Global Drainage Systems
Senior 6 · Geography
Global Drainage Systems
Lesson 4 of 5
25 min
Rain that falls on Rwanda's eastern hills may take months to reach the Mediterranean, passing through Lake Victoria, the swamps of South Sudan and the deserts of Sudan and Egypt. The Nile basin is shared by eleven countries: Burundi, the DR Congo, Egypt, Eritrea, Ethiopia, Kenya, Rwanda, South Sudan, Sudan, Tanzania and Uganda. Rivers and lakes join countries together, and they also divide them: every dam upstream changes life downstream. This lesson surveys the world's major rivers and their impact, then the lakes, seas and oceans into which they flow.
| River | Approximate length | Mouth | Notable features and uses |
|---|---|---|---|
| Nile | 6,650 km | Mediterranean Sea (arcuate delta) | Exotic river through the Sahara; irrigation for Egypt and Sudan; Aswan High Dam; Grand Ethiopian Renaissance Dam on the Blue Nile |
| Amazon | about 6,400 km | Atlantic Ocean | By far the largest discharge in the world, about one fifth of all river water reaching the oceans; drains the largest rainforest |
| Yangtze (Chang Jiang) | about 6,300 km | East China Sea | Busiest inland waterway in the world; Three Gorges Dam, one of the largest hydroelectric stations |
| Mississippi–Missouri | about 6,275 km | Gulf of Mexico (bird's-foot delta) | Barge transport of grain and coal; levees and flood control |
| Congo | about 4,700 km | Atlantic Ocean (estuary) | Second-largest discharge in the world; deepest river; huge hydropower potential at the Inga rapids; navigable in long stretches between rapids |
| Niger | about 4,180 km | Gulf of Guinea (arcuate delta) | Inland delta in Mali; Kainji Dam in Nigeria; oil fields in its delta |
| Zambezi | about 2,570 km | Indian Ocean | Victoria Falls; Kariba and Cahora Bassa dams |
Lengths are approximate because they depend on where the source is taken to be and how the mouth is measured.
| Positive impacts | Negative impacts |
|---|---|
| Water for homes, industry and irrigation | Floods that destroy crops, homes and roads |
| Hydroelectric power | Water-related diseases such as bilharzia (schistosomiasis) and malaria |
| Transport of bulky goods by barge and boat | Barriers to road and rail transport; bridges are costly |
| Fishing and fertile alluvial soils on floodplains | Bank erosion and the silting of reservoirs and harbours |
| Tourism (Victoria Falls, Rusumo Falls) | Disputes over shared water, such as that between Ethiopia, Sudan and Egypt over the Grand Ethiopian Renaissance Dam |
| International boundaries, such as the Akagera between Rwanda and Tanzania | Pollution carried downstream across borders |
Definition
Lake: a body of standing water in a depression in the land, not directly connected to the ocean.
Sea: a large body of salt water, smaller than an ocean and usually partly enclosed by land, such as the Mediterranean or the Red Sea.
Ocean: one of the five great connected bodies of salt water: the Pacific, Atlantic, Indian, Southern and Arctic.
Residence time: the average time water stays in a lake or reservoir: .
| Origin | How it forms | Examples |
|---|---|---|
| Tectonic: rift valley (faulting) | Water fills a long, deep, steep-sided trough (graben) formed as blocks sink between parallel faults | Lake Tanganyika (about 1,470 m deep, the second deepest in the world), Lake Kivu, Lake Malawi |
| Tectonic: downwarping | The crust sags gently into a broad, shallow basin | Lake Victoria, the largest lake in Africa, and Lake Kyoga |
| Volcanic: crater and caldera | Water collects in the crater of an extinct volcano or in a caldera | Crater lakes of western Uganda; Lake Toba, Indonesia |
| Volcanic: lava-dammed | A lava flow blocks a valley and the river ponds up behind it | Lakes Burera and Ruhondo in northern Rwanda, dammed by lava from the Virunga volcanoes |
| Glacial | Ice scours hollows in rock, or moraine dams a valley | The Great Lakes of North America; the thousands of lakes of Finland and Canada |
| Erosion and deposition by rivers and the sea | Meanders cut off, or bars seal off a bay | Ox-bow lakes; coastal lagoons |
| Solution | Rainwater dissolves limestone into hollows that fill with water | Karst lakes in limestone regions |
| Human-made | A dam is built across a river to store water | Lake Volta (Ghana), Lake Nasser (Egypt and Sudan), Lake Kariba |
Worked example 1: how long does water stay in a lake?
Problem. An illustrative lake holds 500 km³ of water and loses 100 m³ s⁻¹ through its outlet river. Ignoring evaporation, estimate its residence time.
Solution. Convert both to the same units. One year has seconds.
A long residence time means pollution entering the lake stays for generations; this is why deep rift lakes are so sensitive to pollution.
Worked example 2: classifying a lake from evidence
Problem. A lake is long and narrow, more than 400 m deep, bordered by steep, straight escarpments, and lies in a line with other similar lakes. A second lake is broad and shallow, with an irregular, finger-like shoreline of flooded valleys. Classify each.
Solution. The first is a rift-valley lake: its depth, straight steep sides and alignment along a line of faults show a graben. The second is a downwarped or drowned-valley lake: gentle sagging flooded the lower ends of river valleys, giving shallow water and a ragged outline, as at Lake Kyoga.
The ocean covers about 71% of the Earth's surface and holds about 97% of its water (NOAA). The Pacific is the largest and deepest ocean; the Atlantic is the second largest. The ocean floor is not flat:
Figure 1: the relief of the ocean floor.
| No. | Feature | Description |
|---|---|---|
| 1 | Continental shelf | The gently sloping, submerged edge of a continent, to about 200 m deep; sunlight and nutrients make it the richest fishing ground; oil and gas are drilled here |
| 2 | Continental slope | The steeper descent from the shelf edge to the deep ocean |
| 3 | Abyssal plain | The flat, deep ocean floor, mostly 3,000–6,000 m deep, covered by fine sediment |
| 4 | Seamount | An underwater volcano that does not reach the surface |
| 5 | Mid-ocean ridge | An underwater mountain chain where plates move apart and new crust forms, such as the Mid-Atlantic Ridge |
| 6 | Ocean trench | A long, narrow, very deep trough where one plate sinks beneath another; the Challenger Deep in the Mariana Trench is nearly 11,000 m deep |
| 7 | Island arc | A curved line of volcanic islands beside a trench, such as the Mariana Islands and Japan |
Should a country upstream on a shared river, such as Ethiopia on the Blue Nile, need the agreement of countries downstream before building a dam? Consider the rights and needs of both sides.
Warning
Common mistakes
Tip
Exam technique. For "explain the formation of a lake", name the process (faulting, downwarping, damming by lava…), describe the resulting depression, and give a named example with one feature that proves its origin, such as the depth and straight sides of a rift lake.
Miacademy and MiaPrep Learning Channel: "Lakes – A Closer Look At Four Types of Lakes". As you watch, add an African example to each of the four types the video describes, using the table above.
Practice
Summary
Sources and credits
Photo: Shore of Lake Kivu – Karongi-Kibuye – Western Rwanda – 01 (Adam Jones, CC BY-SA 3.0), via Wikimedia Commons.
Video: "Lakes – A Closer Look At Four Types of Lakes", Miacademy and MiaPrep Learning Channel, embedded from YouTube.
Figure 1, the worked examples and the exercises were drawn and written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
