A White Nile Megalake

The Nile valley was once home to one of the largest lakes on Earth. The Nile River is the longest river in the world and has its source in two major catchments. The first is in the Ethiopian highlands where monsoon rains supply the Blue Nile with high seasonal flow and most of the sediment that ends up in Egypt. The second is the White Nile which has more reliable flow but doesn’t transport much sediment. At one point in the past, the White Nile was flooded to the point where it couldn’t completely drain and a lake filled the valley to south of Khartoum.

Prof. Martin Williams first recognised geomorphic features indicative of a lake back in 2003. On the grainy satellite imagery of the time he could see cuspate shorelines on the eastern side of the Nile, stretching more than 250 km south of Khartoum. The distance to the equivalent side on the west was up to 70 km. This stretch of the Nile (see map below) is unusually flat for a river valley. The floor only falls about 25 m over 250 km. Dates obtained from sediments associated with the high water level were ambiguous and indicated they could have been deposited up to 400,000 years ago.

When Martin first approached me at an Australian Academy of Science meeting in 2005 about trying to date the flooding event, I was rather apprehensive. Firstly, exposure dating sediments is not straightforward, mainly because sediments arrive at the deposition site with an existing inherited exposure to cosmic rays. Secondly, Sudan was in the middle of a civil war. The south was ceding from the north and Darfur was a war zone. Martin assured me it was safe and I submitted a risk assessment to the university which included an escape plan to the Red Sea or Egypt in case of conflict.

In 2006 Martin and I travelled to Khartoum in Sudan. Without VISAs explaining our trip, the Brigadier in charge of immigration wanted to deport us on the next flight out to the Middle East. Martin made some calls and his colleague Osman El Tom from the Agricultural Research & Technology Corporation arrived with paperwork. Meanwhile we were detained in a small room with a couple of chairs, a mattress and a feral cat.

After a few days of organisation we drove south to the village of Al Jabalayn, which means “place of two mountains”. The trip was relatively uneventful except for some unauthorised roadblocks and food poisoning. In the otherwise completely flat landscape, a number of granite inselbergs rise up out of the plain. Sediment eroding from the weathering mountains was reworked by the waves of the lake into a cuspate shoreline curving away to the north and south (see below), pinning the margin of the lake. We were surprised by the abundance of evidence for a large number of people living at the site during the Neolithic. It was much wetter during the African Humid Period than today and instead of desert, the area was fully vegetated with typical African animals. Scattered around the inselbergs were grinding holes carved into flat surfaces for processing seeds to flour. Also embedded in the sediments were ancient hearths with charcoal and bones of antelope or zebras. Spreading drought in the last few thousand years drove people away from this traditional lifestyle.

I collected samples from the top of the inselberg (or jebel) nearby to measure what the likely background signal was for cosmogenic nuclides in the weathered sediments. We selected part of the shoreline nearby to study, which was up to 1 km wide. To dig a trench, we visited the local road works and arranged to borrow a bulldozer. The next day it arrived driven by a boy who didn’t look over 15, who quickly dug us a 3 m deep trench. The excavation revealed the stratigraphy of the beach ridge, which was largely bedded gravel, indicative of strong wave action. We took a number of photos of the section. After digging out these digital photos I used photogrammetry and made the 3D model of the section above. I didn’t know about structure from motion at the time so was pleasantly surprised when it worked on such old photos. The model reveals the texture of the profile and stitches the photos together so that an orthophoto can be created, scaled to the tape measure.

The results came as something of a surprise1. When you account for the inherited signal, the amount of exposure put the age of the shoreline in the last interglacial period. Every 100,000 years or so there is a brief respite in the ice age when it is much warmer and the continental ice sheets of the northern hemisphere melt. Equally importantly in Africa, the shifts in the Earth’s orbit mean that there was more sunlight hitting northern Africa. Combined with the influence of the ocean, this results in a much “stronger” summer monsoon, bringing more rain into Africa. We were excited about having such strong evidence for this event. Our paper was reported by New Scientist and popular science websites such as Archaeology News Network and Discovery News. A link to a BBC radio interview is below.

Why did a lake form? This seems to be a result of a combination of a number of factors. The valley has a low gradient, so the water flows more slowly in the White Nile valley as opposed to other river valleys. Second, there are geographic features which constrict flow near Khartoum. The enormous Khor Abu Habl alluvial fan from the Nuba mountains abuts the Nile valley from the west. Linear dune fields blown out from the fan encroach on the Nile further to the north, forcing it against the Manaqil Ridge. Lastly, the Blue Nile joins from the east. When flooding, this provides a temporary damming effect. Dating of ancient channels of the Blue Nile reveals that it joined the White Nile much further south than present where the valley is flatter, compounding the damming effect.

After sampling, we travelled north along the western margin of the Nile (see GPS map below). We took the opportunity to sample alluvial sediments and dunes along the way for dating2. The area is difficult to travel to and any new information on the palaeoenvironment of this region is valuable. At one stop in particular near Ad Douiem, we were lucky to find a sandpit cut into a palaeochannel of the Nile. The sediment was coarse sand and had enormous crossbeds dozens of metres across. Dating revealed the channel was cut about 28,000 years ago. In a time of declining precipitation leading up to the maximum of the last ice age, the river was shallower and more energetic. The style of sedimentation here was in stark contrast to the sluggish river that formed a gigantic lake to the south.

1. Barrows TT, Williams MAJ, Mills SC, Duller GAT, Fifield LK, Haberlah D, Tims SG, Williams FM (2014). A White Nile megalake during the last interglacial period. Geology, 42(2), 163-166
2. Williams MAJ, Williams FM, Duller GAT, Munro RN, Tom OAME, Barrows TT, Macklin M, Woodward J, Talbot MR, Haberlah D, et al (2010). Late Quaternary floods and droughts in the Nile valley, Sudan: new evidence from optically stimulated luminescence and AMS radiocarbon dating. Quaternary Science Reviews, 29, 1116-1137

Red dots = sampling sites.

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