Mission 1: At any moment, rivers hold only about 0.006% of Earth's fresh water. So how does a river keep flowing through weeks without rain?

The drainage basin

• A drainage basin is the area of land drained by a river and its tributaries.
• Its boundary is the watershed: high ground separating one basin from the next.
• Tributaries join at a confluence; the river ends at its mouth, in a sea or lake.
• The basin is an open system: water and sediment enter and leave it.

Inputs and outputs

• Input: precipitation. Its type (rain or snow) and intensity (mm per hour) decide what happens next.
• Snow is stored until it melts. Intense rain can fall faster than the ground can absorb it.
• Outputs: evaporation from surfaces and transpiration from plants (together, evapotranspiration), and river flow at the mouth.
• Some water can also leave the basin underground, or be pumped out.

Stores

• Vegetation (interception on leaves), surface stores (puddles, lakes), soil moisture and the channel.
• Groundwater, held in aquifers: permeable rock that stores water and lets it move.
• The cryosphere: snow, glaciers and frozen ground, which can hold water for months to centuries.
• Stores delay water on its way to the river.

Flows

• Infiltration: water soaks into the soil. Percolation: it moves down into the rock below.
• Overland flow: infiltration-excess (Hortonian) flow when rain exceeds the soil's infiltration capacity; saturation-excess flow when the soil is already full.
• Throughflow: water moves downslope through the soil.
• Base flow: groundwater seeps slowly into the channel, keeping rivers flowing between storms.

Where the fresh water is

• Only about 2.5% of Earth's water is fresh.
• Of that fresh water, 68.7% is in ice caps, glaciers and permanent snow, and 30.1% is groundwater.
• Rivers hold about 0.006%: a tiny, fast-moving store, refilled from much larger, slower ones.

Groundwater feeds rivers

• A USGS study of 54 US streams found groundwater contributed on average 52% of streamflow, ranging from 14% to 90%.
• The USGS calls its base-flow method a "conservative estimate", so the true share may be higher.
• So how a basin stores water decides how its river behaves in dry spells.

Ice feeds rivers

• In the upper Indus basin, glacier melt contributed 40.6% of total runoff (1998–2007); in the upper Ganges, 11.5%. Snowmelt is counted separately.
• The Indus is the world's most relied-upon mountain "water tower", and one of the most vulnerable.
• Meltwater arrives in the warm season, often when rain is scarce.

The water balance

• P = Q + E ± ΔS: precipitation = runoff + evapotranspiration ± change in storage.
• Over many years, storage rises and falls but changes little overall, so E ≈ P − Q.
• River Lambourn, southern England (234.1 km², on chalk): mean catchment rainfall 768 mm a year, 1962–2019.
• Its mean flow, 1.743 m³/s, equals a runoff depth of 235 mm a year spread over the catchment.

Mission 2: A silt grain is one-hundredth the width of a coarse sand grain, yet water must flow about three times as fast to pick it up. Why?

River discharge

• Discharge (Q): the volume of water passing a point each second, in m³/s ("cumecs").
• Q = A × v: cross-sectional area (m²) × mean velocity (m/s).
• To measure it, hydrologists divide the channel into vertical slices, usually 25–30, measure width, depth and velocity in each, and add up the slices.

Channel shape and efficiency

• Wetted perimeter (P): the length of bed and banks in contact with the water.
• Hydraulic radius (R) = cross-sectional area ÷ wetted perimeter: R = A/P.
• A larger R means a smaller share of the water touches the bed and banks, so less energy is lost to friction: the channel is more efficient.
• Velocity also rises with gradient and falls with roughness (boulders, vegetation).

Stream flow

• Velocity is lowest against the bed and banks, where friction acts, and highest near the surface in mid-channel.
• On a bend, the fastest flow swings towards the outer bank.
• Most river flow is turbulent: eddies lift and carry sediment.
• Stream flow changes with every storm and season, so discharge, velocity and the work a river can do change too.

