A river starts at its source, typically in mountains or highlands, and ends where it empties into a larger body of water like an ocean, sea, or lake. The entire journey is driven by one force: gravity pulling water from higher elevation to lower elevation. But the specifics of how rivers begin and where they terminate are more varied and surprising than most people realize.
How a River Begins
The starting point of a river is called its source, or headwaters. Rivers can begin in several ways, and most are fed by a combination of these:
- Rainfall and snowmelt. Rain falling on steep slopes runs across the surface and collects into small channels called rills. Melting snow and ice do the same. These tiny streams merge into larger ones, eventually forming a river. Precipitation is the dominant source of water for most rivers on Earth.
- Springs. In places where underground water rises to the surface, a spring can serve as a river’s starting point. Some rivers in volcanic or geologically active regions are even fed partly by hot springs and geysers pushing deep groundwater upward.
- Glaciers. Massive ice fields slowly melt at their edges, producing steady flows of cold water that become the headwaters of rivers. Many major rivers in Asia, South America, and Europe trace their origins to glacial melt.
- Lakes. A river’s source can be a lake with an outflowing stream. Lake Victoria in East Africa, for instance, is often cited as where the Nile begins.
Most rivers in mountains or hills start as a combination of these. Rain and snowmelt collect on slopes, trickle into small streams, and those streams merge with others flowing downhill. As small creeks flow downhill, they join to form larger streams and eventually a recognizable river.
Pinpointing the Exact Source
Finding the precise spot where a major river “starts” is trickier than it sounds. A large river’s watershed can have hundreds of springs, lakes, and tributaries all draining into the main channel. The convention is to trace the river back to the tributary that is farthest from the river’s end. That most distant point of flowing water gets designated as the source.
Even with that rule, disagreements are common. The source of the Amazon River remains contested. Expeditions in the 20th and early 21st centuries pointed to Nevado Mismi, a mountain in the Peruvian Andes, as the Amazon’s most distant source, but not everyone agrees. The Nile has a similar debate. Lake Victoria is its most famous source, but the lake itself is fed by the Kagera River, whose longest headstream traces back to highlands between Congo and Rwanda near Lake Tanganyika. Depending on which tributary you follow, the Nile’s “true” starting point shifts by hundreds of miles.
What Happens Along the Way
Between its source and its end, a river picks up water from an entire network of smaller streams called tributaries. These branching arms extend outward toward the edges of the river’s drainage basin, which is the total land area that funnels water into that river system. Every raindrop that falls within a drainage basin eventually flows toward the same river.
Where a tributary meets the main river is called a confluence. At each confluence, the river grows larger and carries more water. The Mississippi, for example, collects water from tributaries draining 31 U.S. states and two Canadian provinces before reaching the Gulf of Mexico. The shape and size of a drainage basin, along with how many tributaries feed into it, determine how powerful a river becomes by the time it reaches its end.
The land doesn’t need to be dramatically steep for this to work. In most landscapes, the ground slopes downhill in some direction, and flowing water finds its way along that slope. Rivers don’t always flow south, a common misconception. They flow whichever direction gravity takes them, which depends entirely on the shape of the terrain.
How a River Ends
The point where a river meets its final destination is called the mouth. Most rivers end by emptying into an ocean or sea, but the way they do it varies.
Deltas form when a river carries large amounts of sediment and deposits it at the mouth faster than ocean currents can wash it away. The sediment builds up into a fan-shaped landmass that the river splits across in multiple channels. The Ganges-Brahmaputra, Nile, Mekong, Yangtze, Amazon, Irrawaddy, and Mississippi deltas are among the world’s largest, with a combined area of about 265,000 square kilometers. These seven alone account for roughly 57% of all subsiding delta area on the planet.
Estuaries form where river water mixes with tidal saltwater, creating a brackish (partly salty) zone. This happens in regions where the coastline allows ocean water to push inland and mingle with the outflowing river. Estuaries are incredibly productive ecosystems because of the mix of nutrients from both freshwater and saltwater sources. Chesapeake Bay on the U.S. East Coast is a classic example.
Some river mouths are enormous. The Amazon pours an estimated 157,000 cubic meters of water per second into the Atlantic Ocean, a volume so immense that its peak flood flow has been measured at roughly 350,000 cubic meters per second. The tidal influence of the Atlantic actually reaches hundreds of kilometers upstream during low flows.
Rivers That Never Reach the Sea
Not every river ends at an ocean. About 20% of the world’s land area drains into what are called endorheic basins, where rivers terminate on land rather than connecting to the sea. These rivers end at inland lakes or dry depressions, and their water is lost to evaporation rather than mixing with ocean water.
Most endorheic basins sit in arid or semi-arid regions where sand dunes or other landforms block outlet passages to the coast. Central Asia is full of them. The Amu Darya and Syr Darya rivers, for example, flow into the Aral Sea (or what remains of it) rather than any ocean. The Jordan River ends at the Dead Sea, one of the saltiest bodies of water on Earth, with no outlet. In Australia, the rivers of the Lake Eyre basin flow inward toward the continent’s dry center, and in many years they evaporate before reaching the lake at all.
These inland terminations are a reminder that a river’s “end” is defined by where its water can no longer flow downhill. Sometimes that’s a coastline. Sometimes it’s a desert basin where the water simply runs out of momentum and disappears into the ground or the air.