Changing river environments

Cambridge IGCSE Geography 0460 · Paper 1 Physical Geography Syllabus: 2027, 2028 and 2029 Official syllabus points: 1.1.1 to 1.3.9

Topic 1 is one of the five physical topics examined on Paper 1. Every question in this topic is marked on the same ladder: simple statements at the bottom, a named example in the middle, and place specific detail, meaning real names, real figures and real dates, at the top. That last step is what separates a Level 3 answer from a Level 2 one, so learn the two detailed examples in section 15 properly.

1. Characteristics of rivers and drainage basins

A drainage basin is the area of land drained by a river and its tributaries.

Learn these twelve terms. They are listed by name in syllabus point 1.1.1, which means the examiner can ask you to define any one of them, or to identify it on a photograph or a map extract.

Why wetted perimeter matters. A large wetted perimeter means more of the water is touching the bed and banks, so there is more friction, so the river flows more slowly. A deep, narrow, smooth channel has a small wetted perimeter for its size, so it flows faster. Paper 4 mark schemes credit exactly this point.

1. heavy rain
2. channel fills
3. cross sectional area rises faster than wetted perimeter
4. less friction per unit of water
5. velocity increases
6. discharge increases sharply.

The upper course and the lower course

Fieldwork and photograph questions constantly ask you to contrast two points on a river. Learn the contrast as a table, because the marks are awarded one per comparison and a comparison must mention both places.

Exam trap: a very common answer is "the river gets faster because it is steeper at the source". The gradient is steeper at the source, but the river is slower there, because the channel is shallow and rough and the wetted perimeter is large relative to the water in it. Friction, not gradient, wins in the upper course.

2. The Bradshaw model

The Bradshaw model is a diagram showing how eight river and channel variables are expected to change with distance downstream. It is new to the 2027 syllabus at point 1.1.2, so learn it explicitly rather than assuming your textbook covers it.

Reading from source to mouth, the model predicts:

Increase downstream

• discharge
• occupied channel width
• channel depth
• average velocity
• load quantity

Decrease downstream

• load particle size
• channel bed roughness
• gradient (the slope angle of the channel)

How to use it in an answer. The model is a prediction, not a law. Fieldwork results often disagree with it, and Paper 4 questions are frequently built around results that do not fit. Reasons a real river departs from the model include a tributary joining and raising discharge in one step, a weir or dam interrupting the profile, a resistant rock band creating a waterfall well downstream, and human straightening of one reach.

1. tributaries join
2. discharge rises
3. channel widens and deepens
4. wetted perimeter grows more slowly than cross sectional area
5. friction per unit of water falls
6. average velocity rises downstream.

3. How the drainage basin operates within the water cycle

The drainage basin is an open system. Water enters it, moves through it and leaves it, and the syllabus expects you to classify each part correctly.

The watershed is the boundary of this system. Precipitation falling inside the watershed drains to that river. Precipitation falling on the far side of the watershed drains to a different basin.

Exam trap: infiltration and percolation are both flows, not stores, and they are not the same thing. Soil moisture and groundwater are the stores that they feed.

4. Processes which operate in a drainage basin

Syllabus point 1.1.4 names eleven processes. A 4 mark definition question expects two marks per term, so give a definition plus a detail of direction or location.

Speed matters, and examiners test it. The three routes water can take to reach the channel move at very different speeds:

1. overland flow (fastest, hours)
2. throughflow (slower, hours to days)
3. groundwater flow (slowest, weeks to years).

This is the single most useful chain in the topic, because it explains almost every human cause of flooding in section 8.

Permeable and impermeable

• A permeable rock or soil lets water pass through it, so infiltration and percolation are high and overland flow is low. Chalk and sandstone are permeable.
• An impermeable rock or soil does not, so water is forced across the surface. Granite, clay and concrete behave as impermeable surfaces.

5. Erosion

Erosion is the wearing away and removal of the bed and banks of the river. The syllabus names four processes at point 1.1.5, and mark schemes require the mechanism, not just the label.

