River channel processes and landforms Flashcards
Erosion
Wearing away and removal of material from the river bank and beds
It takes up the most energy
Transportation
Moving of sediment
Deposition
Occurs when a river is no longer competent or has lost the capacity to carry its load
Any reduction in river velocity will mean material being deposited, starting with the coarsest sediment as this requires much energy to remain in suspension
It takes up the least energy
River’s energy
Around 95% of a river’s energy is lost to friction
Energy is spent on flow, then transport, then finally erosion
Energy levels in a river vary spatially and temporally
What the amount of energy in a river depends on
The vertical distance to sea level at any point in a river’s course (potential energy)
The volume and velocity of river flow - as they flow they convert potential energy to kinetic energy
Hydraulic Action
The force of air and water on the sides of rivers and in cracks
Evorsion (type of hydraulic action)
The sheer force of water eroding the bank and the bed by swirling vortices of water
Cavitation (type of hydraulic action)
The turbulent water hits current river banks and pushes water into cracks
The air in cracks is compressed, pressure increases and in time the rock will collapse
Most dominant form of hydraulic action
Abrasion (Corrasion)
The rubbing or scouring of the bed and banks by sediment carried along the river
Can vary from fine particles kept in suspension by turbulent flow to heavier boulders rolled along at times of bankfull flow
Major method by which rivers erode both vertically and horizontally
Potholes
Turbulent eddies in the current swirl pebbles around to form potholes that are hollow in the river bed and pebbles are likely to become trapped
Attrition
Reduction in size of the sediment particles as they collide with each other, the bed and banks
Pieces of sediment become smaller and more rounded as they move downstream, so more common to find rounder, smaller rocks downstream and coarser, angular rocks upstream
Solution / Corrosion
Process is independent of river discharge and velocity
Occurs when rocks dissolve in the water and are carried away
Most common when the rocks in the channel are carbonate (limestone, chalk)
3 types of load
Suspended load, bed load and dissolved load
Hydraulic Radius
Area / wetted perimeter
The higher the HR, the more efficient the river is as less energy is lost through friction
Upper course has lowest HR, lower course has highest HR
Saltation
When pebbles, sand and gravel (bedload) are lifted up by currents and bounced along the bed in a hopping motion
Traction
When largest boulders and cobbles (bedload) roll or slide along the bed
Suspension
Very fine sand particles such as clay and silt (suspended load) are dislodged and carried by turbulence in a fast flowing river
Solution
Water flowing within a river channel contains acids (e.g. Carbonic from precipitation) which dissolve the load such as limestone in running water and removed in solution
Soil particle sizes
Clay < 0.002mm
Silt = 0.002mm - 0.05mm
Sand = 0.05mm - 2mm
Gravel = 2mm - 5mm
Pebbles = 5mm - 75mm
Cobbles + Boulders =75mm - 200mm
River competence
Maximum size of load a river is capable of transporting
River capacity
Total volume of sediment a river can transport
River calibre
Measurement of the long axis of sediment in a river
Factors influencing Capacity, Competence and Calibre
Velocity - at low velocity only fine particles may be transported and at high velocity heavier material can be moved
Discharge - related to velocity and also depends on channel size
An increase of 10 times in discharge raises the transporting power of a stream by 10^3 or 10^4 times
When deposition occurs
Sudden reduction in gradient
River enters a lake or the sea
Discharge reduced following a drought
Shallower water
Sudden increase in sediment volume
River overflows its banks
Flocculation
Flocculation
Escalates close to the river’s mouth where fresh water meets salty or brackish water
On meeting the brackish water, clay particles coagulate as the salt activates electrostatic bonds between the clay particles
When they coagulate they increase in mass
This leads to deposition as he river is no longer competent enough to carry its load
This deposition may lead to the formation of a delta at the mouth
Laminar Flow
Uniform flow with all particles moving in the same direction at the same time
Very rare in nature as rivers are dynamic systems so it is more usual to see in sections of a river
Streamlined, uniform current
Requires low fluid velocity or smooth beds
Turbulent Flow
Eddies and Vortices cause turbulent flow
It is more common as water particles flow in random directions
Erosion takes place because of turbulent flow
Discontinuous, distorted flow
Eddy viscosity - internal friction at a larger scale
Variations in microtopography due to rocks
Vortices and small whirlpools can form
Helicoidal Flow
Found in meanders
Corkscrew motion moves material from the outside of one meander bend and deposits it on the inside of the next bend
Thalweg
The path of least resistance in a river and the position of the rivers fastest flowing current
Least friction, greater energy, greatest erosion
In a straighter river, the thalweg stays close to the middle
In a highly meandering river, the thalweg can be found on the outside flow which is why erosion takes place at a greater rate on the outside bend
Swings from side to side following path of least resistance
Straight Channel
Symmetrical channel shape
Thalweg is in centre
May occur in stretches in between meanders
Low levels of turbulent flow - more laminar flow
Can be man made
Meandering Channel
Asymmetrical channel cross section
Thalweg is skewed towards the outside bend causing erosion
Erosion occurs on one side with deposition on the other
Helicoidal flow present
Braided Channel
When the river creates multiple channels inside the main river channel
Lower course dominated by seasonal changes in discharge
High levels of depositional features, sand bars and eyots and regular configuration of converging and diverging streams
River Landforms in upper course
Waterfalls, gorges
(erosional induced landforms)
River Landforms in middle course
meanders (river cliffs, point bars, oxbow lakes), riffle and pool sequences
River Landforms in lower course
bluffs, floodplains, levees and deltas
V-shaped valley formation
A river’s discharge is low in the upper course and so the river only has enough energy to erode downwards (vertical erosion)
The valley’s sides are slowly broken down through weathering
