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The starting point ­ Pools and Riffles
Rivers rarely flow in a straight line.
Even in a straight section, flow is unstable
The origins are uncertain but it is thought that,
in times of flood, POOLS and RIFFLES
develop in relatively straight sections
POOLS are areas of deeper water
RIFFLES are areas of shallower water…read more

Slide 3

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Pool and riffle sequences
It is not known why but these appear in an alternating
pattern along straight sections of river, between 5 and 6
times the width of the channel between each pool, or each
POOLS are areas of greater erosion where the available
energy builds up due to a reduction in friction
This energy is then dissipated across the following
RIFFLE region
Because greater energy is needed to overcome the greater
friction in the shallower RIFFLE, erosive capacity is
decreased and (except in times of flood) DEPOSITION
tends to occur here…read more

Slide 4

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Pool and riffle sequences
The regular spacing of the pools and riffles is
thought to occur due to SECONDARY FLOWS
that exist within the channel
HELICOIDAL FLOW is thought to be the most
important of these, a corkscrew movement
Helicoidal flow is thought to be responsible for
moving material from the outside of one meander
bend and depositing it on the inside of the next
This helicoidal flow, and other secondary flows,
are thought to slowly increase the curving nature
of the channel (SINUOSITY)…read more

Slide 5

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Sinuosity results in a regular wavelength in a
meandering river channel
It is usually about 10 times the channel width
Sinuosity can be represented by a formula:
Actual channel length / straight line distance
The older the meander system, the more
sinuosity there will be…read more



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