Rivers · KS3 Geography

The drainage basin

A drainage basin is the area of land drained by a river. Its edge is the watershed.
The source is where a river begins, a tributary is a smaller river that joins it, a confluence is where two rivers meet, and the mouth is where it reaches the sea or a lake.
In the upper course, the river is steep with a V-shaped valley. In the middle course, it meanders across a wider valley. In the lower course, it is wide and slow, with a flat floodplain and an estuary.

Erosion: hydraulic action (the force of the water), abrasion (stones scraping the bed and banks), attrition (stones knocking together and becoming smaller and rounder) and solution (rocks such as limestone dissolving).
Transport: traction (large boulders rolled along the bed), saltation (small stones bouncing), suspension (fine material carried in the water) and solution (dissolved material).
Deposition happens when the river slows down and drops what it is carrying.

Waterfalls form where hard rock lies over softer rock. The soft rock is eroded faster, making a plunge pool and an overhang, which collapses. The waterfall moves back upstream, leaving a gorge.
Meanders: the water flows fastest on the outside of a bend, eroding a river cliff, and slowest on the inside, depositing a slip-off slope.
Oxbow lakes form when a river cuts through the neck of a meander, usually during a flood, and the old bend is cut off.
Floodplains and levees (raised banks) form from deposition during floods.

Physical causes: heavy or long rainfall, snowmelt, steep slopes and saturated or impermeable ground.
Human causes: building (tarmac and concrete are impermeable) and deforestation make water reach the river faster.
Hard engineering: dams, embankments and flood walls. Soft engineering: planting trees, not building on floodplains and letting some land flood (washlands).