Geoscience Reference
In-Depth Information
The modern confluence of the Blue and White Nile is at Khartoum, and the main
Nile begins at this point. When the unregulated Blue Nile was in flood in modern
times, the waters of the Blue Nile used to travel 300 km up the White Nile to create a
'flood reservoir' or 'pulsating lake' up to 3 km wide near its northern end (Willcocks,
1904
). Once the Blue Nile flood had slackened, the pent-up waters of the White Nile
were released into the main Nile, helping maintain perennial flow in that river. With
the completion in 1935 of the Jebel Aulia Dam on the White Nile 35 km upstream
of Khartoum, the reservoir when full also began to produce a body of slack water
that extends about 300 km upstream. The creation of a seasonal lake in the lower
White Nile Valley has most likely been the rule ever since the White Nile joined
the Blue Nile during the Middle Pleistocene some 300,000 years ago (Talbot and
Williams,
2009
). However, the location of the confluence has shifted over time, as has
the size and northern terminus of the lake. During the LGM at around 21 ka, aridity
prevailed across intertropical Africa (Hoelzmann et al.,
2004
). Lake Victoria was dry,
Lake Albert was low and there was no overflow from Uganda into the White Nile,
which dried up (Adamson et al.,
1980
; Beuning et al.,
1997
). The abrupt return of
the summer monsoon at 14.5 ka led to overflow into the White Nile and widespread
flooding across the valley up to an elevation of 382 m relative to the Alexandria datum
that was in use until recently (Williams et al.,
1982
; Talbot et al.,
2000
; Williams
et al.,
2006c
). Lake Tana in Ethiopia also overflowed at that time (Lamb et al.,
2007
;
Marshall et al.,
2011
), and the enhanced Blue Nile floods would have caused major
flooding in the lower White Nile Valley (Williams et al.,
1982
). The flow of water into
the White Nile was supplemented at that time by overflow from Lake Turkana into
the Pibor and thence into the Sobat, a major White Nile tributary (Harvey and Grove,
1982
).
There is striking evidence of even more extensive flooding in the White Nile Valley
in the form of a lake that was more than 500 km long and up to 50 km wide at
386 m elevation (Williams et al.,
2003
). The 386 m strandline of this lake has recently
been directly dated to last interglacial time using
10
Be cosmogenic nuclides (Barrows
et al.,
2014
). With enhanced flow from both the Blue and the White Nile at that time,
a seasonal lake formed and extended at least as far south as the present Melut bend on
the White Nile. The Blue Nile floods would have propagated about 400 km upstream
in the White Nile, to an elevation of 386 m. With the release of flow as the Blue
Nile flood waned, sandy channel bars and dry-season sand dunes may have acted as
temporary dams. The northern limit of the 386 m lake coincides with a former Blue
Nile channel that bifurcates and joins the White Nile between 70 km and 120 km
upstream of the modern confluence.
A series of former Blue Nile channels radiate north-west across the alluvial plain
west of the Blue Nile towards the White Nile and vanish beneath the dunes located
between Jebel Aulia in the north and Naima in the south (
Chapter 10
,
Figure 10.10
).
The heavy mineral assemblage at six sites within the sand dune complex indicates that
