A.1.4: Formation of typical river landforms

Question 1

What affects the amount of erosion happening at a waterfall

Answer 1

For example: If the drop in height is great, the water will be very fast; therefore, there will be more rapid abrasion/hydraulic action.

Other possible reasons include:
- Geology (rock type eg. sandstone vs chalk): chalk is very easy to erode
- Scale/size of drainage basin: higher volume, bigger plunge pool means more erosion
Variations in discharge levels: seasonal rivers (higher discharge in rivers, more abrasion)
Load causing abrasion in plunge pool: type of bedload causes erosion

Question 2

Waterfall formation

Answer 2

• Form where river processes interact with the geology of landscapes
• Hard rock, on top of the waterfalls. Harder to erode, more resistant
• Soft rock, bottom of the waterfall. Easier to erode, less resistant.
• Water flowing on top, has more energy. GPE converts to KE
• Begins to undercut rock behind the waterfall. Undercuts and creates overhang.
• Will at some point collapse, waterfall will retreat and move upstream.

Question 3

Sinuosity

Answer 3

It is measured by looking at how long the channel is compared to the valley length through which it runs

Sinuosity = length of stream channel ÷ length of valley

Straight channel (<1.5)
Meandering channel (>1.5)

Question 4

Thalweg

Answer 4

Line of maximum velocity

Question 5

Sand Bars

Answer 5

This amount of deposited material in the middle of the channel causes meandering.

Question 6

Friction

Answer 6

A turbulence makes the stream flow unstable. This turbulence comes from rough river bed and banks. The helicoidal flow then raises water on the outside channel and the return flow happens at depth.

Question 7

Levees

Answer 7

Formed in the lower course, due to deposition and repeat flooding

During flooding, the river deposits course material (e.g. gravel and sand) next to the channel’s edge (on the bank) while finer material (slit and clay) is carried further to the floodplain

The river load that is deposited on the floodplain is alluvium - has a lot of materials and nutrients, helps fertilise floodplain

Why?
When a river floods onto its floodplain, it encounters friction and the velocity is reduced due to vegetation on flood plain slowing down) = not enough energy to carry load

Heavy material deposited first, finer load last

Question 8

Floodplains

Answer 8

Flat area of land next to a river or stream made up of fertile grass that experiences occasional or periodic flooding

• Only form in the lower course of the river
• Floods when the river has the most energy
• If the river continues to rise any land that will be underwater is called a floodplain
• As the river spreads over the floodplain there will be a sudden increase in booth the wetted perimeter and the hydraulic radius

Question 9

Oxbow Lake

Answer 9

Usually form in middle to lower course of the river stream

Meanders that form oxbow lakes have two sets of curves: one curving away from the straight path and the river and one curving back

Question 10

Meander

Answer 10

Meander is the winding curved shape of a river

Erosion occurs on the side of the riverbank. As the water flows it breaks down the river

As deposition and erosion occurs on the side of the river bank, the gap between the two arms of the river gets closer and closer

Swan neck = when the meander neck is very thin, find the shorter course

Eventually the river chooses to take the shorter course and causes the large meander to be pinched off

Concave: Inner curve
Convex: Outer curve