1) waterfalls form where a band of hard rock meets a softer rock. The soft rock is eroded more than the harder rock, causing a step in the river bed.
2) water flowing over the step speeds up die 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 the harder rock.
3)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 round by turbulence
4) over time, more undercutting causes more collapse. The waterfall will retreat(move back up the channel) leaving behind a steep sided gorge.
are small circular holes in the river bed.
they're formed by abrasion as turbulence swirls a river bed-load round in a circular motion, causing it to rub and scrape out holes.
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.
Large sweeping curves in a rivers middle and lower stages are called meanders. They're formed by erosion and deposition.
1) meanders form where alternating pools(deep water) and riffles(shallow water) develop at equally spaced intervals along a stretch of river. the distance between the pools is 5-6 times the width of the river bed.
2) as river channel is deeper in pools it's more efficient, so it has greater energy and so more erosive power. energy is lost as the river flows over the riffle(shallow water) due to friction
3) the spacing and distance between the riffles(shallow water) and pools(deep water) causes the river flow to become uneven and maximum flow to be concentrated on one side of the river.
4) turbulence increases in and around the pools(deep water) as the water speeds up, so the flow of water begins to twist and coil.
5) this causes corkscrew-like currents in the river called helicoidal flow,which spiral from bank to bank between pools.
6) the helicoidal flow causes more erosion and deepening of the pools. it also causes eroded material to be deposited on the inside of the bend, where the river looses energy.
7) the combination of erosion and deposition exaggerates the bends until large meanders are formed. the combined process also creates the meanders distinctive asymmetric cross section.
8)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.
1) occurs when the river is carrying a vast amount of erosded sediment
2)if the rivers velocity drops, or the sediment load becomes too much for the river to carry, sediment is deposited into the river channel.
3) this causes the river to divide into many small, wining channels that eventually rejoin to from a single channel.
1) when a river overflows its banks and floods the flat land either side of it theres an increase in the wetted perimeter and reduction in hydraulic radius.
2) This increases friction reducing the velocity of the river and causing fine silt and sand to be deposited across the flood plain.
1)natural raised embankments formed when a river over flows its banks.
2)during a flood, material is deposited across the whole flood plain as the river looses velocity and energy due to increased friction.
3)the heaviest material is dropped first, closest to the river channel.
4)over time, this material builds up on the river banks and creates levee's.
1) when a river reaches the sea, the energy of the river bed is absorbed by the lower moving water of the sea.
2) this causes the river to deposit its load. These deposits build up on the sea bed, until the alluvium(deposited sediment) rises above sea level, partially blocking the mouth of the eiver.
3)the river has to braid into several distributaries in order to reach the sea, forming a delta.