Rivers 2- Landforms Flashcards
Potholes
Cylindrical depressions in the solid rock of a river bed
Found in the upper course, at areas of high-velocity turbulent flow and many pebbles in the bedload
Potholes formation
Pebbles and gravel collect and rotate in a depression
This is a type of abrasion called drilling
More pebbles become trapped in the deepening hollow
Rapids
Areas of very turbulent flow with steep steps and white water
Found in the upper course, at areas where there are changes in geology
Rapids formation
Bands of less resistant rock are eroded by hydraulic action and abrasion, whilst bands of more resistant rock are not eroded
This causes steps in the gradient
Waterfalls
A sudden change in gradient, causing a fall of water that creates a gorge of recession
Found in the upper course, where there is a change in rock type
Waterfalls formation
A band of resistant rock lies above a band of softer rock
The softer rock is eroded by hydraulic action, abrasion and solution
A plunge pool formed which undercuts the resistant rock to leave an overhang, which eventually collapses
Meanders
A sweeping loop or bend in the river channel
Found in the middle/lower course
Meander formation
Deposition produces pools and riffles, and the Thalweg swings around the riffles, eroding the outside bend by hydraulic action and abrasion
Helicoidal flow causes undercutting to form river cliffs
Lower velocities on the inside bend causes deposition to occur, forming slip-off slopes
Meander migration
The whole meander feature moves laterally and downstream
The zone of greatest erosion is just downstream of the midpoint in the meander bend
Oxbow lakes
A curved, horse-shoe shaped body of water, isolated from the river channel
Found in the middle/lower course
Oxbow lake formation
Erosion narrows the neck of the meander
During floods, the neck is broken through and then afterwards deposition cuts off the former meander
Floodplains
Areas of low-lying ground next to rivers that are naturally experience to flooding
Floodplain formation
Meander migration widens the valley floor and creates bluffs at the edge
When the river bursts its banks, velocity falls and alluvium is deposited
Levées
Natural embankments of sediment in long ridges
Found in the lower course, either side of a river channel
Levée formation
During a flood, the river water loses competence and deposits it’s coarsest material next to the channel
Braided rivers
Rivers that have multiple channels and islands of sediment called eyots
Found in the upper course where discharge is variable
Braided rivers formation
Variable discharge (commonly due to glaciers above the river) When discharge is high, the river can transport a lot of material When the discharge falls, the river loses competence so deposits material The sediment can build up into eyots
Deltas
Depositional landforms formed at the mouth of rivers or inland on the approach to lakes
Usually occur in calm seas with a gently sloping seabed (almost tideless)
Delta formation
Rivers with large sediment loads lose competence as they enter the sea due to a fall in velocity
The water spreads out laterally increasing wetted perimeter
Flocculation occurs causing coagulation of particles
Deposits form eyots and the river splits into distributaries
Beds of a delta
Topset beds are larger bedload, deplored first
Foreset beds are made up of middle-sized sediments and are transported further
Bottomset beds are the finest sediments and so are transported furthest
Bird’s foot deltas
Dominated by an extruding finger-like branch of deposition
Have fewer distributes and finer sediments
Formed due to weak ocean processes and high discharge
Arcuate deltas
A gentle sloping shoreline and a distinct pattern of branching distributaries
Dominated by coarser material
Rejuvenation
A river has renewed potential energy so increased erosion
Caused by a fall in sea level due to isostatic rebound, tectonic processes or eustatic change
Isostatic rebound
Uplift of the land following the melting of land-bound ice at the end of a glacial period (localised)
Eustatic change
Lowering of sea level, during an ice age, caused by the accumulation of ice on land (global)
Knickpoints
Sudden changes in a river’s gradient
Marked by rapids or a waterfall
River terraces
Formed when a rejuvenated river cuts down through a floodplain
Paired terraces indicate rapid downcutting
Unpaired terraces form when lateral erosion occurs
Incised meanders
Vertical erosion deepens meanders
Entrenched meanders indicate rapid vertical erosion and resistant valley sides (symmetrical gorge)
Ingrown meanders form when lateral erosion occurs as well (asymmetrical river valley)
