IB Geography HL — Drainage basin hydrology and geomorphology

Key Idea: Topic 7.1 is about how water moves through a drainage basin and how the river then shapes the land. It pulls together three ideas: 7.1.1 — the basin as a system: a catchment is an open system of inputs (precipitation), stores, flows and outputs — and the water balance that links them. 7.1.2 — discharge & hydrographs: how a river's discharge responds to a storm, read off a storm hydrograph (peak discharge, lag time), and what makes a basin flashy or subdued. 7.1.3 — processes & landforms: how erosion, transport and deposition carve waterfalls and gorges upstream and build meanders, floodplains and deltas downstream. This is Option A (Freshwater), examined on Paper 1 — SL answers two options, HL three (same questions). Each option = a short data-response off a figure, a structured Explain/Outline/Suggest, and a [10] Examine extended answer.

🌧️ 7.1.1 — The drainage basin as an open system

A drainage basin is the area drained by a river and its tributaries (the catchment), bounded by the watershed. Study it as an open system: rain enters, water is held in stores, moves along flows, and leaves as evapotranspiration and discharge. The master equation is the water balance: precipitation = evapotranspiration + run-off ± change in storage. Examiners test it by asking you to read a stores-and-flows graph — a value or a duration, with the units.

Tip: Anything that seals the ground or strips vegetation cuts storage and speeds run-off. Trace the chain through to a store or flow: concrete → no infiltration → less soil/groundwater storage → more overland flow. That chain is what earns the development marks.

📈 7.1.2 — Discharge and the storm hydrograph

River discharge is the volume of water passing a point each second (cumecs, m³/s). A storm hydrograph shows how discharge responds to a storm — rainfall bars above, a discharge curve below. Read the peak discharge, the lag time (the gap between peak rainfall and peak discharge) and the rising/falling limbs. A short lag + high peak = flashy (urban, impermeable, steep); a long lag + lower peak = subdued (rural, permeable, vegetated).

Don't just name a factor — explain how it changes the run-off. Urbanisation → impermeable surfaces → less infiltration → faster run-off → shorter lag, higher peak. For a hydrograph reading, the single mark is usually won or lost on the units (cumecs, mm, hours).

🌊 7.1.3 — River processes and landforms

A river always erodes, transports and deposits — which one wins depends on its energy (velocity and discharge). Fast, high-energy stretches erode; slow, low-energy stretches deposit. Name the process first, then the landform it builds: upper-course erosion gives waterfalls, gorges and V-shaped valleys; lower-course deposition gives floodplains, levees, deltas and ox-bow lakes. A meander uses both — erosion on the outer bend, deposition on the inner.

Example: Anchor answers with real places. The lower Mississippi has classic levees and a bird's-foot delta at the Gulf of Mexico; the Nile built a fertile arcuate delta in Egypt; the Ganges-Brahmaputra forms one of the world's largest deltas in Bangladesh; the lower Rio Grande swings across a broad floodplain leaving ox-bow lakes.

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