Fluvial Landforms Flashcards
Upper course characteristics
- steep gradient
- low volume of water
- rough channel - lots of friction
- bed load - large angular fragments
- deep v-shaped valley
- vertical erosion
V-shaped valley
Outcrops of land or hillside which will protrude (the river meanders around them).
Formation of a waterfall
- A river runs over alternating bands of hard and soft rock. The softer rock erodes, forming a step in the channel.
- The erosive power of the water undercuts the softer rock through the processes of hydraulic action, abrasion and solution (define 1). The circular motion of the water undercuts the softer rock, creating a plunge pool at the base of the waterfall.
- This will eventually leave a large block of hard rock overhanging with no support, which will collapse and the process will occur again. In this way, the waterfall retreats upstream leaving a steep-sided valley known as a gorge.
Gorge
A deep, vertical sided river valley cut into the landscape as a waterfall retreats upstream.
Rapids
- areas along a river where the water flows faster and more turbulent
- this is due to an increase in the river gradient or where a river flows over alternating soft and hard rocks.
Potholes
- holes in the riverbed, small or large
- form when rock fragments get caught in circular currents
- as they are swirled around the process of abrasion drills them into the rock
- found downstream of a waterfall as water is fast-flowing and turbulent.
Middle course characteristics
- less steep gradient
- larger water input due to tributaries
- open V or V-shaped valley
- bed load - smaller, less angular particles
- lateral erosion dominant - river meanders
- channel roughness decreases - less friction
Which rivers are meanders more common?
- when bedrock is not too hard or too fragile
- less steep gradient
- load carried is not excessive
What happens on the inside of a meander?
low river velocity -> low river energy -> deposition of load -> slip-off slope
What happens on the outside of a meander?
high river velocity -> high river energy -> erosion (define) -> steep river cliff
What is a riffle?
Sections of channel between two meanders where water is shallow and flows through course bed sediment. They appear to deflect max velocity of a river towards one bank and so causes it to be undercut by erosion processes. On the opposite bank water moves more slowly and deposition occurs. The river channel does not get wider but moves laterally (sideways).
What is a pool?
Found in the channel bed on the outer bank of meanders. They are areas of deep smooth water flow.
Eddies
A current of water flowing in the opposite direction to the natural direction of flow, caused by an obstacle in its path.
Meandering meanders
Meanders are not fixed, they move down valley. The max velocity point is normally a little downstream of the mid-point of the bend due to centrifugal force, therefore meander creeps downstream.
Oxbow lake formation
- The meandering river deposits material on the slip-off slopes of the inner bends. Erosion on the outer banks forms river cliffs and the neck of the meander narrows.
- Erosion, possibly due to a storm episode, breaches the neck of the meander and some of the river’s discharge follows the new direct channel downstream.
- The meander loop becomes cut off from the new channel and the oxbow lake is formed.
- Overtime the oxbow lake will fill in with sediment and plant growth to leave a meander scar on the floodplain.
Lower course characteristics
- very low gradient (flatter land)
- larger volume of water - tributaries
- very small, rounded particles of silt and sand
- smoother river channel - less friction
- some lateral erosion, mostly deposition
- very wide, flat floodplain
What is a floodplain?
A flat strip of land breaching a river channel, regularly flooded.
Why are floodplains flat?
Meanders migrate downstream and erode away any highland, such as interlocking spurs, creating a broad flat plain.
Do floodplains deposit?
It is a depositional feature
- 5-10% flood water deposits in the form of a thin layer of rich fertile alluvium
- 90-95% point-bar deposits ie sand and gravel laid down on the inside of bends of migrating meanders
- alluvium is rich in nutrients for plant growth and is located on surface, accessible by plant roots.
Do floodplains flood?
Occurs to some extent annually on floodplains of natural rivers. Larger floods = every decade, largest flood = once a century. This annual flooding causes conflict with human occupation of the floodplain.
Why do floodplains attract settlement?
- fertile soil for farming rich in nutrients
- water supply for crops, homes and industry
- transport routes alongside and via river - roads and rails
- defensive site in meander core
- flat land easy to build on
- flat land for mechanised farming
- food supply - fish
Levees
- form naturally in river’s floodplain as a result of deposition that occurs when a river overspills. Channel and flood waters spill sediment out on to adjacent land.
- As flood waters leave the main river channel the velocity and carrying capacity decreases rapidly as it flows away from main river channel.
- Material being carried from river is deposited. Largest first.
- This creates a ridge on river bank, obvious when river turns back to normal flow levels.
- deposited material can help to contain the river in the channel.
Braided channels
- In the lower course where river velocity is reduced because of flatter land, braiding will occur.
- This is when the river deposits sediments which piles up within the river channel itself.
- River will then be forced to split or continue on its path to the sea.
- These streams are known as distributaries.
Delta
Flat area of sand and silt built out into sea area.
On reaching the sea…
- river velocity slows down
- river energy decreases
- river deposits sediment into the sea
- layers of sand and silt may accumulate to form new area of land known as a delta
Factors affecting delta formation
- sediment load - deltas are common at the mouths of rivers with large sediment loads eg the nile
- coastal energy - deltas are common in low energy coastal environments eg the Mediterranean, where river sediment gradually builds up on the seabed without being disturbed. They are not common along coast of UK or Ireland due to large tidal ranges and high energy environments, which erode the coastline and prevent deposition.
- flocculation - deposition at coast is aided by the process in which the salt nature of the sea water causes fine clay particles to join together and settle. By the time the river reaches the sea its load will be composed to finer particles (sand, silt, clay) due to attrition.
- sea floor gradient - a gentle and shallow sea floor gradient allows sediment to build upwards to reach the surface as a landform.
Types of deposit at a delta
- lightest, smallest particles of river load are carried into deeper water to settle as bottomset beds.
- medium particles drop as steep wedges of sediment (forset)
- heaviest particles settle rapidly on top of delta (topset). As delta exceeds seawards, the bottomset and forset beds are further buried and incorporated into delta itself.
Arcuate deltas
- onvex, curved, outer edge maintained by the action of longshore drift along its outer sea edge.
- wave action smooths off outer edge, forming the convex shape.
Bird’s food delta
- in low wave energy environments, deposition builds outwards from the higher energy river.
- distributaries form multi-lobed shapes.
