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Rivers, lakes, deserts and grasslands

World physical geography and map work · Part 2 / 3 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

Drainage basins and inland drainage; the major rivers of Asia, Europe, Africa, the Americas and Australia and the points examiners test about them; the record-holding lakes and the shrinking Aral Sea and Lake Chad; the four reasons deserts form and where the hot and cold deserts lie; and the tropical and temperate grasslands with their regional names.

Part 1 laid out the highlands; this part follows the water that drains from them and then turns to the places where water is scarce. Rivers matter for the exam because they carry borders, dams and disputes, and questions often ask which countries a river crosses or into which sea it drains. Lakes are tested for their records: the largest, the deepest, the highest and the ones that are shrinking. Deserts and grasslands are tested by their regional names and by the reasons they lie where they do.

Every river has a source, where it begins, and a mouth, where it ends in a sea, a lake or another river. The streams that join it are tributaries, and the whole area that sends its water into one river is its drainage basin; the high ground separating two basins is called a divide or watershed. Most rivers reach the ocean, but some basins have no outlet to the sea at all, which is called inland drainage. The Volga, Europe's longest river, ends in the Caspian Sea; the Amu Darya and Syr Darya of Central Asia flow towards the Aral Sea; and the Jordan ends in the Dead Sea. Water leaves such basins only by evaporation, so their final lakes become salty.

Asia's great rivers fan out from its central highlands. The Yangtze (Chang Jiang), the longest river in Asia, flows east across China to the East China Sea at Shanghai; the Three Gorges Dam on it is the world's largest hydroelectric station by installed capacity. Further north the Huang He, or Yellow River, carries so much yellow loess silt that its bed builds up and it has burst its banks many times, earning the name 'China's sorrow'. The Mekong rises on the Tibetan plateau and passes through or along six countries, China, Myanmar, Laos, Thailand, Cambodia and Vietnam, ending in a delta in southern Vietnam. In Siberia the Ob, the Yenisei and the Lena flow north to the Arctic Ocean. In West Asia the Tigris and the Euphrates rise in the mountains of Turkey, flow through Iraq and join as the Shatt al-Arab before entering the Persian Gulf.

Europe's rivers are short beside Asia's but carry heavy traffic. The Volga, rising north-west of Moscow, is the longest river in Europe and drains into the landlocked Caspian Sea. The Danube rises in the Black Forest of Germany and flows east to the Black Sea, passing through or along ten countries, more than any other river in the world, and through four capitals: Vienna, Bratislava, Budapest and Belgrade. The Rhine rises in the Swiss Alps, flows north through the rift valley described in Part 1, and reaches the North Sea through the Netherlands; with the port of Rotterdam at its mouth, it is one of the busiest inland waterways in the world.

The Nile, usually counted the longest river in the world, flows north through north-east Africa to the Mediterranean. Its two main branches meet at Khartoum in Sudan: the White Nile, which leaves Lake Victoria, and the Blue Nile, which rises at Lake Tana in the Ethiopian Highlands and brings most of the water and silt in the flood season. The Grand Ethiopian Renaissance Dam on the Blue Nile has been the subject of a long dispute between Ethiopia, Sudan and Egypt. The Congo, second only to the Amazon in the water it carries, crosses the equator twice. The Niger rises in the highlands of Guinea, flows north-east, then bends south-east to a large delta in Nigeria. The Zambezi plunges over the Victoria Falls on the border of Zambia and Zimbabwe before reaching the Indian Ocean in Mozambique.

The Amazon rises in the Andes of Peru and flows east to the Atlantic close to the equator; it carries more water than any other river and drains the largest basin on Earth. The Paraná flows south to join the Uruguay in the Río de la Plata estuary, and the Orinoco drains Venezuela into the Atlantic. In North America the Mississippi and its long tributary the Missouri form the continent's largest river system, ending in a bird's-foot delta on the Gulf of Mexico. The St Lawrence carries the outflow of the Great Lakes north-east to the Atlantic, and the Colorado has cut the Grand Canyon on its way to the Gulf of California. Australia's main system, the Murray-Darling, drains the dry south-east of that continent.