The river also erodes in two directions, and photograph questions use this contrast:

• Vertical erosion cuts downwards. It dominates in the upper course and produces V shaped valleys, waterfalls and gorges.
• Lateral erosion cuts sideways. It dominates in the middle and lower courses and produces meanders, river cliffs and wide floodplains.

Exam trap: attrition and abrasion are constantly swapped. Abrasion attacks the channel. Attrition attacks the load.

6. Transportation

The river carries its load in four ways. Solution appears in the syllabus twice, once as an erosion process and once as a transport process, and the two meanings are different: dissolving the rock is erosion, carrying the dissolved minerals downstream is transportation.

The river's ability to transport depends on velocity, which is why the load is dropped when the river slows.

1. heavy rain
2. discharge and velocity rise
3. more and larger material can be carried
4. load moves downstream
5. velocity falls at the mouth
6. deposition.

7. Deposition and the main landforms

Deposition is the laying down of material a river can no longer carry. It happens wherever velocity falls: on the inside of a bend, on the floodplain during a flood, behind an obstruction, and where the river meets the sea or a lake.

Syllabus point 1.2.1 names eleven landforms and requires characteristics and formation for each. Formation questions are worth 5 or 7 marks and a labelled diagram earns credit in its own right, so always draw one.

Waterfalls, rapids and gorges

A waterfall forms where a band of hard, resistant rock lies over softer, less resistant rock.

1. river crosses hard rock over soft rock
2. soft rock is eroded faster by hydraulic action and abrasion
3. the hard rock is undercut and left as an overhang
4. falling water and swirling load erode a deep plunge pool at the base
5. the unsupported overhang can no longer hold its own weight and collapses
6. the waterfall retreats upstream
7. the steep sided gorge is left behind where the waterfall used to be.

Rapids form where the rock bands are less sharply contrasted or lie at a gentler angle, so the river bed is uneven and turbulent rather than vertical.

Marks note: mark schemes give the collapse mark only when you say the overhang is unsupported or cannot hold its weight, and give the gorge mark only for stating that the gorge marks the former position of the waterfall.

Potholes

Potholes are circular hollows drilled into the bed rock of the upper course.

1. uneven, turbulent river bed
2. loose stones are trapped in a small hollow
3. the swirling current spins them in a circular current
4. they scour and grind the rock by abrasion
5. the hollow deepens and widens into a pothole.

V shaped valleys and interlocking spurs

In the upper course the river has enough energy for vertical erosion but little lateral erosion.

1. river cuts down vertically
2. the valley sides are steepened and left unsupported
3. weathering loosens material on the sides
4. mass movement carries it down to the channel
5. the river removes it
6. a narrow, steep sided V shaped valley develops.

Interlocking spurs are the ridges of higher, more resistant land that the young river winds around because it lacks the energy to erode them. From the valley floor they appear to overlap or interlock.

Meanders

A meander is a pronounced bend in a river channel. Its characteristics, which are what a "describe the features" question wants, are:

• a river cliff (also called a cut bank) on the outer bend, steep, formed by erosion;
• a point bar or slip off slope on the inner bend, gentle and shallow, formed by deposition;
• an asymmetrical cross section, deep on the outside and shallow on the inside;
• a winding, sinuous shape;
• alternating pools (deep) and riffles (shallow) along the channel;
• the fastest thread of water, the thalweg, swinging to the outside of each bend.

1. water flows fastest on the outer bend
2. erosion by hydraulic action and abrasion undercuts the bank
3. a steep river cliff forms
4. water flows slowest on the inner bend
5. the river drops its load there
6. a gently sloping point bar builds up
7. the bend becomes more pronounced over time.

Marks note: IGCSE mark schemes do credit helicoidal flow, the corkscrew or spiralling motion of water that sweeps eroded material from the outer bank across to the inner bank, even though the term is not printed in the syllabus. Use it as a bonus, never instead of the erosion and deposition points.

