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Lake Retreat: African river valley once hosted big lake

The valley of the White Nile, one of two main tributaries of Africa's longest river, may long ago have held a shallow lake that sprawled 70 kilometers across and stretched more than 500 km along the river.

Satellite images clearly show a continuous string of arc-shaped features that extends about 280 km southward from Esh Shawal, Sudan, along the eastern margin of the White Nile valley. Previous ground surveys found that the elevations of those landforms vary by no more than 2 meters, says Martin A.J. Williams, a geographer at Australia's University of Adelaide. West of these contours, the terrain slopes gently toward the river, dropping about 15 centimeters across each westward kilometer. East of the features, the ground rises at a steeper gradient of 3 meters per kilometer.

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