Landforms Flashcards
What landforms does fluvial erosion create?
Waterfalls, rapids and potholes.
Explain how waterfalls are formed.
Waterfalls from where a band of hard rock meets softer rocks. The soft rock is eroded more than the harder rock, causing what looks like 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 the bits of collapsed rock are swirled around by turbulence.
Over time, more undercutting causes more collapse. The waterfall will retreat leaving behind a steep sided gorge.
What is a pothole?
Small circular holes drilled into the river bed. They are formed by high velocity turbulent water loaded with pebbles as it swirls a rivers bed load round in a circular motion, causing it to rub and scrape out holes by abrasion (corrosion). Attrition rounds and smooths the pebbles caught in the hole and helps reduce to the size of the bed loads. Pot holes can vary in width from a few cm to several meters and are generally found in the upper or middle course of a river, where the valley lies well above the base level and therefore there is more potential for downcutting, and where the river bed is more likely to be rocky.
What are rapids?
Rapids are relatively steep sections of river with turbulent flow where there are several sections of hard rock.
Explain the formation of meanders.
- Meanders form where alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river. The distance between pools is 5-6 times the width of the river bed.
- Because the river channel is deeper in pools its more efficient, so it 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 the flow of the water begins to twist and coil, which causes corkscrew like currents (helicoidal) in the river, which spiral from bank to bank between pools.
- The helicoidal flow causes more erosion and deeming of the pools. It also causes eroded material to be deposited on the inside of the next bend, where the river looses energy.
- The combination of erosion and deposition exaggerate the bends until large meanders are formed. The combined processes also create the meanders distinctive asymmetric cross-section.
How are oxbow lakes formed?
A oxbow lake is horseshoe- shaped lake separated from an adjacent river. They are formed when the neck of the loop of a meander is broken through, often during flooding. Deposition dams off the loop leaving the oxbow lake.
What landforms does fluvial deposition make?
Braiding, flood plains, levees and deltas.
Explain braiding..
Braiding occurs when the river is forced to split into several channels separated by islands. It is more likely to occur when rivers have a large load and are carrying a vast amount of sediment e.g. meltwater and when a river has variable discharges. The small, winding channels that are created, eventually rejoin to form a single channel.
Explain flood plains…
When a river overflows its banks it floods the flood plain. This is the flat land on either side of the river, which forms the valley floor in the middle and lower courses of the river. Overtime, the flood plain becomes wider and deeper. As the river floods, there is increase in the wetted perimeter and reduction in hydraulic radius. This increases friction, reducing the velocity of the river and causing fine silt and sand to be deposited across the flood plain.
Explain levees…
Levees are natural, raised embankments formed as a river over flows it bank and deposits its coarsest and heaviest material first e.g. sand and gravel. Other material is already deposited in the flood across the flood plain as the river looses its velocity and energy due to increased friction. As more floods occur, the levees are built up.
Explain deltas…
A delta is a feature of deposition, located at the mouth of a river as it enters a sea or lake. Deposition occurs as the velocity and sediment- carrying capacity of the river decrease on entering the sea or lake, and bed load and suspended are dumped. Flocculation occurs as fresh water mixes with sea water and clay particles coagulate due to chemical reaction. The clay settles on the river bed. Deltas form when the rate of deposition exceeds the rate of sediment removal. In order for deltas to form the following conditions are likely to be met:
- The sediment of the river is large
- The area into the which the river empties has small tidal range and weak currents.
What are the three types of deposit that deltas are usually composed?
Topset beds, forest beds and bottom set beds.
What are top set beds?
The larger and heavier particles are the first to be deposited as the river looses it energy, these form the top set beds.
What are forset beds?
Medium graded particles travel a little further before they are deposited as steep angled wedges of sediment.
What are bottom set beds?
The very finest particles travel the furthest into the lake before depositing.