Oxbow lakes

1. continued erosion on the outer bends
2. the neck of land between two bends narrows
3. during a flood the river has enough energy to cut straight through the neck
4. the river takes the new, shorter, straighter course
5. the flow into the old bend slows
6. deposition seals off both ends of the abandoned bend
7. a crescent shaped oxbow lake is left on the floodplain, which is later colonised by vegetation and silts up.

Describing one on a map: crescent, horseshoe or C shaped; separated from the present channel; typically a few hundred metres long.

Floodplains and levées

The floodplain is the wide, flat area of land either side of a river in its lower course, built of alluvium, the fine sediment left by past floods.

Levées are the natural raised banks of sediment along the edges of the channel.

1. the river exceeds bankfull and spills onto the floodplain
2. the escaping water immediately meets friction and its velocity falls sharply
3. the coarsest material is dropped first, right at the channel edge
4. the finest silt is carried further out and settles across the floodplain
5. repeated floods build the coarse deposits into raised levées and the fine deposits into a flat, fertile floodplain.

The migration of meanders across the valley floor also widens the floodplain by lateral erosion.

Deltas

A delta is a landform of deposited sediment at the mouth of a river, where it enters a sea or lake.

Three conditions are needed: a large sediment load, a sharp drop in velocity at the mouth, and a sheltered receiving body of water with a small tidal range and weak currents, so the sediment is not removed.

1. river reaches the sea
2. velocity drops sharply
3. fresh water meets salt water
4. the fine clay particles flocculate, clumping together and becoming heavy enough to settle
5. sediment builds up at the mouth
6. the channel is blocked and splits into distributaries
7. the delta extends seaward over time.

Features to label on a photograph or sketch map: distributaries (not tributaries), the branching or elongated shape, alluvial deposits, sediment plumes offshore, marsh vegetation, and small islands or eyots.

Exam trap: the branches of a delta are distributaries. A mark scheme for a delta sketch map states explicitly that "tributaries" is not accepted.

8. Opportunities and hazards of living near a river

Syllabus point 1.3.1. Answers are marked on the same level ladder, so a named place is worth two marks.

Opportunities

• Water supply for drinking, washing, industry and irrigation.
• Fertile alluvial soil on the floodplain, giving high crop yields.
• Flat land on the floodplain, which is cheap and easy to build on and to run roads and railways along.
• Navigation and transport, both along the river and along the valley.
• Fishing, both for food and for income.
• Hydro electric power where the gradient and discharge allow.
• Tourism and recreation, giving jobs in the tourist industry.
• A defensive or bridging point site for a settlement.

Hazards

• Flooding of homes, farmland and infrastructure.
• Bank erosion, undercutting buildings, roads and farmland.
• Waterborne disease, such as cholera and typhoid, where water is contaminated.
• Mosquitoes breeding in standing water, spreading malaria and dengue.
• Dangerous animals in some rivers, and drowning risk.
• The river as a barrier to movement, expensive to cross.
• Pollution of the water supply.

9. The causes of river flooding

Syllabus point 1.3.2 splits these into natural and human, and a question asking for both will not reward you fully for only one.

Natural causes

• Prolonged heavy rainfall, saturating the soil so that no further infiltration is possible and all further rain becomes overland flow.
• Intense rainfall over a short period, from a thunderstorm or a tropical cyclone, where rainfall intensity exceeds the infiltration rate even in dry soil.
• Snow and glacier melt, especially rapid melt in spring, or melt falling on already frozen ground.
• Impermeable rock or already saturated soil in the basin.
• Steep valley sides, which speed overland flow into the channel.
• A circular basin shape or high drainage density, so water from all tributaries arrives at the same time.

Human causes

• Deforestation → less interception and less transpiration → more water reaches the ground and reaches it faster → more overland flow.
• Urbanisation → tarmac, concrete and roofs are impermeable → infiltration is almost zero → drains and gutters deliver water to the channel in minutes rather than days.
• Ploughing down the slope, and compaction of soil by machinery or livestock, both of which reduce infiltration.
• Poor river management, such as failure to maintain or dredge channels, or the collapse or failure of a dam.
• Waste and rubbish in the channel, which blocks the flow and reduces capacity.
• Building on the floodplain, which does not cause the flood but converts it into a disaster.

