Drainage in Africa and the World

Lessons

Senior 3 · Geography

Drainage in Africa and the World

4 lessons

Lesson 1 of 4

Rivers and Drainage Basins

25 min

Let's begin

Rain that falls on the hills above Rubavu can end up in the Atlantic Ocean. Rain that falls a few kilometres to the east, on the other side of the same ridge, can end up in the Mediterranean Sea, after a journey through Lake Victoria, Uganda, South Sudan, Sudan and Egypt. Rwanda sits on one of the great water partings of Africa. This lesson explains how rivers organise themselves into basins, and maps the great rivers of Africa and the world.

Definition

Drainage: the way water flows off the land in rivers, streams and lakes. Drainage basin (or catchment): the whole area of land drained by a river and its tributaries. Watershed (or divide): the high ground that separates one drainage basin from the next. Example: the Congo–Nile divide in western Rwanda.

The parts of a river system

Term	Meaning	Rwandan or African example
Source	Where a river begins: a spring, a lake or a marsh in the hills	The Sebeya rises at about 2,660 m in the mountains of the Congo–Nile divide (RWB, 2019)
Tributary	A smaller river that joins a larger one	The Mwogo, Rukarara and Mbirurume feed the Upper Nyabarongo (RWB, 2019)
Confluence	The point where two rivers meet	The Nyabarongo begins at the confluence of the Mwogo and Mbirurume (RWB, 2019); the White and Blue Niles meet at Khartoum
Mouth	Where a river enters a sea, lake or larger river	The Sebeya enters Lake Kivu at Rubavu, at about 1,470 m (RWB, 2019)
Delta	A fan of sediment at a mouth, cut by many channels	The Nile delta in Egypt; the Niger delta in Nigeria
Estuary	A wide, tidal mouth where river and sea water mix	The mouth of the Congo

Drainage patterns

Seen on a map, rivers and their tributaries form patterns. The pattern depends on the rock, the slope and the structure of the land (Figure 2).

Figure 2: six drainage patterns. Drawn by Ganzaa.

Dendritic: branches like a tree, on rock of the same hardness everywhere. The most common pattern, seen in most of Rwanda's hill country.
Trellis: long tributaries run along valleys of soft rock between parallel ridges of hard rock and join the main river at right angles.
Radial: rivers flow outwards in all directions from a high centre such as a volcano or dome, e.g. the streams on the slopes of the Virunga volcanoes or Mount Kenya.
Centripetal: rivers flow inwards to a lake or basin with no outlet, e.g. the rivers draining into Lake Chad.
Rectangular: rivers follow joints and faults in the rock, so they bend sharply at right angles.
Parallel: rivers flow side by side down a steep, even slope, such as the side of a rift valley.

B Hosein, a geography teacher: Drainage Patterns. Watch for how each pattern is linked to the rock type and the structure of the land.

The major rivers of Africa

Figure 1: the main rivers and lakes of Africa.

River	Length (Wikipedia)	Rises in	Flows into	Notes
Nile	over 6,600 km	The highlands around Lake Victoria (White Nile) and Lake Tana in Ethiopia (Blue Nile)	Mediterranean Sea, through a delta	Longest river in Africa; crosses the Sahara with little added water
Congo	about 4,700 km, measured with the Lualaba–Chambeshi headwaters	Highlands of Zambia and the DR Congo	Atlantic Ocean, through an estuary	Basin of about 4,000,000 km²; the world's deepest recorded river
Niger	about 4,180 km	Guinea Highlands, close to the Atlantic	Gulf of Guinea, through a delta	Flows north-east into the Sahel, then turns south-east
Zambezi	about 2,574 km	North-western Zambia	Indian Ocean	Victoria Falls; Kariba and Cahora Bassa dams

The Nile basin covers parts of eleven countries. The White Nile carries a steady flow from the rainy lake plateau; the Blue Nile and Atbara bring a summer flood from the Ethiopian Highlands, which once flooded Egypt every year. The Congo basin straddles the Equator, so one part or another always has its rainy season, and the Congo's flow is large and steady all year.

Major rivers of the other continents

The Amazon (South America) carries more water than any other river. The Mississippi–Missouri drains the interior plains of North America to the Gulf of Mexico. The Yangtze and Huang He water the plains of China; the Ganges and Indus flow from the Himalayas; the Danube and Rhine are the great navigable rivers of Europe; the Murray–Darling drains south-eastern Australia.

Rwanda on the Congo–Nile divide

The Rwanda Water Resources Board describes Rwanda's rivers as draining to two great basins (RWB catchment plans, 2019). West of the Congo–Nile divide, the Sebeya catchment is "one of the most upstream parts of the Congo River basin": its water flows into Lake Kivu, then by the Rusizi to Lake Tanganyika and on to the Congo and the Atlantic. East of the divide, the Upper Nyabarongo catchment lies "within the Nile basin": the Nyabarongo flows to the Akagera River and Lake Victoria, the main reservoir of the White Nile. RWB calls the Upper Nyabarongo catchment Rwanda's "water tower".

