4.6 River Landscape Flashcards
How does river discharge change downstream and why?
It increases downstream because:
•more tributaries have joined
•drained a much larger part of the drainage basin
•the channel has eroded to be a much bigger cross sectional area
How does channel width change down stream and why?
It increases as it goes downstream because:
•there is more discharge
•more tributaries have joined
•velocity has gone up meaning more erosion (sideways)
How does channel depth change downstream and why?
It increases because:
•there are more tributarias
•more discharge
•velocity has gone up
How does velocity change downstream and why?
It increases because:
•its wider and deeper
•its beds and banks are smoother so it is more efficient
How does sediment load size change downstream and why?
It decreases because:
•rivers have more velocity which means it creates attrition so it erodes the the sediment and as a result there is less sediment.
How does sediment particle shape change downstream and why?
It becomes more rounded because:
•Attrition is eroding the sediments meaning the particle shapes would become smoother.
How does slope angle change downstream and why?
It is flatter because in the upper course traction transport erodes downwards and creates vertical. But in the lower course suspension transport erodes sideways making the lower course flatter
What is meant by the long profile of a river?
The long profile of a river shows you how the gradient changes over the different courses.
Compare traction with suspension
Traction is when large particle are pushed along the river bed by the force of the water, but suspension is when smaller particles are carried along by the water.
Compare hydraulic action and abrasion
Hydraulic action is the force of the water that breaks rock particles away from the river channel but abrasion is when picked up rock scrapes and rubs against the channel and wears it away.
What is weathering?
There are three types of weathering:
Physical, chemical and biological.
These all break down rocks on the valley sides in different ways.
Compare freeze-thaw and biological weathering
Freeze-thaw only happens when the temperature is below freezing and then goes back up above 0°C. The water gets into cracks, freezes and then expands widening the cracks to make them break up, but biological weathering is when parts of plants and animals grow in cracks and split the rocks apart.
Formation of a waterfall
Hard rock is next to an area of soft rock with a river flowing through.
The softer rock is eroded by hydraulic action and abrasion.
Water continues to erode the soft rock until a steep drop is created.
The hard rock is eroded and collapses.
The rock swirls at the bottom and erode the area creating a plunge pool.
There would be more undercutting leaving the waterfall to retreat and a gorge is created.
Formation of a v-shaped valley and interlocking spurs
There is a small amount of discharge in the upper course and only has enough energy to erode down.
Freeze-thaw erodes the rock by freezing at night and cracking the rocks and the valley sides are broken down.
Rockfalls occur and take the material towards the river channel and steepens the valley.
A v-shaped valley is created with steep sides and very narrow.
Interlocking spurs form where there is more resistant bands of rocks and cannot be eroded.
Formation of meanders
The current is faster on the outside of the bend because the river channel is deeper.
More erosion and hydraulic action happens on the outside of the bend.
The current is slower on the inside of the bend due to the river channel being shallower.
Eroded materials are deposited on the inside and form a slip-off slope.
Over long periods of time some meanders can loop back ob themselves so only a narrow neck of land is left.
Formation of oxbow lakes
Erosion causes the outer bends to get closer
There is now only a small bit of land left between each bend
The river then goes through the land usually during a flood.
The river flows along the shortest course.
Deposition eventually cuts of the meander.
This forms an oxbow lake.
Formation of flood plains
A floodplain is the valley floor either side of the river and gets flooded.
When a river floods onto the floodplain the water slows down and deposits material. This builds the floodplain.
Meanders move across the floodplain which makes it wider.
They also migrate downstream so it flattens the valley floor.
The deposition from the slip off slope from meanders builds up the floodplain.
Formation of levees
Levees are raised embankments along the river channel.
Eroded material is deposited over the flood plain from a flood.
Heaviest material is deposited closer to the river channel because it gets dropped first from the river.
Deposited material builds up and creates levees along the edge of the channel.
Formation of deltas
When a river reaches the sea or an ocean it will lose velocity and deposit sediment.
The river deposits sediment faster than coastal processes erode, so sediment builds up in a fan-shaped delta. The river then splits into smaller channels called distributaries.
Explain two ways climate, geology and slope processes change the river landscape
In wetter climates there is higher discharge which increases the rate of rate of erosion and vertically erodes to create v-shaped valleys and erodes laterally to create flat floodplains.
Soft rocks have more erosion meaning more sediment load so have gentle sloping valley sides but area with hard rock will have less erosion and have less sediment load meaning it creates steeper valley sides.
Vertical erosion by rivers make valley sides steeper and increases the movement of material. Soil creep moves soil because of gravity. This makes soil move down and dry out and contract lower down.
What is lag time
The delay between peak rainfall and peek discharge
What is the rising limb
The increase in river discharge as rainwater flows into the river
What is peak discharge?
The highest discharge in the period of time you’re looking at.
How can geology affect flood hydrographs
If water cant infiltrate into impermeable rocks there is more surface runoff.
