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Figure 10.5. Alluvial terrace, north of Aqaba, Jordan Desert.
axial fluvial system is important, because there is relatively little sediment deposition
from axial river channels onto their flood-plains, in contrast to that associated with
distributary fluvial systems (Weissmann et al., 2010 ). Leier et al. ( 2005 ) referred to
what they termed megafans , which they considered to be diagnostic of river basins
located in monsoonal or seasonally wet tropical regions with mountainous headwa-
ters. In fact, a bigger sample of river basins (600) than that studied by Leier et al.
( 2005 ) indicates that megafans are included within the broader class of distribu-
tary fluvial systems, which span a wider range of climatic zones (Weissmann et al.,
2010 ).
Another fluvial feature common in arid central Australia and elsewhere are
'floodouts', which may be defined as sites 'where channelized flow ceases and flood-
waters spill across adjacent alluvial surfaces' (Tooth, 1999 , p. 222). The 'floodout
zone' is 'that part of the [ephemeral stream] system where there is a marked reduc-
tion in channel capacity compared with reaches upstream and where overbank flows
become increasingly important' (op. cit., p. 222). Tooth identified four main agents
responsible for the formation of floodouts, all involving barriers to flow (bedrock,
eolian, hydrologic, alluvial). Of these, the most important is channel constriction
upstream from Quaternary river terraces cemented by carbonate, iron or silica, and
the burial of these terraces down valley by younger alluvium, with a decline in
the size and capacity of the trunk stream channel, leading to overflow but without
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