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Rivers in cold mountain areas typically have braided channels with cross sections approchaning a
rectangular
shape. This is due to the relatively steep slopes, the abundance of coarse debris and the high
amplitude in seasonal discharge variation. Coarse material is transported as bedload whereas fine material
is carried in suspension. Meltwater from glaciers with intense basal abrasion is especially rich in fine
material, giving the corresponding rivers and lakes the characteristic milky colour.
In flat areas covered by fine sediments
(Loess in the Qinghai-Tibet Plateau, for instance; Figure 1), meanders
rich in suspended sediments and with asymetric parabola-shaped cross sections develop. Channels on very steep
slopes become straight and are usually V-shaped. Such steep channels commonly involve episodic discharge
and debris-flow activity. |
1 - Rakaia river (New Zealand) showing several braided
water channels, which are typical for glacier forefields with strong
seasonal runoff fluctuations.
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Sediment input into river systems is from
- incision by the river into its own bed
- lateral erosion through weakening, weathering and undercutting steep lateral slopes
- rock fall from cliffs
- larger-scale slope instabilities (landslides of various scale)
- tributary gullies and debris flows
Sediment flux is related to discharge: large rivers carry more than small ones and flood
discharge edvacuates more debris than normal flow. However, there is much scatter in corresponding statistical
relations, because sediment flux depends more on the availability of sediments than on the transport capacity
of the river. The most easily eroded material is sand, because cohesion in clay and gravity in gravel slow
down the process. With increasing discharge - during the development of a flood, for instance - the
typical periglacial rivers with braided channels and heavy bedload tend to pass directly to erosion
without much transition in a transportation mode that is characteristic for meandering channels.
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