1. deforestation
2. interception and transpiration fall
3. infiltration falls as roots no longer open the soil
4. overland flow rises
5. water reaches the channel in hours instead of days
6. discharge peaks higher and sooner
7. the channel is exceeded and the river floods.

10. The impacts of river flooding

The syllabus defines impacts as positive and/or negative, as social, economic and environmental, and at a range of scales. Sorting your answer under those headings is the quickest way to reach Level 3, and mark schemes state plainly that impacts can be positive.

11. Managing river flooding

Syllabus point 1.3.4 asks for an evaluation of the strategies and techniques, including sustainable ones. Naming a method is Level 1. Explaining how it works is Level 2. Naming a place and giving costs or figures is Level 3. Judging the method against its drawbacks is what "evaluation" means.

Hard engineering

Soft engineering, and the sustainable options

How to write the evaluation. Sustainable strategies work with the river system rather than against it, cost less to maintain, and protect habitats, but they need land and act slowly. Hard engineering protects a specific settlement quickly and reliably up to its design limit, but it is expensive, it transfers the problem downstream, and it fails badly when a flood exceeds what it was built for. The strongest answers say that no scheme prevents all floods: management reduces flood risk to an acceptable level.

12. The human causes of river pollution

River pollution is the addition of substances to a river that lower its water quality and harm the organisms living in it or the people using it.

Syllabus point 1.3.5 asks only for human causes, so do not pad the answer with natural sediment or leaf fall.

• Untreated or partly treated sewage discharged from homes, informal settlements and treatment works, adding bacteria, organic matter and nutrients.
• Industrial effluent, including hot water from cooling, heavy metals such as chromium, mercury and lead, acids, dyes and oil.
• Agricultural runoff, carrying nitrate and phosphate fertiliser, pesticide and slurry from fields into the channel.
• Solid waste, litter and plastic dumped on the banks or directly into the water.
• Mining waste and quarry runoff, adding suspended sediment and dissolved metals.
• Oil and fuel spills from boats, road runoff and leaking storage.
• Thermal pollution from power stations and factories returning warm water, which lowers the amount of oxygen the water can hold.
• Water abstraction for irrigation and industry, which reduces flow and therefore concentrates every pollutant already present.

1. fertiliser applied to fields
2. heavy rain produces overland flow
3. nitrate and phosphate are washed into the channel
4. nutrient levels rise
5. algal bloom
6. light is blocked and the algae later decompose
7. oxygen is stripped from the water
8. fish die. This is eutrophication.

13. The impacts of river pollution

Use the social, economic and environmental headings again.

Indicator species. Some invertebrates, such as the mayfly nymph and the stonefly nymph, survive only in clean, well oxygenated water, while others, such as the rat tailed maggot and the sludge worm, tolerate heavily polluted water. Recording which species are present is a standard fieldwork method for measuring pollution without laboratory equipment.

14. Managing river pollution

Point 1.3.7 again asks for an evaluation, and again requires sustainable approaches.

How to write the evaluation. Treatment and regulation deal with the largest point sources but are costly and depend on enforcement and maintenance, which is exactly why several national river schemes have underperformed. Buffer strips, wetlands, education and controlled fertiliser use are cheaper, work with natural processes and are therefore more sustainable, but they act slowly and cannot absorb a large industrial or sewage discharge on their own. The strongest answers argue that pollution management only works when the source is tackled, not just the water.

15. Detailed specific examples

Syllabus points 1.3.8 and 1.3.9 each require one detailed specific example. That is two separate case studies, one for flooding and one for pollution, and each must cover causes, impacts and management including sustainable approaches. Learn names, dates and figures. Mark schemes cap a general answer at 5 out of 7 and describe Level 3 as needing "locational details, specific details, statistics, dates".

15a. Flooding: the Somerset Levels, England, winter 2013 to 2014

The rivers Parrett and Tone drain the Somerset Levels, a very low lying area of southwest England, much of it at or barely above sea level.

