Rivers
Water finding the sea, and reshaping the land on the way.
A river is channelled surface water flowing downhill under gravity. Its drainage basin is all the land whose water drains into it, separated from neighbouring basins by a ridge called the watershed.
The long profile
From source to mouth a river typically steepens least and widens most. The upper course is steep and narrow with vertical erosion dominant; the middle course widens as lateral erosion takes over; the lower course is broad, gentle and depositional.
| Course | Dominant process | Landforms |
|---|---|---|
| Upper | Vertical erosion | V-shaped valley, waterfalls, gorges, rapids |
| Middle | Lateral erosion | Meanders, river cliffs, slip-off slopes |
| Lower | Deposition | Floodplain, levees, oxbow lakes, delta |
Erosion and transport
Rivers erode by hydraulic action, abrasion, attrition and solution, and carry load by traction, saltation, suspension and solution. Load size drops downstream as attrition rounds and shrinks the particles — which is why headwater boulders become lowland silt.
Meanders and oxbows
Flow is fastest on the outside of a bend, eroding a river cliff; slowest on the inside, depositing a slip-off slope. The bend migrates until the neck is cut through in a flood, abandoning the loop as an oxbow lake.