Rivers, Floods and Management: River Landforms

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A Levels Geography (Physical Geography-AS) Mind Map on Rivers, Floods and Management: River Landforms, created by Andrew_Ellinas on 05/06/2014.
Andrew_Ellinas
Mind Map by Andrew_Ellinas, updated more than 1 year ago
Andrew_Ellinas
Created by Andrew_Ellinas almost 11 years ago
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Resource summary

Rivers, Floods and Management: River Landforms
  1. Potholes
    1. Small circular depressions in the riverbed, carved out of solid rock.
      1. Commonly found in upland areas, close to the source of the river, where the channel flows directly over bedrock.
        1. Formed when rock fragments are 'drilled' into holes and cavities by turbulent whisking-like action of the river. This type of erosion is called corrosion.
            1. Rapids and Waterfalls
              1. Tougher, more resistant rocks, are less easily eroded than weaker rocks, and they will often form irregular steps in the long profile of a river. These irregularities may create a very turbulent stretch of river, called rapids.
                1. Form white-water, as the river plunges over jagged rocks and mini-waterfalls to form dangerous whirlpools and fast-flowing tubes of water.
                  1. A waterfall is a single, more pronounced, 'step' in the long profile of a river.
                    1. Most commonly formed when a river flows over a relatively tough band of rock.
                      1. Directly beneath the waterfall is a plunge pool, formed by the erosion of hydraulic action to the less resistant rock underneath.
                        1. Gradually the waterfall will undercut the rock behind it. A narrow steep sided gorge will be formed.
                            1. Meanders
                              1. Sweeping bends made by a river, and are often found in lowland areas in the river's middle course
                                1. Thalweg is the line of greatest velocity or fastest flow within the river.
                                  1. Erosion on the outside bend, deposition on the inside bend.
                                    1. Helicoidal flow is the corkscrew-like flow of water in a meander
                                        1. Alternating sequence of shallow, fast-flowing sections called riffles. And deeper, slower moving sections called pools.
                                          1. Riffles and pools commonly form in channels during low-flow conditions, and are closely linked to the very complex pattern of Helicoidal flow,
                                              1. The old meander loop is called an oxbow lake. This is a lake that has resulted from the deposition and erosional processes working antagonistically.
                                              2. Braiding
                                                1. A river described as being braided when it becomes sub-divided into many separate channels.
                                                  1. Channel becomes overloaded, results in the channel depositing sediment in the channel to form islands.
                                                    1. Common in rivers that experience large variations in discharge, and where the load is relatively large e.g. rivers that flow from the snouts of glaciers.
                                                      1. Floodplains and Levees
                                                        1. A floodplain is an extensive flat area of land on either side of the river which periodically becomes flooded
                                                          1. Floodplains are most extensive in lowland areas, where they can be several km wide
                                                            1. Silt deposits that form the floodplain are fertile, good for farming.
                                                                1. The river flows over sediment that have been deposited on the valley floor by previous floods - every time the river floods, it deposits a fresh layer of silt on top of the existing floodplain.
                                                                  1. When the river floods, the water overtops onto the floodplain and larger and heavier sediment is deposited on top of the banks. This is because water velocity at this point is much less than in the main channel.
                                                                    1. The build up of sediment gradually raises the bank and these are called levees.
                                                                    2. Deltas
                                                                      1. A river loses energy very rapidly when it enters the sea (or lake) and as a result, vast amounts of sediment are deposited in a fan shape.
                                                                        1. The build up of deposited material breaks through the water surface and this forms new land called a delta.
                                                                          1. A river in a delta is forced to split into different rivers called distributaries. This is the rivers response to the massive deposition o sediment and reduction in the gradient.
                                                                            1. An arcuate delta is a gently curved delta.
                                                                              1. A birds foot delta is shaped like a 'birds foot' and represents deposition along the edges of several distributaries.
                                                                                1. A cuspate delta is shaped like an arrow towards the sea.
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