Geoscience Reference
In-Depth Information
Figure 10.6. Run-off and infiltration associated with deep and shallow soils.
the development of distributary channels. He concluded that floodouts are neither
alluvial fans nor terminal fans but a distinctive landform related genetically to
floodplains.
Table 10.1
is a very simple and highly schematic summary of some of the more
important differences between desert river systems and those located in well-vegetated
humid regions, one of which (cited at the start of this chapter) was expressed succinctly
by G.K. Gilbert (
1877
) more than a century ago. Where annual rainfall is sufficiently
high, as in tropical and temperate humid regions, the soil surface along the valley
sides has a dense plant cover, with deep, well-structured soils and abundant soil
pores, so infiltration rates tend to be high and run-off tends to be relatively low
(
Figure 10.6
). In contrast, under the sparse or absent plant cover of arid areas, the
bare soil surfaces are prone to form impermeable surface crusts as a result of raindrop
impact, so that infiltration rates into the often shallow soils are low and run-off rates
are correspondingly high. As a result, desert streams are prone to flash floods and
have typically erratic, ephemeral or highly seasonal flow regimes, in contrast to the
regular perennial stream flow of humid regions, where the streams are maintained by
high rates of base flow.
Because the valley sides in humid regions are protected from erosion by the dense
plant cover, the amount of sediment supplied to the stream channels tends to be
relatively low and often quite fine, again in contrast to desert rivers with their coarse
sediment loads and high rates of sediment influx during the intense but infrequent
storm events. In well-vegetated humid regions the streams tend to carry a sizeable
suspension load of clay- and silt-sized particles and to be sinuous in plan view and
often well-entrenched into their flood-plains. Desert streams, on the other hand, tend
to be bed-load streams with steep, unstable, shifting channels. Obviously, there are
exceptions to each of these generalisations. The highest rates of sediment yield in
rivers come from both semi-arid and seasonally wet tropical regions (Douglas,
1967
;
Milliman,
1997
). In effect, the seasonally wet tropics operate as semi-deserts during
the dry season and as humid tropics during the wet season.
