Geoscience Reference
In-Depth Information
(Butzer et al.,
1972
) and Uganda (Livingstone,
1980
). In addition, as we saw in
Chapter 8
, Saharan desert dunes were active up to 800 km south of their present limits
(Mainguet and Canon,
1976
; Mainguet et al.,
1980
; Talbot,
1980
; Swezey,
2001
). The
White Nile, deprived of the run-off from its headwaters by the closure of the Ugandan
lakes, dried out in the winter months, during which sand dunes migrated across its
former bed (Williams,
2009b
).
To sum up, the late Pleistocene Blue Nile and Atbara rivers were highly seasonal,
bed-load streams that, together with their tributaries, ferried and deposited vast quant-
ities of poorly sorted sands and gravels in central Sudan and southern Egypt (Williams
et al.,
2010a
). With the return of the summer monsoon around 17 ka, strengthening at
15 ka (Williams et al.,
2006c
), run-off increased in the Ethiopian headwaters and Lake
Tana overflowed once more (Lamb et al.,
2007
). From around 15 ka until around 7.5 ka
and perhaps slightly later (Williams,
2009b
), the Holocene Blue Nile was depositing
clays across the low-angle Gezira alluvial fan in the central Sudan. Thereafter, it began
to incise, terminating its upward-fining depositional cycle.
A similar pattern of widespread deposition of late Pleistocene sand and gravel,
followed by terminal Pleistocene to early Holocene fine-grained alluviation culmin-
ating in vertical river entrenchment has been documented for the Son and Belan rivers
in semi-arid north-central India (Williams and Clarke,
1984
; Williams et al.,
2006b
;
Gibling et al.,
2008
), as well as in the subhumid to semi-arid Murray and Murrum-
bidgee river basin in south-eastern Australia (Bowler,
1978a
;Bowler,
1978b
;Page
et al.,
1991
;Pageetal.,
1996
). It thus appears that rivers in semi-arid catchments are
sensitive to changes in plant cover, whether they were once glaciated or not. A sub-
stantial reduction in vegetation cover in their headwaters is conducive to a bed-load
regime, reverting to a suspension-load regime once the plant cover has been restored
and a soil cover has been widely established in the headwaters. In essence, in the
absence of any eustatic, isostatic or other tectonic causes of changes in base level, a
river will tend to aggrade its valley when the ratio of load to discharge is high and
to degrade its valley when the ratio of load to discharge is low. However, care is
needed to avoid falling into the trap of circular argument in which a given type of
climate (wetter, drier, transitional from wet to dry or dry to wet) is inferred from the
presence of a river deposit and the inferred climate is then used to account for the
existence of the same deposit. Some independent check on the purely fluvial evidence
is therefore necessary when seeking to reconstruct climatic changes in deserts (Reid,
2009
).
10.12 Conclusion
River sediments are useful archives with which to reconstruct past hydrologic changes
in arid areas. The sediments themselves provide a guide to the type of erosion and
