1.13 - River Processes
Weathering processes affecting river valleys and channels
Weathering plays a critical role in shaping river landscapes by breaking down rocks along valley sides and within river channels. This process weakens materials, making them more susceptible to erosion and movement.
Types of weathering in river environments
- Mechanical weathering - This involves the physical breakdown of rocks without altering their chemical structure. A key example is freeze-thaw weathering, where temperatures fluctuate around freezing point. Water enters cracks in rocks, freezes, and expands, exerting pressure that widens the cracks over repeated cycles.
- Chemical weathering - Rocks are broken down through changes in their chemical composition. Rainwater, being slightly acidic, reacts with certain minerals in rocks, leading to their dissolution in a process called carbonation weathering. Additionally, some minerals are soluble in water and are washed away through dissolution weathering.
- Biological weathering - Living organisms contribute to rock breakdown. For instance, plant roots grow into cracks in rocks, exerting force that splits them apart.
Mass movement and its impact on river landscapes
Mass movement refers to the downslope transfer of material due to gravity. It often occurs on valley sides and can reshape landscapes over time.
Mechanisms and causes of mass movement
- Types of mass movement - Two primary forms are slides, where material moves in a straight line down a slope, and slumps, where material shifts with a rotational motion.
- Conditions triggering mass movement:
- Undercutting by rivers - When a river erodes the base of a valley side, it destabilises the slope above.
- Water saturation - Heavy rainfall saturates valley sides, acting as a lubricant that reduces friction.
- Prior weathering - Weathering processes loosen material on slopes, increasing the likelihood of mass movement.
- Impact on rivers - Mass movement adds substantial amounts of material to the river's load.
Erosion processes within river channels
Erosion in river channels is the process by which water and transported materials wear away the bed and banks. The extent of erosion is largely influenced by the river's speed, with faster flows causing more significant wear.
Key processes of river channel erosion
- Hydraulic action - The force of water breaks rock particles away from the river channel.
- Abrasion - Rocks carried by the river scrape against the channel surfaces, wearing them down.
- Attrition - Collisions between transported rocks cause them to break into smaller, smoother fragments.
- Solution - Certain rocks, such as limestone and chalk, dissolve in river water.
Transportation mechanisms in rivers
Transportation involves the movement of eroded material along the river. The method of transportation depends on the size and weight of the particles.
Methods of sediment transportation in rivers
- Traction - Large particles like boulders are pushed along the river bed by the force of the water.
- Saltation - Medium-sized particles, such as pebbles, bounce along the river bed.
- Suspension - Fine particles, including silt and clay, are carried within the water column.
- Solution - Dissolved materials are transported within the water itself.
Deposition and the conditions under which it occurs
Deposition is the process by which a river releases the material it has been carrying. It happens when the river's ability to transport sediment diminishes.
Factors leading to deposition in rivers
- Loss of velocity - Deposition occurs when a river slows down, reducing its capacity to carry sediment.
- Specific conditions causing reduced velocity:
- Decreased water volume - Lower water levels result in slower flow speeds.
- Increased sediment load - A higher amount of eroded material can overwhelm the river's transport capacity.
- Shallower water - In areas like the inside of a river bend, shallower depths cause the river to slow.
- River mouth - As a river reaches its endpoint, such as a lake or sea, its speed drops significantly.