Causes

• Natural: an exceptional sequence of Atlantic depressions gave England its wettest January since records began in 1910, on top of an already wet December. The ground was fully saturated, so almost all further rain became overland flow.
• Natural: the Levels are extremely flat and lie at around sea level, so water drains away very slowly, and high tides in the Bristol Channel prevented the rivers discharging at the mouth.
• Human: the Parrett and Tone had not been dredged for around 20 years, so silt had reduced the channel capacity substantially.
• Human: the land is drained, embanked farmland, so pumped drainage was overwhelmed.

Impacts

• Around 600 homes were flooded and about 16,000 hectares of farmland were under water, some of it for more than a month.
• The village of Muchelney was cut off by road for roughly ten weeks, reachable only by boat.
• Around 1,000 livestock had to be evacuated, and pasture was ruined for the following season.
• The main road and rail links across the Levels were closed, with knock on costs to the regional economy.
• Environmentally, sewage and agricultural chemicals were spread across the flooded land, though the silt deposited also enriched the soil.

Management

• Hard: about 8 km of the rivers Parrett and Tone were dredged during 2014 to restore channel capacity; pumping stations were installed and upgraded; roads were raised, including the road to Muchelney.
• Hard and long term: a tidal barrier at Bridgwater was proposed to stop high tides holding back river discharge.
• Sustainable and soft: the £20 million Somerset Levels and Moors Flood Action Plan combined dredging with catchment wide measures, including tree planting and changed farming practice upstream to slow runoff, and the deliberate use of some low value land as washland storage.
• Evaluation: dredging restored capacity quickly and was what local residents demanded, but it must be repeated and does nothing about the rainfall or the tide. The catchment measures are cheaper to maintain and address the cause, but they act slowly and depend on the cooperation of many landowners.

15b. Pollution: the River Ganges, India

The Ganges flows about 2,500 km from the Gangotri Glacier in the Himalayas to the Bay of Bengal. Its basin is the most densely populated in the world, supporting well over 400 million people.

Human causes of the pollution

• Untreated sewage is by far the largest source. The cities along the river generate billions of litres of sewage a day and treatment capacity handles only a fraction of it, so raw sewage enters the channel directly.
• Industrial effluent, above all from the roughly 400 leather tanneries at Kanpur, which discharge effluent containing chromium and other heavy metals.
• Agricultural runoff carrying nitrate and phosphate fertiliser and pesticide from the intensively farmed floodplain.
• Religious and cultural practices, including the immersion of offerings and idols, and cremation remains placed in the river at Varanasi.
• Solid waste and plastic dumped on the banks and washed in.
• Water abstraction for irrigation, which reduces the flow and therefore concentrates every pollutant already in it.

Impacts of the pollution

• Social: waterborne disease. Cholera, typhoid, dysentery and hepatitis are widespread among the people who drink from and bathe in the river. Faecal coliform counts at Varanasi have been measured at many times the safe bathing standard.
• Economic: fish stocks have collapsed in polluted reaches, destroying livelihoods; the cost of treating drinking water rises; tourism suffers from the smell and appearance of the water.
• Environmental: eutrophication, where nutrients cause algal blooms that block light and then decompose, stripping oxygen from the water and killing fish. Heavy metals accumulate in sediment and in the food chain. The endangered Ganges river dolphin has declined sharply.

Management

• The Ganga Action Plan, launched in 1985, built sewage treatment works along the river. It is generally judged a failure: capacity fell far short of the volume of sewage, and many plants were not run reliably because of power cuts and poor maintenance.
• The Namami Gange Programme, launched in 2014 with a budget of about 20,000 crore rupees, roughly US$3 billion, widened the approach to sewage infrastructure, riverfront development, industrial effluent monitoring and afforestation on the banks.
• Regulation: tanneries at Kanpur have been ordered to close or to install common effluent treatment plants, and industrial discharge limits are enforced through monitoring.
• Sustainable approaches: relocating and grouping tanneries so effluent can be treated collectively; promoting electric crematoria to reduce material entering the river; restoring wetlands along the banks to filter runoff naturally; and public education campaigns working with religious leaders to change practices at the ghats.
• Evaluation: treatment plants tackle the largest single source but require reliable power, funding and maintenance to work at all, which is why the 1985 plan underperformed. Regulation only works where monitoring is genuinely enforced. Education and wetland restoration are cheap and sustainable, but they act slowly and cannot cope with the sheer volume of sewage from a basin of this population.

