Fluvial Landforms

Question 1

Upper course characteristics

Answer 1

• steep gradient
• low volume of water
• rough channel - lots of friction
• bed load - large angular fragments
• deep v-shaped valley
• vertical erosion

Question 2

V-shaped valley

Answer 2

Outcrops of land or hillside which will protrude (the river meanders around them).

Question 3

Formation of a waterfall

Answer 3

1. A river runs over alternating bands of hard and soft rock. The softer rock erodes, forming a step in the channel.
2. 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.
3. 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.

Question 4

Gorge

Answer 4

A deep, vertical sided river valley cut into the landscape as a waterfall retreats upstream.

Question 5

Rapids

Answer 5

• 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.

Question 6

Potholes

Answer 6

• 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.

Question 7

Middle course characteristics

Answer 7

• 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

Question 8

Which rivers are meanders more common?

Answer 8

• when bedrock is not too hard or too fragile
• less steep gradient
• load carried is not excessive

Question 9

What happens on the inside of a meander?

Answer 9

low river velocity -> low river energy -> deposition of load -> slip-off slope

Question 10

What happens on the outside of a meander?

Answer 10

high river velocity -> high river energy -> erosion (define) -> steep river cliff

Question 11

What is a riffle?

Answer 11

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).

Question 12

What is a pool?

Answer 12

Found in the channel bed on the outer bank of meanders. They are areas of deep smooth water flow.

Question 13

Eddies

Answer 13

A current of water flowing in the opposite direction to the natural direction of flow, caused by an obstacle in its path.

Question 14

Meandering meanders

Answer 14

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.

Question 15

Oxbow lake formation

Answer 15

1. 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.
2. 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.
3. The meander loop becomes cut off from the new channel and the oxbow lake is formed.
4. Overtime the oxbow lake will fill in with sediment and plant growth to leave a meander scar on the floodplain.

Question 16

Lower course characteristics

Answer 16

• 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

Question 17

What is a floodplain?

Answer 17

A flat strip of land breaching a river channel, regularly flooded.

Question 18

Why are floodplains flat?

Answer 18

Meanders migrate downstream and erode away any highland, such as interlocking spurs, creating a broad flat plain.

Question 19

Do floodplains deposit?

Answer 19

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.

Question 20

Do floodplains flood?

Answer 20

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.

Question 21

Why do floodplains attract settlement?

Answer 21

• 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

Question 22

Levees

Answer 22

• 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.

Question 23

Braided channels

Answer 23

• 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.

Question 24

Delta

Answer 24

Flat area of sand and silt built out into sea area.

Question 25

On reaching the sea…

Answer 25

• 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

Question 26

Factors affecting delta formation

Answer 26

1. sediment load - deltas are common at the mouths of rivers with large sediment loads eg the nile
2. 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.
3. 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.
4. sea floor gradient - a gentle and shallow sea floor gradient allows sediment to build upwards to reach the surface as a landform.

Question 27

Types of deposit at a delta

Answer 27

1. lightest, smallest particles of river load are carried into deeper water to settle as bottomset beds.
2. medium particles drop as steep wedges of sediment (forset)
3. 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.

Question 28

Arcuate deltas

Answer 28

• 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.

Question 29

Bird’s food delta

Answer 29

• in low wave energy environments, deposition builds outwards from the higher energy river.
• distributaries form multi-lobed shapes.