Downstream changes: the Bradshaw model

• Downstream, discharge, width, depth, cross-sectional area, mean velocity and load quantity increase.
• Gradient, channel bed roughness and load particle size decrease.
• Evidence on velocity: it "generally tends to increase downstream", but "there are many reaches in nearly all rivers where mean velocity decreases downstream" (Leopold, 1953).
• Limitation: it is a generalised model. Tributaries, rock type, lakes and dams break the pattern.

Erosion

• Hydraulic action: the force of moving water, including air forced into cracks.
• Abrasion: the load scrapes and grinds the bed and banks.
• Attrition: particles collide, becoming smaller and rounder.
• Solution: soluble rock such as limestone dissolves.
• Vertical erosion deepens the channel; lateral erosion widens it.

Transport

• Traction: the largest particles roll or slide along the bed.
• Saltation: smaller bed-load particles, such as sand, bounce along the bed.
• The bed load "may only be moved during rapid flows under flood conditions".
• Suspension: fine particles carried within the flow. Solution: the dissolved load, on average about 15% of the mass a stream transports.

Competence, capacity and deposition

• Competence: the largest particle a river can move. Capacity: the total load it can carry.
• Both rise as velocity rises, so most transport happens in high flows.
• Deposition happens when velocity or discharge falls, or when the load exceeds capacity.

The Hjulström–Sundborg curve

• The graph links particle size to the velocity needed to erode, transport or deposit it. Read values from it as approximate.
• One textbook reading: a 1 mm sand grain is eroded at around 20 cm/s, and stays in suspension until velocity drops below about 10 cm/s.
• A 0.01 mm silt grain needs about 60 cm/s to be eroded.
• Fine grains, especially clay, are cohesive: they "stick together". They also lie within a thin layer of slow water on the bed, so turbulence barely reaches them.

Spatial factors

• Upper course: steep gradient and large, angular load; vertical erosion dominates.
• Lower course: more discharge and finer load; lateral erosion and deposition dominate.
• Channel characteristics: on a bend the outer bank is eroded and the inner bank receives deposits. Resistant rock and bank vegetation slow erosion.

Temporal factors: seasonality

• Much of a river's erosion and transport happens during high flows, when velocity, competence and capacity peak.
• Ganges at Patna, 1980: about 88% of the year's sediment load moved during the monsoon, which carried 76% of the year's water.
• In dry seasons, low velocity means deposition dominates.
• Exam move: name the process, the place in the channel and the time of year.

Mission 3: Niagara Falls has moved about 7 miles (11 km) upstream in about 12,000 years. How does a waterfall move backwards?

How a waterfall forms

• Waterfalls form where a band of resistant rock lies over weaker rock.
• The weaker rock erodes faster, leaving the resistant rock as an overhanging caprock.
• Hydraulic action and abrasion in the plunge pool undercut the face, and the unsupported caprock collapses.
• Repeated collapse makes the waterfall retreat upstream, leaving a steep-sided gorge.

Niagara Falls

• Resistant dolostones and limestones of the Lockport Group overlie weaker shales and limestones, such as the Rochester Shale.
• About 12,000 years ago water found a single low path; the falls have since migrated about 7 miles (about 11 km) upstream.
• Over the last 500+ years, retreat was 3–5 feet (about 0.9–1.5 m) a year. It is now estimated at about 1 foot (0.3 m), largely because water is withdrawn for hydroelectric power.

Meanders

• On a bend, the line of fastest flow (the thalweg) swings to the outer bank. A corkscrew current (helicoidal flow) carries sediment across the bed towards the inner bank.
• Outer bank: erosion forms a steep river cliff. Inner bank: deposition forms a point bar.
• In cross-section the channel is deep on the outside of the bend and shallow on the inside.
• Over time the channel "migrates around within its flood plain".

Oxbow lakes

• Erosion on the outside of two neighbouring bends narrows the neck of land between them.
• In a flood the river cuts through the neck and takes the shorter, steeper route.
• Deposition seals off the old bend, leaving an oxbow lake.

