Waterfalls, potholes and braided channels...

The formation of some of the landforms of Fluvial erosion and deposition...

Refers to AQA AS Geography (Chapter 1 - Rivers, floods and management)

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  • Created by: Bethany
  • Created on: 17-04-13 18:31
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  • Waterfalls, rapids, potholes and braided channels...
    • Waterfalls and Rapids
      • Occur when there is a sudden change of gradient as the river flows downstream
      • Waterfalls are more dramatic features:
        • May be due to a more resistant band of rock occurring across the rivers course.
        • Due to rejuvenation in an area (renewed erosional power as sea level falls)
        • River falls over a rock edge to a deep plunge pool, and layers of weak rock are excavated more quickly than overlying resistant rocks.
        • Force of swirling water around the rocks/ boulders enlarges and deepens the plunge pool by hydraulic action (force of water)/ abrasiom
        • Undercuts the cap rock above. The overhanging cap rock eventually collapses and the waterfall receives upstream (over thousands of years.) Creates a gorge.
      • Basic Case Study: High force in upper Teesdale
        • Outcrop of igneous rock (Whin Sill - Resistant Cap rock) overlies softer sandstone/ limestone, which are eroded more quickly, leaving the overhang.
        • 22m High - tallest in England
        • Gorge stretched 500m downstream
    • Potholes
      • Cylindrical holes drilled into the rocky river bed by turbulent, high-velocity water loaded with pebbles.
        • Pebbles become trapped in slight hollows and vertical eddies (currents) are strong enough to allow the sediment to grind a hole into the rock by abrasion
        • Attrition rounds and smooths the pebbles caught in the hole, helping to reduce the bedload size.
      • Very in width from a few cms to several metres
      • Found in upper of early middle course where valley lies well above base level (greater down-cutting potential)
    • Braided Channels
      • 'Braiding' occurs when a river is forced to separate into several channels, separated by islands.
      • Depositional feature of rivers supplied with large loads of sand and/or gravel
        • Most likely to occur when a river had variable discharges.
        • Its banks, formed by sand and gravel, are easily eroded (hence unstable)
        • Wide channel in relation to depth.
        • The river can become chocked, with several sandbars and channels that are constantly changing their locations.
      • It occurs in environments with rapidly fluctuating discharges:
        • Semi-arid areas of low relief receiving rivers from mountainous areas.
        • Glacial streams with a variable annual discharge. In Spring, meltwater causes discharge and competence to increase, so a river can transport more particles. As the river falls particles are deposited.

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