Rivers 2- Landforms

Question 1

Potholes

Answer 1

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

Question 2

Potholes formation

Answer 2

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

Question 3

Rapids

Answer 3

Areas of very turbulent flow with steep steps and white water
Found in the upper course, at areas where there are changes in geology

Question 4

Rapids formation

Answer 4

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

Question 5

Waterfalls

Answer 5

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

Question 6

Waterfalls formation

Answer 6

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

Question 7

Meanders

Answer 7

A sweeping loop or bend in the river channel

Found in the middle/lower course

Question 8

Meander formation

Answer 8

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

Question 9

Meander migration

Answer 9

The whole meander feature moves laterally and downstream

The zone of greatest erosion is just downstream of the midpoint in the meander bend

Question 10

Oxbow lakes

Answer 10

A curved, horse-shoe shaped body of water, isolated from the river channel
Found in the middle/lower course

Question 11

Oxbow lake formation

Answer 11

Erosion narrows the neck of the meander

During floods, the neck is broken through and then afterwards deposition cuts off the former meander

Question 12

Floodplains

Answer 12

Areas of low-lying ground next to rivers that are naturally experience to flooding

Question 13

Floodplain formation

Answer 13

Meander migration widens the valley floor and creates bluffs at the edge
When the river bursts its banks, velocity falls and alluvium is deposited

Question 14

Levées

Answer 14

Natural embankments of sediment in long ridges

Found in the lower course, either side of a river channel

Question 15

Levée formation

Answer 15

During a flood, the river water loses competence and deposits it’s coarsest material next to the channel

Question 16

Braided rivers

Answer 16

Rivers that have multiple channels and islands of sediment called eyots
Found in the upper course where discharge is variable

Question 17

Braided rivers formation

Answer 17

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

Question 18

Deltas

Answer 18

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)

Question 19

Delta formation

Answer 19

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

Question 20

Beds of a delta

Answer 20

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

Question 21

Bird’s foot deltas

Answer 21

Dominated by an extruding finger-like branch of deposition
Have fewer distributes and finer sediments
Formed due to weak ocean processes and high discharge

Question 22

Arcuate deltas

Answer 22

A gentle sloping shoreline and a distinct pattern of branching distributaries
Dominated by coarser material

Question 23

Rejuvenation

Answer 23

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

Question 24

Isostatic rebound

Answer 24

Uplift of the land following the melting of land-bound ice at the end of a glacial period (localised)

Question 25

Eustatic change

Answer 25

Lowering of sea level, during an ice age, caused by the accumulation of ice on land (global)

Question 26

Knickpoints

Answer 26

Sudden changes in a river’s gradient

Marked by rapids or a waterfall

Question 27

River terraces

Answer 27

Formed when a rejuvenated river cuts down through a floodplain
Paired terraces indicate rapid downcutting
Unpaired terraces form when lateral erosion occurs

Question 28

Incised meanders

Answer 28

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)