Rivers
Rivers
Candidates should understand river processes, landforms and management. Learn the three processes of erosion - hydraulic action, abrasion, attrition and solution (often listed as four) - plus transportation (traction, saltation, suspension, solution) and deposition. Understand the long profile and the cross profile: steep gradients and vertical erosion in the upper course, lateral erosion and meanders in the middle, deposition in the lower course.
Key landforms: V-shaped valleys, interlocking spurs, waterfalls and gorges (upper); meanders, oxbow lakes, floodplains and levees (middle/lower); deltas and estuaries (mouth). Explain meander formation by helicoidal flow, erosion on the outer bank (river cliff) and deposition on the inner bank (slip-off slope).
Understand flooding: physical causes (heavy rain, snowmelt, impermeable rock, steep slopes, small basins) and human causes (urbanisation, deforestation, floodplain building). Learn hard and soft engineering, and the storm hydrograph.
Traps: reversing the meander banks (erosion is on the outside of the bend); confusing a delta with an estuary; calling levees natural only (artificial levees exist); and muddling attrition (rocks colliding) with abrasion (rocks scraping the bed).
Rule of thumb: fast, steep water erodes; slow, flat water deposits. Follow the energy downstream.
Worked example: A waterfall forms where a band of hard rock overlies soft rock. The soft rock is undercut by hydraulic action and abrasion, creating a plunge pool; the hard caprock collapses, and the waterfall retreats upstream, leaving a gorge.