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Difficulty: Very hardDrainage Systems: Major Rivers and Lakes of Africa

The River Niger exhibits an unusual boomerang-shaped course, rising just 240 kilometres from the Atlantic Ocean in the Fouta Djallon highlands of Guinea, yet flowing northeastward into the arid Saharan margin before bending sharply south to discharge into the Gulf of Guinea. Which of the following geological and hydrological phenomena best explains this anomalous flow path?

  1. The capture of an ancient endorheic upper inland river system by a lower coastal river eroding headward from the Gulf of GuineaAnswer
  2. B
    The structural obstruction of the East African Rift System directing the upper course headwaters toward the Mediterranean Sea
  3. C
    The heavy deposition of deltaic sediments at the upper course causing localized structural inversion toward the Indian Ocean
  4. D
    The rapid down-slope mass wasting along the Jos Plateau boundary that continuously blocked direct Atlantic outlets

Answer

The capture of an ancient endorheic upper inland river system by a lower coastal river eroding headward from the Gulf of Guinea
The unique course of the River Niger (the 'elbow of capture' near Bourem/Timbuktu) is the result of stream piracy (river capture). Historically, the Upper Niger originated in the Fouta Djallon and discharged into a landlocked inland lake (Lake Juf) in the Sahara. Meanwhile, a lower coastal stream eroding headward from the Gulf of Guinea cut through the highlands at Tosaye and captured the inland lake waters, diverting the entire upper river system southward into the Atlantic Ocean.

Step-by-Step Solution

1
Analyze the geological history of the River Niger's course.
Identified that the River Niger is composed of two distinct historical sections: the Upper Niger and the Lower Niger.
Understanding river evolution helps explain why a river rising close to the coast flows deep inland before returning south.
2
Evaluate the mechanism of River Capture (Stream Piracy) at Tosaye.
The Upper Niger used to empty into an endorheic (inland drainage) lake near Timbuktu. The Lower Niger eroded headward until it breached the sill at Tosaye, capturing the upper basin.
Headward erosion by a energetic coastal stream capturing an inland system is the established geological explanation for the River Niger's elbow of capture.

Key Concept

River Capture and Drainage Evolution of Major African Rivers
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Topics

Drainage Systems: Major Rivers and Lakes of Africa

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