Geoscience Reference
In-Depth Information
Table 10.2. Erosion processes
Agent
Erosion process
Sediment produced
Raindrop impact
Rainsplash
Colluvium
Overland flow
Slopewash
Colluvium
Sheet-floods
Slopewash
Colluvium
Slow mass movement
Soil and rock creep
Colluvium
Rapid mass movement
Landslides, mudflows
Colluvium
Rills and gullies
Channelled flow
Alluvium
Streams and rivers
Channelled flow
Alluvium
Frost and ice
Gelifraction, solifluction
Periglacial deposits
Glaciers, ice caps
Glacial erosion
Glacial deposits
Wind
Deflation, wind abrasion
Eolian sand, loess
Waves
Beach erosion, long-shore drift
Beach deposits
Groundwater
Leaching and solution
Precipitates
10.3 Sediment sources
In most desert rivers originating from upland regions, the bulk of the sediment comes
from the mountainous headwaters, as for example in the Nile Basin, where the
Ethiopian headwaters supply most of the load and much of the wet season discharge.
The same situation holds true for large tropical rivers like the Amazon, in which
roughly 90 per cent of the dissolved and suspended loads comes from 10 per cent of
the basin area, namely the Andean headwaters (Gibbs, 1967 ; Meade, 2007 ). Given the
importance of the headwaters as sources of sediment, we will begin with a brief survey
of weathering and erosion processes in this critical sector. Even in deserts, erosion
by running water is the most effective agent of geological erosion; wind erosion is
limited to local undercutting of the softer rocks and sediments ( Chapter 9 , Figure 9.2 ).
Desert dunes are themselves reworked sediments laid down initially by rivers or as
beach deposits of former lakes.
Erosion is the detachment and transportation of earth materials. Geological erosion
(or denudation) involves the wearing down and/or wearing back of upland areas
to achieve eventually a surface of low relief in tectonically stable areas with a
long history of weathering and erosion. Such erosion surfaces have been variously
termed peneplains, peneplanes, pediplains, pediplanes or planation surfaces, all of
which have in-built assumptions about how they might have formed (Davis, 1909 ;
Davis, 1912 ; Penck, 1924 ; Penck, 1953 ). In order to avoid genetic connotations,
it seems wiser simply to use the term erosion surface . Accelerated erosion is the
destruction and removal of soil at a rate that is perceptibly faster than the geo-
logical rate characteristic of that region. For soil to form, there must be a rough
balance between weathering and erosion. In seasonally wet tropical regions, soil
formation may take thousands to tens of thousands of years (10 3 -10 4 years), while
soil loss may only take decades or centuries (10 1 -10 2
years). Table 10.2 lists
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