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.
Read the key first, then read a value straight up from the time axis to the right line — overland flow appears only when the soil is saturated.
🔒 Interactive diagram
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The four parts of the drainage-basin system
| Part | Examples | What it does |
|---|---|---|
| INPUT | Precipitation (rain, snow, hail) | The only major input — water entering the basin |
| STORES | Interception, surface, soil water, groundwater, channel | Where water is held before it moves on |
| FLOWS (transfers) | Infiltration, throughflow, overland flow, base flow | How water moves between stores towards the channel |
| OUTPUTS | Evapotranspiration, river discharge | Water leaving the basin — to the air and to the sea |
Key terms — the basin system
- Drainage basin — the area of land drained by a river and its tributaries (the catchment).
- Watershed — the boundary of the basin; the high ground separating one basin from the next.
- Open system — both water and energy cross the boundary in and out (rain in; discharge + evapotranspiration out).
- Water balance — precipitation = evapotranspiration + run-off ± change in storage.
- Overland flow — the fastest route to the river; highest when the ground is impermeable, saturated or frozen.
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).
What makes a basin flashy or subdued
| Factor | Flashy (short lag, high peak) | Subdued (long lag, lower peak) |
|---|---|---|
| Relief (slope) | Steep — fast overland flow | Gentle — water moves slowly |
| Geology | Impermeable rock — no infiltration | Permeable rock — water soaks in |
| Vegetation | Sparse — little interception | Dense forest — intercepts + uses water |
| Land use | Urban — concrete, drains, gutters | Rural / farmland — more infiltration |
Key terms — hydrographs
- Discharge — the volume of water passing a point per second (cumecs, m³/s).
- Peak discharge — the highest discharge the river reaches after the storm.
- Lag time — the gap between peak rainfall and peak discharge (how fast the basin responds).
- Rising / falling limb — the steep climb of the curve, then the gentler recession as the river drains.
- Hydraulic radius — channel efficiency; it and discharge both increase downstream (the Bradshaw model).
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.
Erosional vs depositional landforms
| Feature | Erosional (upper course) | Depositional (lower course) |
|---|---|---|
| Energy | High — fast, steep, turbulent | Low — slow, flat, sluggish |
| Dominant process | Hydraulic action, abrasion, attrition, solution | Deposition as velocity falls (heaviest load first) |
| Landforms | Waterfall, gorge, V-shaped valley, potholes | Floodplain, levees, delta, ox-bow lake |
| Meander | Outer bend — river cliff (erosion) | Inner bend — slip-off slope (deposition) |
Key terms — processes & landforms
- Erosion — hydraulic action, abrasion (corrasion), attrition, solution wearing the channel away.
- Transport — traction (rolling), saltation (bouncing), suspension and solution carry the load.
- Deposition — the river drops its load when velocity falls — heaviest first, finest last.
- Waterfall → gorge — hard rock over soft, undercut lip collapses, the fall retreats upstream.
- Meander / floodplain / delta — built by erosion and deposition acting together over time.
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.
✍️ IB-style questions
Explain three ways that building on a drainage basin could change how rainwater is stored and flows through it.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Examine the view that human activities matter more than physical factors in shaping a river's storm hydrograph.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick self-check
Tap each card to reveal the answer.
What makes a drainage basin an open system? Both water and energy cross its boundary in and out — rain comes in; water leaves as river discharge and as evapotranspiration. A closed system would let only energy cross.
What is lag time, and how do you find it? The gap between peak rainfall and peak discharge. Read the time of each off the axis and subtract — a short lag means a flashy basin.
Why does urbanisation make a basin flashy? Impermeable concrete stops infiltration and drains carry run-off straight to the river — less storage means a higher peak and shorter lag time.
How does a waterfall form? Soft rock under a hard caprock is eroded faster, drilling a plunge pool; the undercut lip collapses and the fall retreats upstream, leaving a gorge.
Where do erosion and deposition act on a meander? Erosion on the fast, deep outer bend (a river cliff); deposition on the slow, shallow inner bend (a slip-off slope / point bar).
🎯 Highest-yield exam reminders
Exam Tips
- The basin is an OPEN system: inputs → stores → flows → outputs; water balance = precipitation = evapotranspiration + run-off ± storage change.
- Reading a figure (stores-and-flows graph or hydrograph): read off the correct line/axis and ALWAYS quote the units (%, cumecs, mm, hours).
- Lag time = peak rainfall to peak discharge. Short lag + high peak = flashy (impermeable, steep, urban).
- Explain land use by tracing it to a store or flow — concrete → no infiltration → more overland flow → higher, faster peak.
- Name the PROCESS first (erosion / transport / deposition), then the landform: waterfalls upstream, meanders/floodplains/deltas downstream.
- On Paper 1, the [10] Examine essay needs two+ developed points (both sides), a named river, a weighing of relative importance, and a clear judgement.