Desert rivers - Climatic Change in Deserts - page 181

author from the bottom of the Blue Nile gorge near Debre Markos in the highlands of
Ethiopia (Williams and Adamson, 1973 ). 14 C ages obtained previously on oyster and
gastropod shells, charcoal and carbonate samples within the paleochannel sediments
indicated that the Blue Nile paleochannels were at least seasonally active between
>

40 ka and 8 ka to 5 ka (Adamson et al., 1982 ). Current work has extended the
age of these channels back to at least 100 ka. The abandonment of the channels
was a direct result of incision by the main Blue Nile channel, which began at least
8,000 years ago (Arkell, 1949 ; Arkell, 1953 ; Adamson et al., 1982 ; Williams et al.,
1982 ). This incision effectively beheaded the distributary channels and deprived them
progressively of their flood discharge. As the Nile cut down into its former floodplain,
a series of shallow drainage channels remained seasonally active and finally dried out.
The final stages of infilling of these paleochannels and adjacent floodplains involved
deposition of a thin layer of dark grey-brown clay. The fining-upwards sequence
reflects a change from the transport of pale yellow medium and coarse quartz sands
and fine quartz and carbonate gravels to grey-brown silty clays, sandy clays and
clays in the upper 50-150 cm. Clay deposition in the back-swamps and flood-plains
of these channels dwindled and finally ceased about 5,000 years ago, when the
seasonally flooded swampy plains gave way first to acacia-tall grass savanna and
finally to semi-desert steppe. The fossil snail fauna within the upper two metres of
Holocene Gezira clay shows a progressive change from permanent water species to
semi-aquatic species with lungs and gills to land snails (Tothill, 1946 ; Tothill, 1948 ;
Williams et al., 1982 ). One of these, the large land snail Limicolaria flammata , today
inhabits the acacia-tall grass savanna region to the south of Sennar, where the annual
rainfall is at least 450-500 mm, in contrast to the 175 mm that now falls at Khartoum.
Limicolaria was at its most widespread in this region around 5.2 ka.

10.8 A late Pleistocene and early Holocene depositional model
for the Blue Nile
Thirty years earlier, when far fewer radiometric ages were available for this region,
Adamson et al. ( 1980 ) and Williams and Adamson ( 1980 ) proposed a simple depos-
itional model linked to climate and plant cover to account for these changes. During
cold, dry glacial intervals, the headwaters of major Ethiopian rivers would be sparsely
vegetated, hill slope erosion would be accelerated and rivers would become highly
seasonal, low-sinuosity, bed-load streams which carried and deposited large volumes
of poorly sorted gravels and sands ( Figure 10.12 ). Conversely, with a return to warm,
wet conditions and re-establishment of a dense plant cover in the headwaters, we
should see a change to high-sinuosity, suspended-load streams that carried and depos-
ited silts and clays ( Figure 10.13 ). A fining-upwards alluvial sequence from coarse
basal gravels through sands to horizontally bedded silts and clays is thus a predictable
outcome of a change from a bed-load to suspended-load regime, related to a change