1.15 - How Erosion & Deposition form River Landforms
The role of erosion and deposition in river landform creation
Rivers shape the landscape through two primary processes: erosion and deposition. These processes work together to create distinctive landforms, particularly in the middle and lower courses of a river.
Processes shaping river landforms
- Erosion - This involves the wearing away of river banks and beds by the force of water and the materials it carries. It is more dominant in high-energy environments where the current is stronger.
- Deposition - This occurs when the river loses energy and drops the material it is carrying, often in low-energy environments where the current slows down.
The development and characteristics of meanders
Meanders are large, looping bends in a river, typically found in the middle and lower courses where the river channel has both shallow and deep sections. Their formation results from the interplay of erosion and deposition, influenced by differences in water velocity across the river channel.
How meanders form through erosion and deposition
- Velocity differences - Water flows faster on the outside of a bend due to deeper channels and less friction, creating a high-energy environment. Conversely, it flows slower on the inside of a bend due to shallower channels and more friction, resulting in a low-energy environment.
- Erosion on the outside - The higher velocity on the outside of bends leads to significant erosion through:
- Abrasion - The wearing away of surfaces by particles carried in the water.
- Hydraulic action - The direct force of water against the river bank.
- Deposition on the inside - The slower current on the inside of bends causes deposition of eroded materials, forming gentle slopes.
- Resulting features - Erosion carves steep-sided river cliffs on the outside of bends, while deposition creates crescent-shaped features called point bars (also known as slip-off slopes) on the inside, made of sand or stones.
The processes forming ox-bow lakes from meanders
Over time, the continuous processes of erosion and deposition can transform meanders into isolated water bodies known as ox-bow lakes. This occurs through a specific sequence of events, often accelerated during high-flow conditions such as floods.
Stages in the formation of ox-bow lakes
- Growth of meanders - Meanders gradually become larger and more pronounced as erosion continues on the outside of bends.
- Narrowing of land - Erosion causes the outside bends of adjacent meanders to move closer together, leaving only a thin strip of land, known as the neck, between them.
- Breakthrough during floods - During periods of high water flow, such as floods, the river may breach this narrow neck, taking the shortest path.
- New river course - The river now flows along this new, straighter route, bypassing the old meander loop.
- Cutoff by deposition - Deposition eventually seals off the ends of the abandoned meander loop, isolating it from the main river channel.
- Formation of ox-bow lake - The cutoff loop becomes a standalone, curved water body known as an ox-bow lake.
Key terminology related to river landforms
Understanding the specific terms associated with river landforms helps in identifying and describing the features created by erosion and deposition processes.
Important terms and alternative names
- Point bars (slip-off slopes) - Crescent-shaped deposits of sand or stones on the inside of meander bends, formed by deposition.
- River cliffs - Steep, eroded banks on the outside of meander bends, shaped by high-energy water flow.
- Ox-bow lakes (billabongs) - Isolated, curved lakes formed from abandoned meander loops, with the alternative term 'billabongs' used in Australia.