Hydrology and fluvial geomorphology

The drainage basin system

Inputs (precipitation), stores (interception, surface, soil moisture, groundwater, channel), flows (throughfall, stemflow, infiltration, overland flow, throughflow, percolation, groundwater flow) and outputs (evapotranspiration, runoff).

• Water balance: P = Q + E ± ΔS.

Storm hydrographs

Show river discharge after a storm: rising limb, peak discharge, lag time, falling limb and baseflow.

• Flashy hydrographs (short lag, high peak): impermeable rock, steep slopes, urbanisation, saturated soils, deforestation.
• Subdued hydrographs: permeable rock, vegetation, gentle slopes.

River processes and landforms

• Erosion (hydraulic action, abrasion, attrition, solution), transportation (traction, saltation, suspension, solution) and deposition. The Hjulström curve links velocity and particle size with erosion, transport and deposition.
• Upper course: V-shaped valleys, interlocking spurs, waterfalls and gorges (Victoria Falls and the Batoka Gorge formed along lines of weakness in basalt).
• Lower course: meanders, ox-bow lakes, floodplains, levées, braided channels and deltas.

River management

Dams (Kariba, Tokwe-Mukosi) provide water, hydroelectricity and irrigation but displace communities and trap sediment. Flood management: hard engineering (dams, levées) and soft engineering (floodplain zoning, afforestation, early warning).

Check yourself

1. State the water balance equation.
2. Give two factors that produce a flashy hydrograph.
3. Explain how Victoria Falls has retreated upstream over time.