River Sediment/Load can be classified into:
- Clay < 1/256
- Silt:1/16mm-1/256mm
- Sand:1/16-2mm
- Gravel:2mm-4mm
- Pebbles:4mm-16mm
- Cobbles:16mm-256
- Boulders:> 256mm
Boulders,Cobbles,Pebbles and Gravels are classified as Gravel whereas Silt and Clay is classified as Mud Most erosion in rivers occurs in the upper course where steep gradients increase the rivers velocity and in turn energy to dislodge material.Huge velocities usually after heavy storms are capable of lifting large boulders.
Corrasion / Abrasion
Corrasion involves large rocks(cobbles) rubbing against the bed and sides of a river. This mechanism creates small holes called potholes on the rivers bed.
Eddies and turbulent currents fuel the cobbles to start rotating, drilling the bed in the process. Strong turbulent currents can enlarge these potholes to become large holes.
Attrition
Attrition involves rocks bumping and colliding with each other causing them to become smaller and smoothened. This erodes the load in the river rather than the river itself.
Hydraulic Action
Strong currents in a river dislodges soil and rock particles on the sides of a river.
Cavitation
Air bubbles penetrates into tiny pore spaces on the sides of a river were upon popping creates small shockwaves or vibrations which dislodges soil or rock particles. With time the sides of a river gradually wear down. This may be hard to bear but it works in eroding rivers especially the banks.
Corrosion/Solution
Involves the dissolving of certain rocks in the river. Rocks such as limestone are susceptible to solution.
Three distinctions of river erosion
Headward Erosion
Involves a river cutting back at its source e.g. the underlying soft basaltic rocks along Victoria Falls have caused the river to recede backwards for the past years creating a gorge nearby. Headward erosion mainly occurs in the upper course near the source e.g along a limestone terrain.
Vertical Erosion
Involves a river cutting down its bed. Vertical erosion creates steep sided and narrow valleys e.g. gorges. When the bed of a river is resistant the main type of erosion is abrasion which involves the drilling of the hard surface. Vertical erosion is common in the upper course where a river is fast and efficeint.
Lateral Erosion
Is when a river erodes sideways causing widened streams. Occurs around middle and lower courses.
Transportation
Transportation depends on load and velocity. Large cobbles and boulders are not easy to lift than small pebbles and sand therefore rolled along the river’s bed.If velocity is high enough then the river is capable of lifting and moving the load even large boulders can be lifted.
Saltation
This involves the bouncing or leaping of particles in the river. When a pebble or a cobble is lifted it dislodges and disturbs the inertia of resting particles causing them to move.
Suspension
Small particles are lifted up and transported while in the air.
Traction
Large boulders are too heavy to be uplifted, therefore rolled along the bed.
River Processes
Definitions
A river’s capacity is a measure of how much load it can hold and carry.
Competence is the maximum size of a sediment that can be carried by a river.
Deposition
Deposition of a river occurs in the lower course where the load is too big to be carried anymore by the river.
The sediment will be larger than the river’s velocity.
Velocity and energy is reduced by:
- River flowing on a flat terrain
- Obstacles such as shrubs
- River flowing along permeable terrain so that water percolates down
- River flowing through arid regions
- Permanent bodies of water e.g. an ocean which cause a river to deposit it’s load.
Four types of deposition are:
- Channel Deposition(e.g. of boulders)
- Valley Margins deposition(eg colluvium).
- Floodplain deposition(eg levees)
- Channel margin deposition (e.g point bars along inner bends of meanders).
