Edexcel iGCSE > River Environments > Fluvial processes
Rivers have been instrumental in shaping the landscape of drainage basins, primarily through the fluvial erosion, transportation, and deposition processes. However, weathering and mass movement are two additional vital processes shaping the landscape in drainage basins.
Weathering is the in-situ (without movement) breakdown of rocks. Examples include:
- Physical weathering, like freeze-thaw action
- Chemical weathering due to slightly acidic rainwater interacting with rocks
- Biological weathering, especially by plant and tree root systems gradually splitting rocks, also plays a vital role in drainage basins.
Mass movement involves the large-scale displacement of weathered materials under gravity. This movement supplies weathered material to rivers, which:
- Adds more material to a river’s load
- Increases erosion in the upper course
- Enhances deposition in the middle and lower courses. The two primary types of mass movement in drainage basins are slumping and soil creep.
Energy and Processes
- Approximately 95% of a river’s energy is expended to overcome friction.
- The remaining 5% is used for the erosion of the river channel and the transportation of eroded material downstream. A river’s energy levels depend on a) the volume of water in the river and b) the speed of the water flow.
- Most friction occurs when the water contacts the bed and the banks. Rocks and boulders on the bed increase the amount of friction.
River channels are shallow and narrow near the source, with beds often cluttered with boulders and highly uneven, resulting in high friction and slower water flow than downstream where the channel is a) wider, b) deeper, and c) less uneven.
There are four key erosion processes:
- Hydraulic action: The removal of loose material from the river bed and banks by the sheer force of the river water.
- Abrasion: The wearing down of the bed and banks by the river’s load.
- Attrition: The process where rocks and stones collide with each other and the bed and banks in swirling water, gradually becoming smoother and smaller.
- Solution: Certain rocks like limestone slowly dissolve in river water.