The longest river depends on where it starts
The NILE and the AMAZON have argued over the title of the world's longest river for years. The Nile is usually given as about 6,650 kilometres, and the Amazon about 6,400, but some researchers who traced the Amazon back to a different SOURCE high in the Andes of Peru have measured it as longer than the Nile. The problem is that it is surprisingly hard to decide exactly where a river begins, among all its streams, and where it ends in a maze of channels at the MOUTH.
There is no argument about size, though. The Amazon carries more water than the next several biggest rivers combined, about a fifth of all the fresh water that flows from the world's rivers into the sea. And although millions of people live along it, not a single bridge crosses the main river anywhere along its length.
What is going on in a river
A river is always rearranging its own valley. Water flows fastest on the outside of a bend, wearing the bank away, and slowest on the inside, where it drops sand and mud. Over years the bends grow bigger and loopier, and that is how a river comes to MEANDER. Sometimes a loop grows so tight that the river cuts straight across its neck in a flood, leaving the old loop behind as a curved oxbow lake.
Near its source a river is usually steep, narrow and fast, tumbling over RAPIDS and carving downwards. Further on, joined by each TRIBUTARY, it slows and spreads out. At the end it may widen into an ESTUARY, where fresh water meets the salty tide, or drop so much mud that it builds a DELTA, a fan of land split into many channels; the Nile delta and the MEKONG delta are both vast farmland. Every BANK and CURRENT is part of the same system. Rivers are also journeys for animals: our fishing page covers the salmon that swim up them to breed.
Rivers that cross borders and deserts
The DANUBE flows through or along the borders of ten countries, more than any other river in the world, and passes through four capital cities: Vienna, Bratislava, Budapest and Belgrade. The Nile flows north, through desert, and nearly all of Egypt's people live along its banks; our ancient Egypt page covers the civilisation it made possible. The CONGO is the deepest river known, with parts more than 220 metres deep. The NIGER begins only about 240 kilometres from the Atlantic Ocean, then flows inland, away from the sea, for thousands of kilometres before curving round to reach it.
The THAMES in London used to freeze over in the coldest winters of the 1600s and 1700s, and Londoners held frost fairs on the ice, with stalls, games and even an elephant walked across it in 1814. The river flows faster now, partly because the old London Bridge, with its many narrow arches, was replaced, and it has not frozen in the city centre since. The GANGES, YANGTZE, VOLGA, RHINE, TIGRIS and ZAMBEZI complete the grid.