Great rivers and deserts

The Nile and the Amazon differ in length by about 214 kilometres, and how you measure a river decides which one comes first.

The longest rivers

The World Factbook lists the Nile in Africa as the longest river at 6,650 kilometres, followed by the Amazon in South America at 6,436 kilometres. Third is the Yangtze in China at 6,300 kilometres, and fourth the Mississippi and Missouri together in North America at 6,275 kilometres. Asia then takes most of the rest of the top ten: the Yenisey and Angara, the Huang He or Yellow River, the Ob and Irtysh, the Amur and the Lena. The Congo, at 4,700 kilometres, is the only other African river in the list.

Why do some books put the Amazon first? A river’s length depends on choices. Where exactly is the source: the farthest trickle of the farthest tributary, or the spring local people name? Where is the mouth, when the Amazon ends in a wide estuary full of islands? Do you follow the main channel or the longest one through a delta? Some river names also join two rivers into one, as the Mississippi and Missouri do. Different surveys make different choices, so some atlases and encyclopedias put the Amazon first.

Rivers carry rain and snowmelt from the land to the sea. The area that drains into one river is its watershed, or drainage basin. The Factbook notes that the oceans take in the drainage from 83 percent of all land. The other 17 percent drains into inland basins with no outlet, where water collects in lakes or dries up. The Caspian Sea is the largest of these inland lakes and the largest lake of any kind, at about 374,000 square kilometres; it is salty, while the next largest, Lake Superior at 82,100 square kilometres, is fresh.

What makes a desert

A desert is defined by dryness, not by sand or heat. USGS describes deserts as arid land with little rainfall that supports only sparse plants and few people and animals, and says about one third of Earth’s land surface is desert. A widely used system from 1953, by the geographer Peveril Meigs, sorts dry land by precipitation. Extremely arid land goes at least 12 months in a row without rain. Arid land gets less than 250 millimetres a year. Semiarid land gets 250 to 500 millimetres and is usually called steppe, not desert.

By that rule the largest desert on Earth is Antarctica, at about 14.2 million square kilometres, and the Arctic is the other great polar desert. The largest hot desert is the Sahara, which National Geographic puts at about 9.4 million square kilometres, stretching across North Africa from the Atlantic to the Red Sea. Cairo, Tripoli and Timbuktu all lie within it. Many hot deserts, including the Sahara, sit near the Tropics of Cancer and Capricorn, where sinking dry air keeps rain away.

The driest places on the Factbook’s list show how extreme deserts can be. The McMurdo Dry Valleys in Antarctica average no measurable precipitation at all. Arica, a port city in the Atacama Desert of Chile, averages 0.76 millimetres a year, less than the thickness of a coin. At the other end, Mawsynram in northeast India gets about 11,871 millimetres, more than 15,000 times as much as Arica.