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

The River Niger exhibits a unique hydrological regime featuring a vast inland delta (the Macina) in Mali, where severe transmission losses occur due to high evapotranspiration and seepage in the semi-arid Sahel. Which of the following hydrological factors is primarily responsible for sustaining its downstream volume as it flows through Niger Republic and into Nigeria?

  1. The seasonal flood surge generated by heavy summer monsoon rains in its humid headwaters in the Fouta Djallon highlands, augmented by the influx of the Bani River.Answer
  2. B
    Subterranean seepage and direct overflow discharging westward from the Lake Chad endorheic basin into the upper Benue River.
  3. C
    High-altitude snowmelt and torrential upper-course tributaries originating from the Ahaggar and Tibesti mountain massifs.
  4. D
    Continuous subterranean inflow from the Nile River catchment moving westward across the Sudanian zone into the Niger basin.

Answer

The River Niger's flow into its middle and lower courses is sustained by the seasonal flood surge originating from heavy summer monsoon rainfall in the Fouta Djallon highlands of Guinea, combined with significant tributary inflow from the Bani River.
The upper River Niger originates in the high-rainfall Fouta Djallon plateau of Guinea, receiving heavy monsoonal precipitation. This generates a massive seasonal flood wave. As the river enters Mali, the Bani River contributes critical additional volume before the main flow enters the Macina inland delta, allowing sufficient water to survive extreme evaporative losses and sustain the middle and lower river courses.

Step-by-Step Solution

1
Analyze the hydrological behavior of the River Niger across its distinct course sections.
Identified that the upper course receives tropical monsoonal rainfall in the humid Fouta Djallon highlands, initiating a major annual flood wave.
Understanding headwater climatic controls explains how large volumes of water enter the system before reaching arid zones.
2
Examine the effect of the Macina inland delta on river discharge.
Recognized that while the Macina causes substantial water loss via evaporation and infiltration, the primary flood wave takes months to navigate the wetland.
Explains why a delayed secondary high-water peak (the 'black flood') emerges downstream in Niger and Nigeria during the dry season.
3
Evaluate the role of major tributaries in maintaining flow south of the inland delta.
Determined that the Bani River joins the Niger at Mopti, adding substantial volume drained from the humid southern savanna zone.
Confirms the primary hydrological mechanism sustaining downstream discharge into its middle course.

Key Concept

Hydrological regimes and discharge characteristics of major African rivers
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Topics

Drainage Systems: Major Rivers and Lakes of Africa

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