Figure 3: the Congo–Nile divide, simplified. Heights from RWB (2019); drawn by Ganzaa.

Worked example

Worked example 1: catchment areas. RWB (2019) gives Rwanda's total area as 26,338 km². The Upper Nyabarongo catchment covers 3,348 km² and the Sebeya catchment 336.4 km². What percentage of Rwanda does each cover?

Upper Nyabarongo: $\frac{3{,}348}{26{,}338} \times 100 \approx 12.7\%$ .
Sebeya: $\frac{336.4}{26{,}338} \times 100 \approx 1.28\%$ . (The RWB plan prints 1.38%: always check a printed figure against your own calculation.)
So the Upper Nyabarongo catchment is almost ten times as large as the Sebeya catchment.

Worked example

Worked example 2: gradient of a river. The Sebeya falls from about 2,660 m at its source to 1,470 m at Lake Kivu over 48 km (RWB, 2019). Calculate its average gradient.

Fall: $2{,}660 - 1{,}470 = 1{,}190\ \text{m}$ .
Gradient: $\frac{1{,}190\ \text{m}}{48{,}000\ \text{m}} \approx \frac{1}{40}$ so about 1 m of fall for every 40 m of length, or 25 m per kilometre.
That is steep for a river: the Sebeya flows fast and carries much sediment after heavy rain.

Warning

"Watershed" has two meanings. In British and East African geography it means the divide between basins, as used here; in American usage it often means the basin itself. In your exam, define the term before you use it.

Tip

Read the pattern, then name the rock. Tree-like: uniform rock. Right angles between ridges: folded rock (trellis). Right angles everywhere: joints and faults (rectangular). Spokes out: a volcano or dome. Spokes in: a basin or lake.

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Practice

Define a drainage basin and a watershed.
Name the drainage pattern found (a) on the slopes of a volcano, (b) around Lake Chad.
Name the sea or ocean into which each flows: (a) the Nile, (b) the Congo, (c) the Zambezi.
Distinguish between a delta and an estuary, with an African example of each.
Describe the route of water from the Nyabarongo to the Mediterranean Sea.
Why is the flow of the Congo steadier through the year than that of the Blue Nile?
The Mwogo is 81.1 km long and the Mbirurume 51.6 km (RWB, 2019). How much longer is the Mwogo, as a percentage of the Mbirurume's length?
Explain how the rock structure of an area can produce a trellis pattern.

Show answersHide answers

Drainage basin: the area drained by a river and its tributaries. Watershed: the high ground separating one basin from another.
(a) radial; (b) centripetal.
(a) Mediterranean Sea; (b) Atlantic Ocean; (c) Indian Ocean.
A delta is a fan of deposited sediment at a mouth, cut by many channels, e.g. the Nile delta; an estuary is a wide tidal mouth where river and sea water mix, with little deposition, e.g. the Congo's mouth.
Nyabarongo → Akagera → Lake Victoria → (Victoria Nile) → White Nile → Nile → Mediterranean Sea.
The Congo basin straddles the Equator, so some part of it is always in its rainy season and tributaries from both hemispheres feed it. The Blue Nile depends on one summer rainy season in the Ethiopian Highlands, so it floods in summer and shrinks in the dry season.
$81.1 - 51.6 = 29.5$ km; $\frac{29.5}{51.6} \times 100 \approx 57.2\%$ longer.
Where layers of hard and soft rock are folded into parallel ridges and valleys, the main river cuts across the ridges, while tributaries erode the soft rock and run along the valleys between the ridges, joining the main river at right angles.

Sources and credits

Rwanda Water Resources Board (RWB): Sebeya Catchment Management Plan 2018–2024 and Upper Nyabarongo Catchment Management Plan, published on the RWB Water Portal in 2019. Facts used as stated.
Nile, Congo River, Niger River, Zambezi and Drainage system (geomorphology): Wikipedia contributors, Wikipedia, CC BY-SA 4.0, adapted by Ganzaa.
Video: DRAINAGE PATTERNS, B Hosein, embedded from YouTube.
Figures 1–3: drawn by Ganzaa; rivers, lakes and outlines in Figure 1 from Natural Earth, public domain.
Explanations, worked examples and exercises: written by Ganzaa.

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

1The parts of a river system

2Drainage patterns

3The major rivers of Africa

4Major rivers of the other continents

5Rwanda on the Congo–Nile divide

The parts of a river system

Drainage patterns

The major rivers of Africa

Major rivers of the other continents

Rwanda on the Congo–Nile divide