Floodplains and levees

• A floodplain is the wide, flat valley floor that floods. It is built sideways as meanders migrate and point bars grow (lateral accretion), and upwards by layers of alluvium left by floods (vertical accretion).
• When a river overflows, velocity falls sharply at the bank edge, so the coarsest sediment is dropped first, close to the channel.
• Flood after flood, this builds raised banks: natural levees. Finer silt and clay settle further out on the floodplain.

How a delta forms

• A delta forms where a river enters a sea or lake, slows, and deposits sediment faster than waves and tides remove it.
• It needs a large sediment supply. "The formation of a delta depends on the interaction between the river flow and sediment supply" and the waves and tides that redistribute it.
• The balance of river, wave and tide energy then sets the delta's shape, so deltas also form on stormy or strongly tidal coasts (screen 3.9).
• In salt water, fine clay particles clump together (flocculation) and settle faster.
• The river splits into distributaries; sediment builds outwards in bottomset, foreset and topset beds.

Delta types (Galloway, 1975)

• Galloway's classification uses the balance of river, wave and tide energy:
  • river-dominated, a bird's-foot shape (the Mississippi);
  • wave-dominated, a smooth, cuspate shoreline (the São Francisco, Brazil);
  • tide-dominated, estuary-like (the Fly River, Papua New Guinea).

Deltas need a supply of sediment (synthesis)

• Coastal Louisiana, on the Mississippi delta, lost about 4,833 km² of land between 1932 and 2016: about 25% of its 1932 land area. The USGS maps the change without assigning one cause; this screen follows one of them.
• Dam construction in the Mississippi basin has cut the river's sediment load by about 50% (Blum and Roberts, 2009).
• Elsewhere too, dams trap sediment: about 7 billion m³ of the High Aswan Dam's reservoir has filled with it, and on the Mekong "dams trap sediment" that maintains the delta's fertility.
• So human interference in one part of a basin changes landforms in another: a spatial interaction.

Reading landforms on a topographic map

• Contours close together mean a steep slope; far apart, gentle.
• A V-shaped valley: contours make Vs pointing upstream, towards higher ground.
• A waterfall: contours bunch together where they cross the river.
• A floodplain: a wide, flat strip with few contours either side of a meandering river, often with oxbow lakes.
• Locate features with grid references and measure them with the scale.

Mission 4: On 23 December 2013, 43–45 mm of rain fell on two river basins in southern England. Within a day, one river surged to one of its highest flows since records began; the other barely rose, and peaked only in mid-February after weeks more rain. Why did the same storm have such different effects?

The storm hydrograph

• A storm hydrograph plots a river's discharge against time after a rainfall event; rainfall is shown as bars.
• Rising limb: discharge increases. Peak discharge: the highest flow. Falling (recession) limb: flow declines.
• Lag time: the time from peak rainfall to peak discharge.
• Base flow: the slow groundwater contribution beneath the storm; storm flow (quickflow) sits on top.

Two rivers, one storm

• Beult, Kent: 277.1 km², mean annual rainfall 691 mm (1961–90). Lambourn, Berkshire: 234.1 km², 736 mm (1961–90 standard average; screen 1.9 used the 1962–2019 mean, 768 mm).
• 23 December 2013 catchment rainfall: Beult 42.6 mm; Lambourn 44.9 mm.
• Beult: daily mean flow rose from 32.0 to 79.4 m³/s on 24 December, the highest daily mean in its daily series from 1958. The series has gaps.
• Lambourn: from 1.42 to 2.39 m³/s. After about 317 mm more rain between 24 December and 15 February, it reached 9.53 m³/s on 15 February 2014, its highest daily mean.

Chalk: the Lambourn

• The Lambourn drains the Chalk of the Berkshire Downs: about 97% of its bedrock is highly permeable.
• Rain infiltrates and percolates into the chalk aquifer, then reaches the river slowly as base flow.
• Its NRFA drainage-density index is low (0.17): few surface channels.

Clay: the Beult

• The Beult drains a "predominantly clay catchment": about 74% of its bedrock has low permeability.
• Water cannot soak in far, so it reaches the river quickly as overland flow and throughflow.
• Its NRFA drainage-density index is 1.11, about six times the Lambourn's: more channels carry water away faster.

