Lakes and How They Were Formed

25 min

Let's begin

Lake Victoria and Lake Tanganyika are neighbours on the map, yet they could hardly be more different. Victoria is a vast, shallow saucer: its average depth is only about 40 m. Tanganyika is a narrow trench about 673 km long and about 1,470 m deep, the second-deepest lake on Earth. The difference comes from the way each basin was made. To explain a lake, ask first how its hollow was formed, and then where its water comes from.

Definition

Lake: a large body of water lying in a hollow in the land. Freshwater lake: one with an outlet, so salts are carried away (e.g. Victoria). Salt lake: one with no outlet, where evaporation leaves salts behind (e.g. Lake Assal in Djibouti).

Lakes formed by earth movements (tectonic lakes)

Rift valley lakes. Where the crust is pulled apart, a block sinks between two parallel faults, leaving a long, narrow, deep trough with steep escarpments (Unit 2). Water fills the trough. Examples: Tanganyika, Malawi, Albert, Edward and Kivu on the Western Rift; Turkana on the Eastern Rift. Rift lakes are long, narrow, very deep, and have straight shores.
Downwarp lakes. Where the crust sags gently between two uplifted areas, a wide, shallow basin forms. Lake Victoria, about 59,947 km², the largest lake in Africa, lies in such a sag between the Western and Eastern Rifts; Lake Kyoga is another. Their shores are irregular, with many bays and islands.

Lakes formed by volcanoes

Crater lakes fill the crater of an extinct volcano, e.g. the small lake in the summit crater of Bisoke in the Virunga range.
Lava-dammed lakes form when a lava flow blocks a river valley and water collects behind it. Lakes Burera and Ruhondo in northern Rwanda are usually explained this way, by lava from the Virunga volcanoes. Lake Kivu is a rift lake, but volcanic activity about 13,000 to 9,000 years ago blocked its old outlet towards the Nile, so it now overflows southwards by the Rusizi into Lake Tanganyika.

Figure 4: four ways of making a lake. Drawn by Ganzaa.

Other origins

Glacial lakes fill hollows scoured by ice sheets or dammed by glacial deposits: the Great Lakes of North America, and thousands of lakes in Finland and Canada.
Inland drainage lakes lie at the lowest point of a basin with no outlet to the sea, e.g. Lake Chad, fed by the Chari. They are shallow and change size with the rain.
Man-made lakes (reservoirs) form behind dams: Lake Nasser behind the Aswan High Dam on the Nile, Lake Kariba on the Zambezi, Lake Volta in Ghana.
Oxbow lakes are cut off from a meandering river on its flood plain; lagoons are cut off from the sea by a sand bar, e.g. along the coast of West Africa.

Waypoint Teaching: The Wonderful African Rift Lakes. Watch for how faulting made the deep troughs of the Western Rift, and compare their shape with shallow Lake Victoria.

Definition

Rift valley lake: a long, narrow, deep lake in a trough that sank between two faults, e.g. Tanganyika. Downwarp lake: a wide, shallow lake in a basin where the crust sagged, e.g. Victoria. Lava-dammed lake: a lake held back by a lava flow across a valley, e.g. Burera.

Worked example

Worked example 1: identify the origin from a description. A lake is 673 km long, about 50 km wide and 1,470 m deep, with steep, straight shores. Name its type and explain.

Very long compared with its width, very deep and straight-sided: these are the marks of a trough between faults.
So it is a rift valley lake. (The figures are Lake Tanganyika's.)
A downwarp lake would be wide and shallow; a crater lake would be small and round.

Worked example

Worked example 2: a shrinking lake. The open water of Lake Chad fell from about 25,000 km² in 1960 to about 2,000 km² in 1980 (Wikipedia). By what percentage did it shrink?

Loss: $25{,}000 - 2{,}000 = 23{,}000\ \text{km}^2$ .
Percentage: $\frac{23{,}000}{25{,}000} \times 100 = 92\%$
A shallow inland-drainage lake reacts quickly to drought and to water taken for irrigation, because it has no deep store of water.

Warning

Do not call Lake Victoria a rift lake. It lies on the plateau between the two branches of the rift, in a downwarp. Kivu, Tanganyika, Malawi, Albert and Turkana are the rift lakes.

Tip

Shape tells origin. Long and deep: rift. Wide and shallow: downwarp. Round and small on a mountain top: crater. Behind a wall of rock in a valley: lava dam or man-made dam.

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Practice

Name the type of lake of: (a) Lake Victoria, (b) Lake Tanganyika, (c) Lake Kariba.
Describe how a rift valley lake is formed. A labelled sketch will help.
Give three differences between Lake Victoria and Lake Tanganyika.
Why does Lake Kivu drain to the Congo rather than to the Nile?
Name two lakes formed by volcanic activity in or near Rwanda.
Explain why lakes with no outlet are often salty.
Lake Victoria's average depth is about 40 m and Lake Tanganyika's maximum depth about 1,470 m. How many times deeper is Tanganyika at its deepest than Victoria on average?

Show answersHide answers

(a) downwarp; (b) rift valley; (c) man-made (reservoir behind the Kariba Dam).
Tension pulls the crust apart; two parallel faults form; the block between them sinks, leaving a deep trough between steep escarpments; rain and rivers fill the trough with water.
Any three: Victoria is wide and shallow, Tanganyika long, narrow and very deep; Victoria lies in a downwarp, Tanganyika in a rift valley; Victoria has an irregular shore with many bays and islands, Tanganyika straight, steep shores; Victoria is larger in area.
Volcanic activity about 13,000 to 9,000 years ago blocked its old outlet towards the Nile, so the lake rose until it overflowed southwards by the Rusizi into Lake Tanganyika, which drains to the Congo.
Burera and Ruhondo (lava-dammed); the crater lake on Bisoke.
Rivers bring small amounts of dissolved salts; with no outlet, water leaves only by evaporation, which leaves the salts behind, so they build up over time.
$\frac{1{,}470}{40} \approx 37$ times.

Sources and credits

Lake Victoria, Lake Tanganyika, Lake Kivu, Lake Chad and Lake Burera: Wikipedia contributors, Wikipedia, CC BY-SA 4.0, adapted by Ganzaa.
Video: The Wonderful African Rift Lakes - Formation & Ecosystems Explained, Waypoint Teaching, embedded from YouTube.
Figure 4, explanations, worked examples and exercises: written and drawn by Ganzaa.

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

1Lakes formed by earth movements (tectonic lakes)

2Lakes formed by volcanoes

3Other origins

Lakes formed by earth movements (tectonic lakes)

Lakes formed by volcanoes

Other origins