Where does a river begin, and which one is the longest?

A river begins wherever rain, melting snow or spring water first gathers into a channel that keeps flowing. That spot is called the river's headwater, or source. There is no single magic moment: rainwater, meltwater and spring water all collect into small streams, and the streams join into bigger ones.

Every river drains a patch of land called a drainage basin: the area where all the surface water runs toward one point, such as a river mouth. Ridges and hills form the edge of the basin, the drainage divide, and neighbouring basins sit on the far side of it. In North America a drainage basin is usually called a watershed. Inside a big basin there are smaller basins that merge where rivers meet, like twigs joining branches.

Two familiar starting points are springs and glaciers. A spring is a place where groundwater emerges from an aquifer and flows across the surface. Many glaciers store water as ice in the cold months and release it as meltwater in summer, which matters when other sources are scarce. Glacial ice, together with the ice sheets, holds about 69 percent of the world's freshwater, so what melts off the high ground is a big part of the story.

Where a river begins is also a surprisingly fierce argument. Geographers usually define a river's source as the most distant point in its basin from which water runs all year round. By that rule the Nile starts not at Lake Victoria but further away, in a tributary of the Rukarara River in Rwanda's Nyungwe National Park. The Amazon's source was long placed on a Peruvian peak called Nevado Mismi, until a 2014 study pointed to a different headwater. Because the two rivers' sources are so hard to pin down, the contest over which is longer is still disputed.