30.0°N 31.2°ECairo, on the Nile
The parts of a river
- Source
- Where the river begins: a spring, a lake, a glacier or a boggy upland.
- Tributary
- A smaller river or stream that joins a larger one.
- Confluence
- The point where two rivers meet.
- Channel
- The bed and banks that hold the flowing water.
- Floodplain
- Flat land beside the river that floods from time to time.
- Mouth
- Where the river ends, in the sea, a lake or another river.
Drainage basins and divides
A drainage basin, also called a watershed in American usage, is all the land that drains into one river. The boundary between basins is a divide, usually a line of high ground. In North America, the Continental Divide runs along the Rocky Mountains, splitting water headed for the Pacific from water headed for the Atlantic and the Gulf of Mexico. The Mississippi basin alone drains roughly 40 percent of the lower 48 states.
Upper, middle and lower course
| Stretch | What the river does | Features you see |
|---|---|---|
| Upper course | Steep and fast, cutting down into rock | V-shaped valleys, waterfalls, rapids |
| Middle course | Gentler, wider, eroding sideways | Meanders, wider valley floors |
| Lower course | Slow and broad, dropping sediment | Floodplains, levees, oxbow lakes, deltas, estuaries |
Meanders and oxbow lakes
Rivers on flat land rarely run straight. Water flows fastest around the outside of a bend, eroding that bank, and slowest on the inside, where it drops sand and gravel. Over time the bends grow loopier. Eventually the river cuts across the narrow neck of a loop during a flood, and the abandoned loop becomes a curved oxbow lake.
Deltas and estuaries
When a river reaches still water it slows and drops its load of sediment. If the sea's waves and tides cannot carry the sediment away, it builds out into a delta, like those of the Nile and the Mississippi. Where tides are strong, the river mouth instead becomes a wide estuary where fresh and salt water mix.
Why river lengths are disputed
The Nile and the Amazon are the two longest rivers, each roughly 6,400 to 7,000 km (about 4,000 to 4,300 miles) depending on the source used. Which is longer is still argued, because the answer depends on which stream counts as the true source, where the mouth is drawn, and how finely the bends are measured. The Amazon is far larger by volume and carries something like a fifth of all the river water that reaches the oceans.
River patterns on a map
Look at a river network on a map and its shape tells you about the land. A branching, tree-like pattern suggests fairly uniform rock. Streams radiating out in all directions suggest a dome or volcano in the middle. A pattern of parallel streams joined at right angles suggests alternating bands of hard and soft rock. On a topographic map, the V's in the contour lines point up every valley.
Floods and floodplains
Floodplains are flat and fertile, which is why so many towns and farms are built on them, and they flood because that is how they formed. If you are choosing somewhere to stay or live near a river, check the official flood maps for that area.
River FAQ
Do all rivers flow south?
No. Rivers flow downhill, in whatever direction that is. The Nile flows north, and so do several large rivers in Russia and Canada.
What is the difference between a river and a stream?
Mostly size, and there is no fixed rule. Small flowing waters are usually called streams, creeks or brooks, and larger ones rivers.
Can a river run dry?
Yes. Some rivers in dry climates flow only after rain, and heavy use of water for farming and cities can reduce others so much that they no longer reach the sea in some years.