The eastern Sahara Desert is one of the most climatically sensitive areas on Earth, varying from lake-studded savanna woodland to hyper-arid desert over the course of a glacial-interglacial cycle. In presently arid Sudan there is widespread evidence that a very large freshwater lake once filled the White Nile River valley. Here we present the first quantitative estimate for the dimensions of the lake and a direct age for the emplacement of one of the shorelines. Using a profile dating approach with the cosmogenic nuclide 10 Be, we estimate an exposure age of 109 ± 8 ka for this megalake, indicating it formed during the last interglacial period. This age is supported by optically stimulated luminescence dating of Blue Nile palaeochannels associated with the lake. Using a high-resolution digital elevation model we estimate that the lake was more than 45,000 km 2 in area, making it comparable to the largest freshwater lakes on Earth today. We attribute the lake's existence to seasonal flood pulses as a result of local damming of the White Nile by a more southerly position of the Blue Nile and greatly increased precipitation associated with a super monsoon.

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