River landforms

Question 1

How are oxbow lakes formed?

Answer 1

As the outer banks of meanders continue to be eroded (hydraulic action), the neck of the meander becomes narrower and narrower.
Eventually, the two outer bends meet and the river cuts through the neck of the meander (usually when river energy is highest).
Deposition will then seal off the old meander and a new straighter river channel is formed.

Question 2

What is a slip off slope?

Answer 2

It is on a rivers meander, and forms where the slowest current and greatest deposition is (inner bend)

Question 3

What is a river cliff?

Answer 3

Its on a rivers meander, and forms where the fastest current and erosion is (outer bend)

Question 4

Why do point bars form?

Answer 4

From deposition on the inside of the meander, where velocity is least.

Question 5

What is a river riffle?

Answer 5

A shallow section of a channel (more friction=slower current)

Question 6

What is a river pool?

Answer 6

A deep section of the channel (less friction= more efficient)

Question 7

State the formation of a waterfall

Answer 7

Soft rock below is eroded more quickly than hard rock, forming an overhang over the bed below.
- The ‘step’ continues to develop as the river flows over the hard rock step
- The drop gets steeper by processes such as abrasion and hydraulic action
- Plunge pool forms at the bottom
- Erosion gradually undercuts the hard rock and the plunge pool gets bigger. Eventually the hard rock is unsupported and collapses

Question 8

What is a gorge?

Answer 8

A steep sided valley, that is formed following the formation of waterfalls

Question 9

What is a bluff?

Answer 9

Where rivers erode the lowest part of a river bank, and the upper part breaks off due to it being unsupported.
This leaves an exposed bank called a bluff.

Question 10

What does a bluff line indicate?

Answer 10

The outer limits of a rivers floodplain

Question 11

What is a bluff also knows as?

Answer 11

A valley wall

Question 12

How is a floodplain formed?

Answer 12

As a result of deposition and erosion by overbook flow.
They are composed of alluvium (silts and clays).

Question 13

What is a width of a floodplain determined by?

Answer 13

By the amount of meander migration and lateral erosion that has taken place.

Question 14

What is a meander scar?

Answer 14

Occurs where an oxbow lake has dried up.

Question 15

What is a levee?

Answer 15

A deposit of sediment, built up at the edge of a river channel.

Question 16

How is a levee formed?

Answer 16

They are formed during a flood stage, which causes a build up of material.
Coarse material will form at the channel edge, and finer/thinner material will form on the floodplain.
Levees form after repetitive flooding

Question 17

What is a river delta?

Answer 17

A low lying plain or landform at the mouth of a river, where the river flows into the ocean or another body of water

Question 18

What leads to the development of deltas?

Answer 18

Result from the deposition of fluvial sediment as the flow exits the mouth and enters slower-moving or standing water.
Slower/standing water causes capacity and competence of a river to decrease.

Question 19

Why does deposition increase as a river enters saltier water?

Answer 19

As salt causes clay particles to coagulate into flocs (flocculation)

Question 20

When can deltas form?

Answer 20

When the rate of deposition exceeds the rate of sediment removal.
This also influences the shape and size of a delta

Question 21

Where can deposition rate exceed sediment removal rate?

Answer 21

• When load size of a river is large, and requires lots of energy to transport
• A coastal area that has low tidal range and weak currents

Question 22

What is a arcuate delta?

Answer 22

A delta in the shape of a fan, usually caused by the formation of distributaries with coarser materials.

Question 23

Where are arcuate deltas found?

Answer 23

In coastal areas where long shore drift trims the edges of the delta

Question 24

What is a bird foot delta?

Answer 24

A delta that features long distributary channels, which branch outwards

Question 25

Where are bird foot deltas found?

Answer 25

They are river dominated, and found where rivers are sediment-rich which are not subjected to wave or tidal action

Question 26

What is a cuspate delta?

Answer 26

A delta that juts out, like an arrow, into the sea

Question 27

Why do cuspate deltas form?

Answer 27

Due to sediment colliding with waves, resulting in an even distribution of sediment on either side of its channel

Question 28

What is an inland/inverted delta?

Answer 28

A delta that is located inside a large valley, or one where are river divides into multiple branches in an inland area.