Which is the longest river in the world?

The Nile is the traditional answer at about 6,650 kilometres, running from the highlands of Burundi and Rwanda through eleven countries to the Mediterranean. The Amazon is usually given as a few hundred kilometres shorter, and the gap is small enough that it has never really been settled.

By volume there is no contest at all. The Amazon discharges more water than the next seven largest rivers combined, something like a fifth of all the river water reaching the sea, and its outflow freshens the Atlantic for hundreds of kilometres offshore. In the wet season it is over forty kilometres wide in places.

Egypt is often described as the gift of the Nile, and the phrase is close to literal. Before the Aswan High Dam the river flooded every summer and left a layer of silt that made the valley farmable in the middle of a desert - which also made the Egyptian year, the Egyptian calendar and Egyptian surveying, since flooding erased field boundaries and they had to be measured out again annually.