Rivers Flashcards
What is the input in the drainage basin system?
Precipitation
What are the flows in a drainage basin system?
Surface runoff
Throughflow
Groundwater flow
Percolation
Infiltration
What are the stores in a drainage basin system?
Surface storage
Soil storage
Groundwater
Interception
What are the outputs in a drainage basin system?
Transpiration
Evaporation
River run-off
How does the long profile of a river change downstream?
The long profile is high in the upper course, but decreases in the upper and middle before becoming flat in the lower course
How does The Valley cross section change downstream?
Upper course:
V-shaped valley, interlocking spurs, steep gradient
Middle:
More open U shape, flatter valley floor, sloping sides
Lower:
Flat, wide
What are the features of a river as it changes downstream?
Upper:
Pools, rapids, waterfalls
Middle:
Meander, river cliffs, slip-off slopes
Lower:
Ox-bow lakes, deltas, mud flats, salt marshes
What is attrition?
Material in river collides and breaks into smaller pieces and becomes more rounded
What is abrasion?
Material carried in suspension rubs against the banks of the river in a sand papering action. Wears away outside of bends
What is corrosion (solution)?
Acids in the river dissolve rocks, such as limestone, which form the bed and banks
What is hydraulic action?
The sheer force of the river dislodge particles from the river’s banks.
What is traction?
Boulders and pebbles are rolled along the river bed at times of high discharge
What is saltation?
Sand sized particles are bounced along the river bed by the flow of water
What is suspension?
Fine clay and sand particles are carried along with the water, even at low discharge.
What is solution?
Some materials dissolve in water, such as calcium carbonate
Why does a river deposit its load along the course of the river?
Increase in friction, reduction in gradient, leading to reduced velocity and energy
How is a v-shaped valley formed?
- River erodes downwards, leaving steep sides
- Steep sides are attacked by weathering, breaking up and loosening soil and rock
- The loosened material enters the river
- A v-shaped valley is left
How is a waterfall formed?
- There are layers of hard and soft rock
- The underlying soft rock is eroded more quickly
- The force of swirling water enlarges and deepens the plunge pool at the base of the waterfall and processes such as abrasion and hydraulic action cause undercutting
- Harder rock is left unsupported and overhangs and eventually collapses on the river bed
- These processes repeat as the waterfall retreats up stream
- A steep sided valley called a gorge is created
How is a meander formed?
- Rivers often bend around things in their path, resulting in areas of slower and faster water movement
- The river flows faster on the outside and erodes the outside bends of the river channel by abrasion and hydraulic action, forming a river cliff
- The river flows more slowly on the inside of the bend, depositing some of its load and forming a river beach
- Continuous erosion on the outer bank and deposition on the inner bank forms a meander in the river, over time becoming larger and more recognisable
How is an ox-bow lake formed?
- In a meander water is pushed to outside bend, greater velocity = more energy to erode
- Continual erosion on the outside bend narrows the meander neck
- The river floods and takes the shortest route, cutting through the neck
- Deposition occurs on banks of river, meander becomes cut off, leaving an ox-box lake
- The lake will slowly dry up
How is a levee formed?
- A river floods and the velocity of the water drops and the friction on the banks of the flood plain increases
- Due to the reduced velocity, deposition occurs.
- Over time, natural embankments called levees form
How is a flood plain formed?
- It is the land either side of a river, where it floods. They are made of silt
- They form when channel velocity falls, so the river deposits it’s material.
- They are food for farming because they contain lots of minerals
How does a delta form?
- Over time, deposition of silt occurs in distributaries to form islands and levees
- Necessary conditions for a delta are:
- substantial amount of silt in river
- lake or sea with no strong tides
- supply of sediment is greater than removal of it by tides - Sea water is salty and helps sand particulars to bind together, being heavier and sinking to the sea bed
- On reaching the sea, velocity decreases, loss of energy so deposition happens
- Sometimes deposition happens in main channel and distributaries are used to reach the sea
- Located at mouth of river as it enters sea or lake
- Eventually, delta grows upwards and outwards to form new land colonised by vegetation
What is the background of the Ganges delta?
- 100,000km2
- were the Ganges, Jamuna and Meghna river meet