Lakes are mostly tested by their records. The Caspian Sea, between Europe and Asia, is the largest lake in the world by area; its water is salty and its surface lies below sea level. Lake Superior, on the US-Canada border, is the largest freshwater lake by area and the biggest of the five Great Lakes: Superior, Michigan, Huron, Erie and Ontario, of which only Michigan lies wholly within the USA. Lake Baikal in Siberia, a rift lake more than 1,600 m deep, is the deepest and oldest lake on Earth and holds about a fifth of the world's unfrozen surface fresh water. In Africa, Lake Victoria, shared by Uganda, Kenya and Tanzania, is the largest lake, and Tanganyika in the rift is the second deepest in the world. Lake Titicaca, on the Altiplano between Peru and Bolivia, is often described as the highest navigable lake.

Some lakes are in the news because they are shrinking. The Aral Sea, on the border of Kazakhstan and Uzbekistan, was once among the largest lakes in the world, but from the 1960s the Soviet Union diverted the Amu Darya and Syr Darya to irrigate cotton. Starved of inflow, the lake split into smaller remnants, leaving a salty, dusty former seabed and ruined fishing towns. Lake Chad, where Chad, Cameroon, Niger and Nigeria meet, has also shrunk sharply since the 1960s through drought and growing water use. The Dead Sea, fed by the Jordan, is many times saltier than the ocean, and its level keeps falling as the Jordan's water is drawn off upstream.

A desert is defined by scarce rain, not by heat: most receive less than about 250 mm a year. Four causes explain where deserts lie. First, between roughly 15° and 30° of latitude, air sinking in the subtropical high-pressure belts is warmed and dried, so the great hot deserts, the Sahara, the Arabian and the Australian deserts, lie in these belts. Second, cold ocean currents off western coasts chill the air above them so that it yields fog rather than rain; this is why the Atacama (beside the Peru current) and the Namib (beside the Benguela current) lie on west coasts. Third, mountains stop moist winds and leave a rain-shadow area behind them, as the Andes do for Patagonia. Fourth, places very far from the sea receive little moisture; such inland deserts with freezing winters are called cold deserts.

The Sahara, stretching across North Africa from the Atlantic to the Red Sea, is the largest hot desert; only the polar deserts of Antarctica and the Arctic are larger. The Arabian Desert continues the same dry belt across the Red Sea, and the Thar, on the border of India and Pakistan, lies at its eastern end. In southern Africa the Kalahari covers much of Botswana, while the narrow Namib runs along the coast of Namibia. The Atacama of northern Chile is the driest non-polar desert on Earth. North America has the Sonoran and Mojave, and Australia the Great Victoria and Great Sandy deserts. The Gobi of Mongolia and China and the Taklamakan of China's Tarim basin are cold deserts, far from the sea and walled off by mountains, while the Patagonian desert of Argentina lies in the rain shadow of the Andes. India's Ladakh is also a cold desert.

Grasslands grow where there is too little rain, or too long a dry season, for forest but enough for grass. In the tropics, between the rainforests and the hot deserts, lies the savanna: tall grass with scattered trees, a wet summer and a long dry winter. The best-known savannas are in East Africa; in South America the savanna of the Orinoco basin is called the Llanos and that of the Brazilian Highlands the Campos. In the middle latitudes, deep inside the continents, lie the temperate grasslands, with shorter grass, hot summers and cold winters. Each has a regional name: the Prairies of North America, the Pampas of Argentina and Uruguay, the Steppes from Ukraine across Russia to Kazakhstan, the Veld of South Africa, the Downs of Australia, the Canterbury plains of New Zealand and the Puszta of Hungary. Their deep, fertile soils have made the Prairies, Pampas and Steppes great wheat lands.

The four themes link up. Rivers rise in the highlands of Part 1 and either reach the sea or end in inland lakes such as the Caspian and the Aral. Belts of climate then decide what covers the land: rainforest near the equator, savanna on either side of it, hot deserts under the subtropical highs and along cold-current coasts, temperate grasslands deep in the continents, and cold deserts where mountains and distance shut out the rain. The last part of the chapter moves out to sea, to the seas, straits, canals and chokepoints that link these lands.

