Geoscience Reference
In-Depth Information
Traditional
landsystem
Figure 24.15
The character and
contemporary hazards of the
traditional alpine landsystem.
Climatic and
economic hazards
Mountain snowfield,
cirque and valley
glaciers; permafrost
Alpine tundra, Alpine
rockfalls, moraine
and colluvial debris
Global warming
- enhanced glacier
melt and retreat.
Increased rockwall
and debris exposure.
Krummholz
Forest-tundra
ecotone/high
pasture.
Landslides and
debris flows.
Accelerated erosion.
Tourist development
- replacement of
traditional farming,
heavy construction,
slope and vegetation
damage; accelerated
erosion.
Timberline
Natural/planted
coniferous forest.
Valley floor slope-
channel system.
Intensive
pasture/arable
farming
Fluvial erosion, soil
loss and loss of
farmland
Earth's highest. They are also responsible for deep inci-
sions in areas of rapid uplift beyond the glacial environ-
ment. The erosive effect of mountain rivers is an order of
magnitude higher than that of lowland rivers. Their
regimes are markedly seasonal, with 'flashy' hydrographs,
responding to seasonal and diurnal melt episodes and the
high moisture fluxes, high relief, fast runoff-generating
character of mountain soil-vegetation-slope systems.
Snow and ice melt typically contribute 50-70 per cent of
annual discharge in alpine mountain rivers, with a similar
percentage occurring in just two or three summer months.
Jökulhlaupur, or glacier lake bursts, promote occasional
but exceptional flood events.
Mountain river channels show marked disequilibria in
both sediment movement and channel form. Sediment
delivery to upstream reaches is highly episodic, dependent
on sediment transfers through the talus-colluvial slope
and processes such as major rockslides and debris flows
which override it. This, and the immature development
of rock slopes, tend to create irregular beds. They are rock-
bound and stepped in places and armoured by large blocks
in others. As a result, suspended sediment loads tend to
be significantly higher than elsewhere. Timberlines and
lakes buffer downstream reaches from slope sediment
yields. Braiding is frequently found in mountain rivers
where they enter flat valley-floor reaches or, especially, as
Plate 24.12 Talus cones, talus sheets and a paraglacial
landslide (left) below frost-weathered rock pinnacles in Nant
Ffrancon, North Wales. Although now largely relict forms,
their surfaces are scarred by recent and contemporary debris
flow tracks, lined by levees and terminating in debris fans.
Photo: Ken Addison
 
 
 
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