchments; the Upper Nyabarongo is the "water tower" and holds the most distant sources of the Nile.
Major rivers: Nyabarongo, Akagera, Akanyaru, Mukungwa, Muvumba, Base (Nile); Rusizi, Sebeya, Koko (Congo).
Drainage density about 0.245 km per km² (6,462 km of rivers, 2008 inventory); steep, erosive rivers in the west; slow rivers with swamps and lakes in the east.

Sources and credits

Sources: Upper Nyabarongo Catchment Plan 2018–2024, Rwanda Water Resources Board (catchment levels, basins, the Nyabarongo and its tributaries); catchment areas from the RWB IWRM GeoPortal (Catchment Level 0 and Level 1 layers); Rwanda State of Environment and Outlook Report, REMA, 2009 (river inventory, older basin shares); Geography of Rwanda, Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa (length of the Nyabarongo, Rusizi, basin share).

Figure 1 was drawn by Ganzaa: catchment boundaries simplified from the RWB IWRM GeoPortal; rivers and lakes © OpenStreetMap contributors (ODbL); Lake Kivu, countries and the national outline from Natural Earth (public domain) and geoBoundaries (CC BY 4.0).

Photos: RwandaNyungwe, BluesyPete, CC BY-SA 3.0; Nyabarongo RIVER, Bugesera-Kicukiro bridge, Mugisha Don de Dieu, CC BY 2.0; via Wikimedia Commons.

Video: "The source of the White Nile, Rwanda (taken from the TV-doc. The Nile Quest)", Terje Tvedt on global history, the Nile, and water, embedded from YouTube. The rest of the lesson, the worked examples and the exercises were written by Ganzaa.

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

1Two raindrops, two oceans

2Two basins, nine catchments

3The major rivers

4How the rivers behave

Two raindrops, two oceans

Two basins, nine catchments

The major rivers

How the rivers behave