Exam traps

• The Volga, Europe's longest river, flows into the Caspian Sea, not the Black Sea; the Danube is the one that reaches the Black Sea.
• The Danube passes through or along ten countries, more than any other river; the Mekong passes through or along six.
• The Blue Nile rises at Lake Tana in Ethiopia and the White Nile leaves Lake Victoria; they meet at Khartoum, not Cairo.
• It is the Congo that crosses the equator twice; the Amazon is the river that carries the most water and drains the largest basin.
• The Caspian is the largest lake by area but is salty; Superior is the largest freshwater lake by area, and Baikal the deepest and largest by volume of fresh water.
• Of the five Great Lakes only Lake Michigan lies wholly within the USA; the others are shared with Canada.
• A desert is defined by low rainfall, not heat: Antarctica is the largest desert, and the Gobi, Taklamakan and Ladakh are cold deserts.
• The Atacama and the Namib are coastal deserts on western coasts, kept dry by cold currents (Peru and Benguela); Patagonia is dry because of the Andean rain shadow.

Recap

• Source, mouth, tributary, drainage basin, divide; inland drainage: Volga to the Caspian, Amu Darya and Syr Darya to the Aral, Jordan to the Dead Sea.
• Asia: Yangtze (longest in Asia, Three Gorges Dam), Huang He ('China's sorrow'), Mekong (six countries), Ob, Yenisei and Lena (Arctic), Tigris and Euphrates (Shatt al-Arab).
• Europe: Volga (longest, to the Caspian), Danube (Black Forest to the Black Sea, ten countries, four capitals), Rhine (Alps to the North Sea, Rotterdam).
• Africa: Nile (White from Lake Victoria, Blue from Lake Tana, meet at Khartoum; GERD dispute), Congo (crosses the equator twice), Niger (delta in Nigeria), Zambezi (Victoria Falls).
• Americas and Australia: Amazon (largest flow and basin), Paraná, Orinoco, Mississippi-Missouri, St Lawrence, Colorado (Grand Canyon), Murray-Darling.
• Lakes: Caspian (largest, salty), Superior (largest freshwater), Baikal (deepest, oldest), Victoria (largest in Africa), Tanganyika (second deepest), Titicaca (high, navigable).
• Shrinking: Aral Sea (cotton irrigation from the 1960s) and Lake Chad; the Dead Sea level is falling.
• Deserts form under subtropical highs, beside cold currents, in rain shadows and far inland; hot: Sahara, Arabian, Thar, Kalahari; coastal (cold current): Namib, Atacama; cold: Gobi, Taklamakan, Patagonia, Ladakh.
• Grasslands: savanna, Llanos and Campos (tropical); Prairies, Pampas, Steppes, Veld, Downs, Canterbury and Puszta (temperate).

Practice questions

Sources

• USGS Water Science School: rivers, watersheds and drainage basins, and the world's largest rivers by discharge (US government) — https://www.usgs.gov/special-topics/water-science-school
• NASA Earth Observatory: satellite images and explainers on world rivers and lakes, including the shrinking Aral Sea, Lake Chad, the Dead Sea, Lake Baikal, the Three Gorges Dam and the Nile; and 'Mission: Biomes' (desert and grassland biomes) (US government) — https://earthobservatory.nasa.gov
• USGS publication 'Deserts: Geology and Resources' (A. S. Walker): definition of deserts, causes of aridity, and the major deserts of the world (US government) — https://pubs.usgs.gov/gip/deserts/
• NOAA Great Lakes Environmental Research Laboratory and National Ocean Service: the five Great Lakes, their connecting rivers and the St Lawrence outflow (US government) — https://www.glerl.noaa.gov
• NOAA National Weather Service, JetStream online school: global circulation, the subtropical high-pressure belts and ocean influences on climate (US government) — https://www.noaa.gov/jetstream/global