Fluvial Landforms - from Fluvial Erosion Flashcards
Explain the characteristics and formation of Waterfalls + Gorges.
Waterfalls form 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 and hydraulic action at the foot of the waterfall as the bits of collapsed rock are swirled round by turbulence.
Over time, more undercutting causes more collapse. The waterfall will retreat (move back up the channel), leaving behind a steep-sided gorge.
Explain the characteristics and formation of Potholes.
Potholes are small circular hollows in the river bed. They’re formed by abrasion as turbulence swirls a river’s bedload round in a circular motion, causing it to rub and scrape out holes - vertical eddies in the water are strong enough to allow the sediment to grind a hole into the rock by abrasion (corrasion).
Potholes vary in depth and diameter from a few centimetres to several metres.
The process of water swirling around the Potholes is called Eddying.
Explain the characteristics and formation of Rapids.
Rapids are relatively steep sections of river with turbulent flow where there are several sections of hard rock. They’re a bit like ‘mini-waterfalls’.
Different resistance amongst rocks causes the formation of Rapids. They are formed when the water goes from one hard rock that resists the water’s erosion to a softer rock that is eroded easier.
Explain the characteristics and formation of Meanders + Oxbow lakes.
Meanders are formed by combined erosion and deposition.
Meanders are large, sweeping curves found in a river’s middle and lower stages. They’re formed by erosion and deposition.
Meanders form where alternating pools (areas of 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 it’s 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 river’s 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 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-section.
Oxbow lakes are features of both erosion and deposition. An oxbow lake is a horseshoe-shaped lake seperated from an adjacent river.
Oxbow lakes are formed when the neck of the loop of a meander is broken through, often during flooding. Deposition dams off the loop, leaving an Oxbow lake.
