Rivers

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Waterfalls and Rapids

Occur when there is a sudden change in the gradient of a river.

  • Rivers flow over bands of soft and hard rock.
  • The softer rock erodes more quickly, leaving a plateau of the more resistant rock.
  • This creates an undercut, leaving an overhang of the hard rock. Where the water falls down over this hard rock, it creates a plunge pool below. The force of the water landing in the plunge pool and the weaker rock in its load deepens the plunge pool.
  • Eventually, the resistant rock collapses, falling into the plunge pool.
  • The waterfall is moved upstream, leaving a gorge.
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Potholes

Cylindrical holes drilled into the bed of a river by water at high velocity with pebbles in its loadThey occur in the upper/middle course.

  • Vertical eddies in the water are strong enough that sediment grinds a hole in the rock.
  • Can be from a few centimetres to a metre wide.
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Braided channels

Braided channels are when the river is forced by islands of deposited sediment to split into several channels. They occur due to large levels of sand/gravel and a variable dischargeBraiding also occurs in semi arid areas that recieve rivers from mountainous areas and glacial streams with variable annual discharge.

  • The channel becomes very wide in relation to its depth.
  • The river becomes choked with sandbars
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Meanders

Meanders are sinuous bends in a river.

  • In low flow conditions, straight channels have alternating bars of sediment on their beds, forcing the water to move around them.
  • This creates alternating shallow sections (riffles) and deeper areas (pools).
  • The maximum velocity is therefore directed towards one of the banks, making an outer concave bank. It erodes, and the river's sinuosity increases.
  • The cross section of a river is asymmetrical.
  • The outer bank forms a river cliff/bluff with a deep pool close to the bank, which is undercut by erosion. The inner bank becomes a point bar.
  • Meanders flow across to the concave outer band with a compensatory subsurface return - helicoidal flow.
  • Eroded material on the outer bank is transported along and deposited on the inner.
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Oxbow lakes

A horseshoe lake separated from the river, created where there are meanders.

  • The river is eroding laterally
  • The meander becomes narrower, due to high discharge -The river erodes the outside of the bends and deposits on the inside so its course is changing.
  • This erosion narrows the neck of the meander
  • To take the path of least energy usage, during a flood, the river cuts through the neck of the meander, creating the new channel.
  • A horseshoe shape is left behind. New deposition seals off the ends and the cut-off becomes an ox-bow lake

 

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Levees

  • On the middle and lower courses, a river is at risk from flooding during times of high discharge.
  • If it floods, the velocity falls, resulting in deposition and forming small raised banks along the sides of the channel.
  • Subsequent floods increase the size of these banks.
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Flood Plains

Flood plains are created as a result of erosion and deposition. A floodplain is the area around a river that is covered in times of flood. 

  • They are relatively flat areas of land on either side of the river which form the valley floor in the middle and lower courses.
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Deltas

Deltas are a feature of deposition at the mouth of a river.

  • They occur as fresh water mixes with sea water and clay particles coagulate. Clay then settles on the riverbed.
  • Deltas only form when the rate of deposition exceeds that of sediment removal.

They are usually composed of:

  • larger particles (the first to be deposited), which make the topset beds,
  • medium grade particles (deposited as steep angled wedges), which make the foreset beds,
  • the finest particles (which travel furthest), which form the bottomset beds.
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Types of delta

  • Arcuate - the land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect. curving shoreline, dentritic (branched form resembling a tree) pattern of drainage
  • Bird's foot - the river splits on the way to the sea, each part of the river juts out into the sea, rather like a bird's foot
  • Cuspatethe land around the mouth of the river juts out arrow-like into the sea. Shaped by gentle opposing currents/longshore drift
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