16. Fieldwork links for Paper 4

The syllabus suggests river hypotheses that Paper 4 uses directly, and river fieldwork appears repeatedly in past papers.

• Does river velocity increase downstream?
• Does the bedload of a local river become smaller and more rounded downstream?
• How far do a river's characteristics fit the Bradshaw model?
• Does pollution increase as a river flows downstream?

Methods to be able to describe

• Width: stretch a tape measure across the channel from bank to bank at the water's edge, holding it taut and horizontal, and read to the nearest centimetre.
• Depth: hold a metre rule vertically on the bed at fixed intervals across the channel, for example every 25 cm, and record each reading.
• Velocity: time a float over a measured distance, for example 10 metres, and repeat several times to take an average, or use a flowmeter held facing upstream just below the surface.
• Bedload: select a sample of pebbles at random, measure the long axis with callipers, and assess roundness against a standard chart.

Reliability points that earn marks

• Repeat each measurement and take an average, because a single float run can be caught in an eddy or an obstruction.
• Increase the sample size, for example 40 pebbles rather than 20, to reduce the effect of an unusual pebble.
• Have two students measure independently and compare, to remove one person's bias.
• Random selection avoids bias but can miss part of the range, so a systematic sample across the channel may be more representative.
• Measure at the same time of day and after similar weather, or discharge will differ between sites.

17. Common exam mistakes

• Saying the river is fastest at the source. It has the steepest gradient there, but the greatest friction, so it is slowest.
• Confusing abrasion (wears the channel) with attrition (wears the load).
• Calling delta branches tributaries. They are distributaries.
• Naming a landform process without the mechanism. "Erosion happens" earns nothing; "hydraulic action forces water into cracks and compresses air until fragments break away" earns the mark.
• Giving no named example on a 7 mark question. The mark scheme caps you at 5.
• Naming a place but giving no detail. Level 3 needs figures, dates or specific locations within the place.
• Listing flood management methods without saying how each works, or without any drawback, on a question that says "evaluate".
• Writing only about negative impacts of flooding. Mark schemes state explicitly that impacts can be positive.
• Answering "human causes of flooding" with natural ones, or the reverse, when the question asks for both.
• Treating the two detailed examples as one. Syllabus points 1.3.8 and 1.3.9 require a flood example and a separate pollution example.

18. Quick revision

• Discharge = cross sectional area × velocity, measured in cumecs.
• Large wetted perimeter → more friction → slower flow.
• Bradshaw: discharge, width, depth, velocity and load quantity all increase downstream; particle size, bed roughness and gradient all decrease.
• Basin system: input precipitation, stores interception, surface, soil and groundwater, flows overland, throughflow, groundwater and channel, outputs evaporation, transpiration and discharge.
• Erosion: hydraulic action, abrasion, attrition, solution. Transport: traction, saltation, suspension, solution.
• Waterfall: hard over soft → undercutting → overhang → plunge pool → collapse → retreat → gorge.
• Meander: erosion outside, deposition inside → river cliff and point bar.
• Oxbow: neck narrows → cut through in flood → sealed by deposition.
• Levées: coarse material dropped first at the channel edge as velocity falls.
• Delta: large load, sharp drop in velocity, sheltered water with a small tidal range.
• Flooding: natural causes are rainfall, snowmelt, relief and geology; human causes are deforestation, urbanisation and poor management.
• Pollution: sewage, industrial effluent, agricultural runoff, solid waste, cultural practices.
• Two detailed examples required, one flood and one pollution, each with causes, impacts and management including sustainable.
• A named example is worth two marks. Place specific detail is worth the top mark.

Not the topic you were looking for? Describe what you are stuck on in your own words and we will take you to the notes that answer it.