The weathered material is transported via gravity and rainfall towards the river channel, steepening the valley’s sides
The steep-sided and narrow valley shape that is created is common in upper course of rivers
Interlocking Spurs
Forms because the river is forced to follow a winding course around the protrusions of the surrounding highland, resulting in spurs interlocking
Pothole Formation
River water is swirled around in irregularities in the river bed creating vertical eddies
Rocks get swept into the small depressions and abrade the hollow
These rocks are called grinders
The process continues deepening and enlarging the pothole
Waterfall Formation
Occur when a band of hard rock overlies a softer rock
The softer rock is eroded quicker by HA and abrasion, washing the rock away downstream
This creates a plunge pool where water is swirled around and potholing can occur as any rocks swept into the plunge pool can cause it to rub against the bed
Over time, an overhang is created as the soft rock is eroded faster
The hard rock eventually collapses as it is unsupported, causing it to retreat up the river, creating a gorge of recession
Rapids
Develop where the gradient of a river bed increases without a sudden break of slope or where stream flows over a series of gently dipping bands of harder rock
Rapids increase the turbulence of a river and hence its erosive power
Pools and Riffles
Sequencing of pools and riffles is very regular
Pools always follow riffles and riffles always follow pools
The spacing of pool to pool or riffle to riffle is regular at 5 times the bed width
They are the starting point of meanders and oxbow lakes
Pools
Areas of deep water and greater erosion
There is energy build up due to less friction
Riffles
Areas of shallow water caused by deposition of coarse sediment
Water flows slower here because of more friction
Formation of Meanders
The erosion and deposition by helicoidal flow creates meanders
A sequence of pools and riffles develop
Once this sequence has developed, the water is forced to flow in a curved shape around the riffles
The riffles direct the water flow towards the deepest water thus producing erosion
This undercuts the channel bank forming a river cliff
Erosion on the outside of the bank is accompanied by deposition of a point bar on the inside ban
A positive feedback system means the erosion and deposition continue to increase
Meander Scars
Once cut off, the water in oxbow lakes becomes stagnant
It slowly silts up over time becoming a stretch of marsh called a meander scar
River Cliffs
Small steep cliff that has been undercut by lateral erosion
Influenced by hydraulic action and abrasion
Mass movement can occur due to undercut base of river cliff
Susceptible to weathering as the ground is more exposed to water and wind
Slip-off Slopes / Point Bars
Take place on the inside bend due to deposition
Sinuosity
Curving nature of a river’s course
Actual channel length / straight line distance
Bluff Lines
Erosion also produces bluffs along the wider floodplain of a river
Over thousands of years, a meandering river gradually shifts from side to side across its floodplain
Where the meanders of the rivers reach valley walls, the water may carve bluffs
A bluff line defines the outer limits of a river’s floodplain, and is often another name for valley wall
Floodplains
Floodplains are initially cut out by the erosive force of the migrating river channel
Contained by bluff lines
Flat area of land on either side of the river and is made up of deposited alluvium from successive floods
Alluvium
Eroded material which is then deposited by rivers
Consists of silt, sand, clay and gravel as well as much organic matter
Meander Migration
Bluff lines get pushed back by erosion
This widens the meander and causes it to ‘migrate’ downstream
Levees
When a river overflows its banks, the increase in friction produced by the contact with the floodplain causes material to be deposited
The coarsest material is dropped first to form a small, natural embankment (levee) alongside the channel
During subsequent periods of low discharge, further deposition will occur within the main channel causing the bed of the river to rise and the risk of flooding to occur
Braided Channels
e.g. Rakaia River (New Zealand)
For short periods of a year, some rivers carry a high load in relation to their velocity
When a river’s level falls rapidly, competence and capacity are reduced and the channel becomes blocked with material
This means it will divide into a series of diverging and converging segments
Factors needed for Braiding
Highly variably seasonal discharge
Abundant sediment availability
Easily eroded banks made of soft unconsolidated material
Deltas
Composed of fine sediment which is deposited when a river loses energy and competence as it flows into an area of slow moving such as a lake or the sea
Prove the world’s more fertile land, while shallow and frequently changing distributaries hinder navigation
Lagoons, marshes and tidal mudflats lie in wetlands between distributaries of deltas
Delta formation
Heaviest material is deposited first and the lightest last
Deposition occurs as the river loses velocity when it enters the sea
Distributaries form as the main river channel splits into smaller channels
Built up in layers
Factors affecing delta development
Sediment availability
Extent of continental shelf
Low energy wave environments
Low tidal range
Flocculation
Shallow offshore gradient
Threats to deltas
Sediment starvation
Reduced river flow
Rising sea level
Storm surges
Abstraction of water, oil and gas causing sinking land
Downwarping of crust due to weight of sediment
Types of bed in a delta
Topset
Foreset
Bottomset
Topset Bed
Sand particles and gravel
Top part of the Delta
Material gets deposited quicker as it is the heaviest
Foreset Bed
Silts and fine sands
Middle part of the Delta
Bottomset Bed
Fine sand and gravel
Bottom part of the Delta
Aggradation
The layering up of sediment on the sea
Vertical direction
Progradation
The pushing out of sediment into the sea
Horizontal direction
Types of delta
Arcuate
Cuspate
Bird’s foot
Arcuate delta
Fan shaped
Rounded, convex outer margin
e.g. Nile delta
Cuspate delta
Tooth shaped
Where material brought down by a river is spread out evenly on either side of its channel by seasonal wind changes
e.g. Tiber delta
Bird’s foot delta
Shaped like a Bird’s Foot
Where the river has many distributaries bounded by sediment and which extend out to sea like the claws of a bird’s foot
Occurs in very sheltered deltas where distributaries can extend far out to sea
e.g. Mississippi delta
