Water Resources, Rivers, Lakes and Drainage Systems
The drainage basin
A basin is drained by a river and its tributaries and bounded by a watershed. The hydrological cycle has inputs of precipitation, outputs of evapotranspiration and runoff, and stores in soil, groundwater and lakes.
River processes and landforms
Erosion processes are hydraulic action, abrasion, attrition and solution. Transport processes are traction, saltation, suspension and solution. Deposition occurs when velocity falls. Landforms include V-shaped valleys, waterfalls where hard rock overlies soft rock, meanders with erosion on the concave bank and deposition on the convex bank, oxbow lakes, floodplains, levees and deltas.
Lake Malawi and the Shire
Lake Malawi is a deep rift lake draining south through the Shire River and its cataracts to the Zambezi. It supports fisheries and hydropower but suffers overfishing, pollution and fluctuating levels.
Worked example
Waterfall formation: a band of resistant rock overlies softer rock; the softer rock erodes faster and undercuts the cap; the cap collapses; the fall retreats upstream, leaving a gorge. The sequence erosion → undercutting → collapse → retreat must be explicit.
Traps
Mixing erosion with deposition; placing deposition on the concave bank; confusing a lake with a river; ignoring lag time on a storm hydrograph.