Chapter 5: Landforms Made by Running Water
This chapter focuses on the role of rivers as the most important agent of denudation. It describes how a river shapes the landscape from its source in the mountains to its mouth at the sea.
1. The Mechanism of River Erosion
Rivers wear away the land through four main processes:
- Corrasion (Abrasion): The mechanical grinding of the river’s load (rocks, pebbles) against the banks and bed.
- Corrosion (Solution): The chemical solvent action of water on soluble rocks like limestone.
- Hydraulic Action: The sheer force of moving water breaking off rock fragments.
- Attrition: The wear and tear of the transported rocks themselves as they collide and break into smaller, rounded particles.
2. The Upper or Youthful Stage (Mountain Course)
In this stage, vertical erosion is dominant as the river flows down steep slopes.
- V-Shaped Valleys: Deep, narrow valleys carved by rapid vertical erosion.
- Gorges and Canyons: Very steep-sided, narrow valleys (e.g., the Grand Canyon).
- Waterfalls and Rapids: Formed when a river falls over a resistant rock ledge.
- Potholes: Circular depressions in the river bed ground out by swirling pebbles.
- River Capture (Piracy): When a more powerful river cuts back and “steals” the headwaters of a neighbor.
3. The Middle or Mature Stage (Valley Course)
The gradient is gentler, and lateral (sideways) erosion begins to widen the valley.
- Meanders: The river begins to swing in loops.
- River Cliffs and Slip-off Slopes: Water cuts into the outer bank (cliff) and deposits sediment on the inner bank (slip-off slope).
- Interlocking Spurs: Projections of high ground which the river meanders around.
4. The Lower or Old Stage (Plain Course)
The river moves slowly across a flat plain; deposition is now the primary activity.
- Floodplains: Large, flat areas of fertile alluvium deposited during floods.
- Ox-bow Lakes: A meander loop that has been cut off from the main river channel.
- Levees: Raised banks formed by the deposition of heavy sediment along the river edges during floods.
- Deltas: Fan-shaped alluvial deposits at the river mouth (e.g., Ganges Delta, Nile Delta). Types include:
- Fan-shaped (Arcuate)
- Bird’s-foot
- Estuarine
5. Quick Revision Table: River Features
| Stage | Main Process | Key Landforms |
|---|---|---|
| Youth (Upper) | Vertical Erosion | V-shaped valleys, Gorges, Waterfalls |
| Maturity (Middle) | Lateral Erosion | Meanders, Wider Valleys |
| Old Age (Lower) | Deposition | Ox-bow lakes, Levees, Deltas |
[!NOTE]
Key Fact: The Base Level is the lowest level to which a river can erode its bed, which is usually sea level.
