Landforms of fluvial erosion and deposition Flashcards
What may waterfalls be a result of?
3
- A resistant band of rock occurring across the course of the river.
- The edge of a plateau (high ground edge).
- The rejuvenation of the area - gives the river new erosional power as sea level falls.
Explain the formation of a waterfall.
The river falls over a rock edge into a plunge pool at the foot of the fall.
Resistant rock - basalt.
Non resistant rock - sandstone.
Upper course.
Layers of non resistant rock are eroded more quickly than overlying resistant rock.
Hydraulic action and abrasion deepens the plunge pool
This undercuts the resistant (cap) rock above.
Eventually the overhanging cap rock collapses.
The waterfall retreats upstream, leaving a gorge ahead of it.
Waterfall EXAMPLE.
High Force in upper Teesdale.
Igneous resistant rock - Whin Sill.
Soft rock - sandstone/limestone - eroded by corrosion.
These erode more quickly, leaving an overhang.
Waterfall - 22m high.
Gorge - 500m.
Largest (by volume) in England.
Explain the formation of a pothole.
+ EXAMPLE.
They are cylindrical holes drilled into the rocky bed of a river by turbulent high velocity water loaded with pebbles
- Sediment accumulates within naturally occurring small depressions on the rock surface on the river bed.
- The sediment/pebbles become trapped in slight hollows and vertical eddies in the water are strong enough to allow the sediment to grind a hole in the rock by abrasion.
- Attrition smoothes pebbles (grinders) caught in the hole - reduce load size.
Upper-middle course.
EXAMPLE: The River Tee.
Explain the formation of braided channels.
Lower course.
Occurs at times of variable discharge e.g snowmelt.
The river is forced to split into several channels separated by sediment islands.
Discharge decrease - load deposited (low HR) - creates smaller channels called distributaries.
Island deposits - eyots.
Vegetation stabilises them - bioconstruction.
Bars are washed away when the discharge increases again.
What is an example of a braided channel, and where are they commonly found?
Areas of variable discharge/easily river channel banks.
The Franz Joseph River drains the meltwater of the Franz Joseph glacier, New Zealand.
Highly variable discharge - daytime melting - nighttime freezing - seasonal temperature variation.
Explain the formation of meanders.
+ EXAMPLE.
Straight channels seem to have alternating bars of sediment on their beds - water bar weaving.
Alternating shallow section - riffles - more friction.
Deeper sections - pools.
The thalweg directs the maximum velocity towards one of the banks, resulting in undercutting erosion (abrasion and hydraulic action) on that side.
A concave outer bend is then created - river cliffs.
Deposition (sand and gravel-alluvium) - inside - convex bank - point bar.
The flow of water in the meanders is kept going by the helicoidal flow.
As erosion (hydraulic action) occurs on the outside bend, the meander begins to migrate slowly, both laterally and downstream.
EXAMPLE: River Holford, Holford village.
Explain the formation of levees.
+ EXAMPLE
Middle-lower courses.
Flood - fall in velocity.
Results in deposition- reduced river capacity.
The biggest material is deposited first, creating levees (or raised banks) along the sides of the channel.
Subsequent floods helps build up these levees.
E.g Levee strengthening - Mississippi River (USA) - began in 1699.
1990’s engineered levees length - 3200km.
Explain the formation of floodplains.
+ EXAMPLE.
Middle - lower courses.
Areas of flat land either side of the river.
Composed of alluvium.
Becomes wider over time - width is determined by how much meander migration and lateral erosion takes place.
Lateral erosion is more powerful just downstream of the apex of the meander bend - causes meander migration.
Interlocking spurs are removed by lateral erosion over time - wider floodplain.
Point bars and old meander scars add to the alluvial deposits of the floodplain - completed with levees.
These become stabilised by vegetation over time.
EXAMPLE: The River Tee.
Explain the formation of a delta.
Mouth of river as it enters the sea - lowest course.
Occurs as the velocity and sediment carrying capacity of the river decrease, causing the suspended load to be deposited.
Flocculation occurs, causing clay to settle on the river bed - sinks.
Due to the alluvium build up, the river has to braid into several distributaries in order to reach the sea.
Bottomset, foreset and topset beds.
What are the conditions that have to be met in order for a delta to form? 2
- The sediment load of the river is very large e.g Mississippi/Nile.
- Small tidal range and weak currents in coastal area e.g Gulf of Mexico.
What are the types of delta deposits? 3
- Topset beds - the larger/heavier load is first to be deposited.
- Foreset beds - medium graded particles travel a little further before being deposited - unstable - can cause sediment mass movement (turbidity currents).
- Bottomset beds - finest particles travel the furthest before being deposited.
What are the different types of deltas?
Arcuate e.g Nile - coarse grained.
Bird’s foot e.g Mississippi - fine grained.
Cuspate.
Explain the formation of rapids.
+ EXAMPLE.
More turbulent waters.
Linked with pool/riffle sequences.
Pools - deeper - hydraulic action - faster.
Riffles - shallower - deposition - slower.
EXAMPLE: The Colarado River, USA.
What are rapids caused by? 2
- Localised increase in gradient along the river’s gradient.
- The river flows over resistant/non resistant bands of rock - linked with pool/riffle sequences.
