978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

figure 1.20 Horseshoe Falls, part of Niagara Falls on the USA/Canadian border.
figure 1.21 Retreat of Niagara Falls, 1678–2015
A low sinuosity river has a value of 1.0 (straight) whereas a high sinuosity
river may have a value above 4.0.
A meander is the term used for a bend in the river with a sinuosity greater
than 1.5. Though no agreed explanation for their formation occurs, it is generally
considered to relate to the energy balance of the river and not the result of an
obstruction within the channel or floodplain.
figure 1.22 A sweeping meander
Meander form
Meanders have an asymmetric cross section (Figure 1.23). On the outside of
the bend, where flow is fastest, erosion deepens the channel. On the inside of
the bend, where flow is slower, deposition occurs. Helicoidal flow occurs where
surface water flows towards the outer banks while the bottom flow is towards
the inner bank. Variations in the flow create differences in the river cross
sections. The most characteristic features of meanders are river cliffs and slip-off
slopes or point bars.
River cliffs are formed on the outside of the bend where erosion is greatest.
The combined effect of hydraulic action and abrasion weaken the riverbank
causing it to collapse. Over time a steep bank will be formed with some of the
collapsed material remaining on the riverbed.
Conversely, on the inside of the meander bend where discharge is at a minimum
and friction is at its greatest, deposition is greatest. Sediment accumulates to create
a gentle sloping bar known as a slip-off slope or point bar. The particles are usually
graded in size with the largest material being found on the upstream side of the bar.
Riffles and pools are a sequence of alternating fast and slow flows as a result
of the differing energy states of the river. Riffles are shallow areas of fast flowing
oxygenated water. Pools are deeper areas with slow moving water.
Not all meanders have a regular form but they do have several key characteristics:
• The meander wavelength tends to be 10 times the channel width (λ ≈ 10 – 14 W).
• Riffles and pools are spaced 5–7 times the channel width (riffle spacing
≈ 5 – 7 W or ≈ ½ λ).
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Hydrology and fluvial geomorphology