A drainage basin is an open system

A drainage basin is the area of land drained by a river and its tributaries. Its boundary is the watershed — the high ground separating one basin from the next. Key features and terms:

• Source — where a river begins; mouth — where it meets the sea or a lake.
• Tributary — a smaller stream joining the main river; confluence — the point where they meet.
• Precipitation in, then evaporation and river discharge out — so the basin is an open system with inputs, stores, flows and outputs.

Name that feature

How water moves through a basin

Water reaches the river by several routes. You must know these transfer terms:

• Precipitation — water reaching the ground as rain, snow or hail (the main input).
• Interception — precipitation caught by leaves before it reaches the soil.
• Infiltration — water soaking into the soil surface.
• Surface runoff (overland flow) — water flowing over the ground straight to the river — fast.
• Throughflow and groundwater flow — water moving to the river through soil and rock — slow.

Why it matters: when infiltration is low (e.g. saturated, frozen or tarmac-covered ground), more water reaches the river as surface runoff, and it arrives quickly — raising flood risk.

Runoff vs infiltration

The river's changing shape

The long profile is the gradient of a river from source to mouth — steep at the top, gentle at the bottom. As you travel downstream the channel changes:

Common surprise: a river actually flows faster on average in its lower course, not the upper. The channel is smoother and deeper, so less energy is lost to friction — even though the slope is gentler.

Downstream changes

Four types of erosion

Rivers wear away their bed and banks in four ways:

• Hydraulic action — the sheer force of moving water forces air into cracks and breaks off rock.
• Abrasion (corrasion) — the load rubs and scrapes against the bed and banks like sandpaper.
• Attrition — rocks in the load knock into each other, becoming smaller, rounder and smoother.
• Solution (corrosion) — slightly acidic water dissolves soluble rock such as chalk and limestone.

Erosion ≠ weathering: weathering breaks rock down in place (e.g. freeze–thaw on a valley side). Erosion is the wearing away and removal of material by the moving river. Don't muddle them in the exam.

Carrying and dropping the load

A river transports its load in four ways:

• Traction — large boulders rolled along the bed.
• Saltation — pebbles bounced/hopped along the bed.
• Suspension — fine sand and silt carried within the water, making it look cloudy.
• Solution — dissolved minerals carried invisibly in the water.

Deposition happens when the river loses energy and can no longer carry its load — for example where velocity falls (shallow water, inside of a bend, or entering the sea). The heaviest material is dropped first.

Process ↔ definition

Tap a process on the left, then tap its matching definition on the right.

Which process?

Upper course: V-shaped valleys, waterfalls & gorges

In the upper course vertical erosion dominates. The river cuts down, and weathering plus runoff on the valley sides create a steep V-shaped valley. Where a river crosses a band of hard rock over soft rock, a waterfall forms and retreats to leave a gorge.

Forming a waterfall

Meanders and ox-bow lakes

In the middle/lower course the river swings in meanders. Water flows fastest on the outside of the bend, causing erosion and a steep river cliff. On the inside the water is slow, so it deposits sand to form a gentle slip-off slope.

Ox-bow lake: erosion on the outside of two bends narrows the neck until a flood cuts straight through. Deposition then seals off the old loop, leaving a crescent-shaped ox-bow lake.

Reading a meander

Floodplains, levees & deltas

In the lower course deposition builds distinctive landforms:

• Floodplain — the wide, flat valley floor. When the river floods it drops sediment (alluvium), leaving fertile land.
• Levees — natural raised banks. In a flood the coarsest material is dropped first, right at the channel edge, building up over time.
• Delta — where a river meets the sea/lake and loses energy, huge amounts of sediment are deposited faster than the sea can remove them, building a delta (e.g. the Nile, the Ganges).

Link it up: levees are built by the same flooding that spreads alluvium across the floodplain — coarse material near the channel (levee), fine material spread further out.

Erosion or deposition?

Tap a landform, then tap the box for the process that mainly forms it.

How a river responds to a storm

A storm (flood) hydrograph plots rainfall and river discharge against time after a rainstorm. The key measure is the lag time — the gap between peak rainfall and peak discharge.

Misconception: lag time is not "how long the storm lasts". It is the time between peak rainfall and peak discharge. A short lag time and a high, steep peak ("flashy" hydrograph) mean a higher flood risk.

Read the hydrograph

What makes a flood more likely?

Both physical and human factors shorten lag time and raise discharge:

• Physical: prolonged or intense rainfall; steep slopes (fast runoff); impermeable rock or clay soils; saturated or frozen ground; snowmelt.
• Human: deforestation (less interception), urbanisation (tarmac and drains speed water to the river), and building on floodplains.

Contrast: a well-vegetated, permeable basin has a long lag time and low peak; a steep, urban, saturated basin has a short lag time and a high peak — the "flashy" flood-prone shape.

Case study: the Cumbria floods, UK (2015)

Edexcel expects a named, located example of a river flood — causes, effects and responses. Learn these facts:

• Where & when: Carlisle and Cumbria, north-west England, on the River Eden, during Storm Desmond, 5–6 December 2015.
• Causes: a record 341 mm of rain in 24 hours at Honister fell on ground already saturated by a wet autumn; steep upland relief gave rapid runoff and a short lag time.
• Effects: around 5,200 homes flooded in Cumbria; power cut to tens of thousands; roads and bridges damaged; the clean-up cost was estimated at over £500 million.
• Responses: emergency rescue and evacuation; later investment in improved flood defences and a mix of hard and soft management along the Eden.

Exam tip: always name the place, date and specific figures — vague answers ("a river flooded and houses were damaged") score few marks.

Case-study recall

Hard vs soft engineering

Rivers are managed to reduce flood risk. Edexcel splits methods into two approaches:

• Hard engineering — built structures that control the river: dams & reservoirs, flood walls/embankments, channel straightening, flood-relief channels. Effective but expensive and can look unnatural.
• Soft engineering — works with nature: afforestation (planting trees to raise interception), river restoration, washlands/flood-storage areas, and floodplain zoning (planning controls on building). Cheaper and more sustainable, but slower to take effect.

Misconception: "soft" does not mean weak or useless. Soft methods are often cheaper, more sustainable and better for wildlife — they just manage risk differently from concrete walls.

Choosing a strategy

Water as a resource

Rivers and their basins are vital resources, but supply and quality raise issues:

• Uses: drinking water, irrigation for farming, industry, hydroelectric power, transport and recreation.
• Supply issues: demand can outstrip supply — managed by reservoirs, dams and water transfer schemes; abstraction for cities and farming can lower river flow.
• Quality issues: water can be polluted by sewage, industrial waste and farm fertilisers/pesticides (which cause eutrophication), harming ecosystems and human health.

Watch out: building a dam or abstracting too much water for supply changes the river regime downstream — it can reduce discharge and affect wildlife, so management always involves trade-offs.

Water quality

The essentials to know

You've covered the whole Edexcel 4GE1 River Environments unit. Press Finish to see your score.