river landforms Flashcards
Formation of a Waterfall
1) River flows over alternative types of rocks.
2) River erodes soft rock faster creating a step.
3) Further hydraulic action and abrasion form a plunge
pool beneath.
4) Hard rock above is undercut leaving cap rock which
collapses providing more material for erosion.
5) Waterfall retreats leaving steep-sided gorge.
how do floodplains form
during a flood water containing large quantities of alluvium pours out over the flat valley floor
the water slowly soaks away, leaving behind deposited sediment
over hundreds of years, repeating flooding forms a thick alluvial deposit which is fertile and often used for farming
how do floodplains become wider
floodplains become wider due to the lateral erosion of meanders
when the outside bend of a meander meets the edge of the river valley, erosion will cut into widening the valley at this point
as meanders slowly migrate downstream the entire length of the valley will eventually be widened
formation of levees
as water overtops the river banks there is a reduction in the velocity of the water
this causes sediment in suspension to be deposited at the river bank
coarse sediment is deposited first and this traps the finer sediment
with each flood the deposited sediment rises the river banks by as much as a few metres
Formation of Ox-bow Lakes Steps
Erosion of outer bank forms river cliff. Deposition inner bank forms slip-off slope.
Further hydraulic action and abrasion of outer banks, the neck gets smaller
Erosion breaks through the neck, so the river takes the fastest route, redirecting the flow
Evaporation and deposition cuts off main channel leaving an oxbow lake
How are interlocking spurs formed?
In the upper course, rivers aren’t powerful enough to erode laterally, so they wind around the hillsides creating a zig-zag shaped flow.
- formation of Gorges
a gorge is narrower, steep sided valley found downstream of a retreating waterfall.
gorges can form in other ways- at tje end off the last glacial period masses of water from melting glaciers poured off upland areas forming gourges
on lime stone, when large underground caverns can accomodate an entire river.
formation of estuaries
incoming tides force seawater and sediment up the river channel
velocity falls dramatically resulting in significant deposition
over time, these muddy sediments break the water surface to form extensive mudflats
vegetation colonises the mudflats to form saltmarshes
following environmental conditions for meanders
gentle gradients
relatively fine sediments
steady precipitation throughout the year
describe the common features of meandering rivers
a common feature of meandering rivers is an alternating pattern of shallows and deeps. these features are associated with complex corkscrew like motion of velocity within the river called helicoidal flow
define and describe riffles
these shallow areas are associated with straighter sections of rivers between meanders. they usually have rocky beds and turbulent flow due to friction with the river bed
define and describe pools
these deeper areas are associated with the meander bends. they usually have finer sediment and less turbulent flow due to the smoother river bed
describe high flow for riffle and pool
riffle- greater friction in the shallower riffles results in slower more turbulent flow
pool- water tends to flow faster through the deeper pools due to a reduction in friction with the bed and banks
describe low flow for riffle and pool
riffle- on entering a riffle, the reduction in channel since often results in slightly faster flow
pool- the reduced volume of water tends to slow down on entering a deep pool, where the channel is larger
formation of meanders
erosion of the outer banks forms a river cliff due to decreased friction
deposition on the inner bend forms a slip of slope due to friction
Over time the horseshoe become tighter, until the ends become very close together.
