River Landforms Flashcards
1
Q
how does a waterfall form
A
- where a band of hard rock meets softer rock. the soft rock is eroded more than the harder rock, causing a step in the river bed
- the water flowing over the step speeds up due to the lack of friction as it drops over the step. this increase in speed gives the water greater erosive power, causing further erosion of the soft rock and undercutting of the harder rock
- as the hard rock is undercut, it can collapse. a deep plunge pool is carved out by abrasion at the foot of the waterfall as bits of collapsed rock are swirled round by turbulence
- over time, more undercutting causes more collapse. the waterfall will retreat leaving behind a steep sided gorge
2
Q
what are potholes and how are they formed
A
- small circular hollows in the river bed
- formed by abrasion as turbulence swirls a rivers bedload round in a circular motion, causing it to rub and scrape out holes
3
Q
what are rapids
A
- relatively steep sections of the river with turbulent flow where there are several sections of hard rock
- mini waterfalls almost
4
Q
how are meanders formed
A
- when alternating pools (areas of deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river. the distance between pools in 5-6 times the width of the river bed
- because the river channel is deeper in pools its more efficient, so has greater energy and more erosive power. energy is lost as the river flows over a riffle because of friction
- the spacing and distance between riffles and pools causes the rivers flow to become uneven and maximum flow to be concentrated on one side of the river
turbulence increases in and around pools as the water speeds up, so flow of water begins to twist and coil - this causes corkscrew like currents in the river called helicoidal flow which spiral from bank to bank between pools
- the helicoidal flow causes more erosion and deepening of the pools. it also causes eroded material to be deposited on the inside of the next bend, where the river loses energy
- the combination of erosion and deposition exaggerates the bends until large meanders are formed. the combined processes also create the meanders’ distinctive asymmetric cross sections
- oxbow lakes are formed when the neck of the lop
5
Q
how are meanders formed
A
- when alternating pools (areas of deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river. the distance between pools in 5-6 times the width of the river bed
- because the river channel is deeper in pools its more efficient, so has greater energy and more erosive power. energy is lost as the river flows over a riffle because of friction
- the spacing and distance between riffles and pools causes the rivers flow to become uneven and maximum flow to be concentrated on one side of the river
turbulence increases in and around pools as the water speeds up, so flow of water begins to twist and coil - this causes corkscrew like currents in the river called helicoidal flow which spiral from bank to bank between pools
- the helicoidal flow causes more erosion and deepening of the pools. it also causes eroded material to be deposited on the inside of the next bend, where the river loses energy
- the combination of erosion and deposition exaggerates the bends until large meanders are formed. the combined processes also create the meanders’ distinctive asymmetric cross sections
- oxbow lakes are formed when the neck of the loop of a meander is broken through, often during flooding. deposition dams off the loops leaving an oxbow lake
6
Q
4 features caused by fluvial deposition
A
braiding
levees
deltas
flood plains
7
Q
4 features caused by fluvial deposition
A
braiding
levees
deltas
flood plains
8
Q
how does braiding occur
A
- when rivers are carrying a vast amount of eroded sediment
- if the rivers velocity drops, or the sediments load becomes too much the the river to carry, sediment is deposited in the channel
- this causes the river to divide into many small, winding channels that eventually rejoin to form a single channel
9
Q
how to levees form and what are they
A
- they are natural embankments formed as a river overflows its banks
- during a flood, material is deposited across the whole floodplain as the river loses velocity and energy due to increased friction
- the heaviest material is dropped first, closest to the river
- overtime this material builds up on the riverbank creating a levee
10
Q
How do deltas form
A
- the river reaches the sea or lake, the energy of the river is is absorbed results by the slow-moving water of the sea - this causes the river to deposit load. these deposits buildup on the seabed, until the alluvium Rises above sea level, partially blocking the mouth of the river
- the river has to braid into several distributaries in order to reach the sea, forming delta
11
Q
How does a river rejuvenate
A
- its base level is lowered. This can be caused by either the ground level rising (crustal uplift) or by a drop in sea level
- the drop in baselevel gives the River greater potential energy, increasing its vertical erosion potential
- the long profile of the river is extended and knickpoint (a sharp point in gradient, often a waterfall) Will form and mark the junction between the original long profile and the new one
12
Q
Which landforms associated with rejuvention
A
- River terraces are former floodplains which have been left above the level of present-day flooding following increased vertical erosion
- incised meanders are formed when a river keeps its meandering course as vertical erosion increases. The result is a deep, winding valley with steep sides. the river is left far below the level of the former floodplain
