The Middle and Lower Course of a River

Starter:

1. Draw a simple diagram of the river cross section at the following areas:

• Upper course

• Middle course

2. Now draw on the direction of erosion (lateral or vertical).

Fluvial Landforms: Depositional

Today we will study the main landforms formed by deposition.

These landforms will tend to be found in the middle and lower course of a river.

What do you think the relief be like in the lower course?

Long Profile

Discussion

• How does the gradient of a river change along its course?

• How does the balance between erosion and deposition change?

• How does the size of material change along the profile?

• What are the 4 processes of transportation?

How is material transported downstream?

Saltation

Solution

Traction

Suspension

Middle course, R. Tees

Valley opens out, more gentle slopes, wider valley bottom

First signs of meanders

Floodplain

River channel wider, deeper, greater velocity and discharge

What landforms are found in the Middle Course of a River?

1) Braiding2) Meanders3) Oxbow Lakes

• Braiding occurs when a river is a river deposits all or some of its load when there is too much material to carry.

• This is likely to happen when extra material is added into the river and/or when the river loses velocity.

• Braiding occurs when the river splits into two or more channels due to the deposition. The ‘islands’ formed are called eyots.

Braiding

eyots

MEANDERS

•A stream consisting of successive Meanders– A meander in general is a bend in a sinous

watercourse.

Meanders• Meander bends occur in the middle section when the river has a

shallower slope (gradient) so its power does not erode vertically, but laterally.

• Simulations with laboratory models of rivers have shown that, above a critical minimum velocity, meanders will form. Friction with the channel bed and banks causes turbulence in the water flow, which promotes the development of alternating bars of sediment along the channel.

• These bars of sediment are called riffles

• Alternating with the riffles are pools of deeper water.

• The riffles disrupt the water flowing over it, swinging the water towards the outer banks.

• A meander starts to form at the deeper side of the river where it flows faster and has more power to erode sideways.

• http://www.cleo.net.uk/consultants_resources/_files/meander4.swf

FLOW IN MEANDERS

• Once the meander starts to develop, helicoidal flow (corkscrew shaped flow) is established, with the water surface being elevated on the outer bank of each curve, and return currents at depth directing the flow towards the opposite bank.

• The outer bank is eroded as a result of the higher flow velocity, whereas material is carried by the flow and deposition takes place on the inner bank, and forms a point bar.

• It is the major flow in the meander bends.

Helicoidal flow

http://upload.wikimedia.org/wikipedia/commons/2/25/Meander.svg

Point bar deposits

Meanders are formed because the current swings to the outside of a bend and concentrates the erosion there. Deposition occurs on the inside of the bed where there is not enough energy to carry load.

EROSION TYPE: Lateral

Also known as the ‘Mature’ stage Meanders 1

(Aerial View)

Middle Course: Meanders

Meander, R. Lavant, Chichester

Floodplain

Point bar deposits on the inner meander bend where there is low energy

River Cliff Slip-Off Slope

WHAT DO THE ARROWS POINT TO?WHAT DO THE ARROWS POINT TO?WHICH WAY IS THIS

MEANDER MOVING?WHICH WAY IS THIS MEANDER MOVING?

Meanders• Constant erosion (undercutting) erodes the outer

(concave) bank of the meander, forming a river cliff• Constant deposition at the outer (convex) bank deposits

sediments, forming a river slope

Meander = a bend in a river

Cut bank erosion (River Cliff)

Point bar deposits }Meander

loop

WHERE IS EROSION TAKING PLACE?WHERE IS EROSION TAKING PLACE?

WHERE IS DEPOSTION TAKING PLACE?WHERE IS DEPOSTION TAKING PLACE?

A

B

C

D

E

F

Maximum erosion

Maximum deposition

FORMATION SEQUENCE OF OXBOW LAKES

Stage # 1

Stage # 2

Stage # 3

Stage # 4

Stage # 5

Stage # 6

Stage # 7

Stage # 8

Stage # 9

Oxbow Lake

Oxbow cuttoff

Meander scarsFinal Stage

The Sebaskachu, Canada

Oxbow Lakes

Ox-bow lake

Ox-bow Lake• A U-shaped body of water formed when a

wide meander from the main stem of a river is cut off to create a lake

Ox-bow Lake• River constantly erodes

the outer banks of two adjacent meanders causing the meanders to move closer together forming a loop

• Overtime, the loop becomes more distinct and is separated by a narrow neck of land

Ox-bow Lake• As erosion and

deposition continue to take place along the inner and outer banks respectively, the two meanders eventually meet

• The sediments deposited will eventually dam up the water in the cut off, forming an ox-bow lake

River Landforms

Oxbow Lakes

Meander neck becomes smaller

new course of the river

oxbow lake

When the river floods it breaks through the thin meander neck and the river takes the easier, straight course. This leaves the meander loop ‘cut off’ as an oxbow lake. Over time, the oxbow lake will become colonised by vegetation.

Questions

Meander Bend on the River Conwy

A B

Explain why there is more deposition at ‘A’ rather than at ‘B’.

Sketch this plan view of a meander bend and label the line of highest velocity.

1. 4.3.2.

Draw the following four diagrams in the correct order of oxbow lake formation. Match 3 of the diagrams with the most appropriate explanation from ‘A’, ‘B’ and ‘C’.

A. The river floods and erodes through the thin meander neck. The river now takes the easier, straight course. The meander loop is left as an oxbow lake.

B. The river meanders. The fastest flow is therefore on the outside of the bend. This fast current erodes the meander neck through the processes of erosion.

X XXXXX = erosion on the outside of the meander bend

C. The meander neck is further eroded until only a thin piece of land separates the two channels.

Name the river landform shown in this aerial photograph.

What else can you identify?

How can you tell that this is not the Upper Course of a river?

This is a cross section of a meander bend. Sketch the diagram and mark on the following

Slip off slope River Cliff

Area of depositionUndercutting

Fastest velocity

Questions1. What is braiding?2. Explain how a meander may form and then

migrate along the course of a river across . Use the following words:

• Riffles• Pools• Helicoidal flow• Point Bar• Cliff• Slip off Slope• Erosion• Deposition• Convex• Concave

3. Using a series of diagrams, explain the formation of ox-bow lakes.

To finish…

• We will now play a short game to see how many of the key terms we can remember.

• You will be given a card with a key term written at the top.

• Under the key term, write down 5 other key words you would use to help you explain what that term is

To finish…

• Now that the cards are finished, we will play a game of taboo.

• You have to explain what the term is on your card, without using any of the words on the card!

• We will split into two teams to do this…