The base flow index

• Base flow index (BFI): the share of a river's long-term flow that comes from stored sources.
• Lambourn 0.97; Beult 0.23.
• The same geology sustains low flows: the flow exceeded 95% of the time is 0.77 m³/s on the Lambourn, but 0.057 m³/s on the Beult.
• Permeable rock both slows floods and protects against drought.

Flow regimes

• Monthly mean flows (calculated from NRFA daily data): the Beult is highest in January (5.53 m³/s) and lowest in July (0.22). The Lambourn is highest in March (2.59) and lowest in autumn (1.11 in September, 1.10 in October).
• Monthly rainfall varies far less: 47–84 mm on the Beult and 53–79 mm on the Lambourn.

Why the regimes differ

• Summer flows fall because evapotranspiration is higher.
• The chalk aquifer fills in winter and drains slowly, so the Lambourn peaks late, in spring.
• In cold climates, snowmelt gives a spring or summer peak, and frozen ground behaves like an impermeable surface.

Other natural influences

• Basin shape: in a round basin, water from all parts arrives together, giving a higher, earlier peak than in an elongated one.
• Slope speeds runoff. Soils: clay soils have a low infiltration capacity.
• Antecedent conditions: rain on wet ground runs off faster. Before 23 December the Beult was already high after rain on 16–22 December.
• Vegetation: interception and transpiration reduce and delay runoff.

Exam move: describe, then explain (skills)

• Describe with values from the graph: peak discharge, lag time, the steepness of the rising limb, base flow.
• Explain with named factors, then weigh them. Here rainfall and basin size were similar, so geology was decisive.
• The guide asks how water flows "can be represented graphically": draw and annotate the hydrograph.

Mission 5: A study found that urbanization made Hurricane Harvey-scale flooding in Houston's basins about 21 times as likely in 2017. How can a city make a flood worse?

Flood risk: magnitude, frequency, distribution

• A flood happens when discharge exceeds bankfull capacity and water covers land that is normally dry.
• Magnitude: how big (peak discharge, depth). Frequency: how often a flood of that size occurs. Distribution: where in the basin floods happen.
• Flood risk combines the hazard with who and what is exposed, and how vulnerable they are.

Urbanization

• Impermeable surfaces (roofs, roads, car parks) reduce infiltration and increase overland flow.
• Drains and sewers carry water to rivers quickly.
• Result: shorter lag time, steeper rising limb, higher peak discharge.
• Building on floodplains also raises exposure.

Hurricane Harvey, Houston, 2017

• Harvey stalled over or near the Texas coast "for four days"; the highest storm total was 60.58 inches (about 1,540 mm), near Nederland, Texas (National Hurricane Center).
• A study in Nature found that urbanization "exacerbated not only the flood response but also the storm total rainfall".
• It estimated that the probability of such extreme floods in the basins studied rose by about 21 times on average because of urbanization (25–30 August 2017).
• One modelling study of one event: strong evidence, but test it against others.

Deforestation: the mechanism

• Trees intercept rain, roots open up the soil, and transpiration dries it, so more water infiltrates and is stored.
• Removing them, especially on steep slopes, increases overland flow and soil erosion; eroded sediment can fill channels downstream.
• FAO and CIFOR (2005): forests can reduce flooding from "frequent, low-intensity, short-duration storms".
• But they "cannot stop catastrophic large-scale floods": in prolonged rain, forest soils saturate and water runs off the surface.

Deforestation and floods: a research debate (AO3)

• The ecologists C.J.A. Bradshaw et al. (2007), using data from 56 developing countries, 1990–2000: flood frequency "is negatively correlated with the amount of remaining natural forest".
• van Dijk et al. (2009): "population density alone already explains up to 83% of the variation in reported flood occurrences", and they judged the conclusion "unsupported".
• Lesson: a correlation in reported floods may reflect how many people are there to report them.

Channel modification: the Kissimmee River, Florida

• Straightening, deepening and lining a channel speeds water away from one place, but can pass the flood peak downstream faster.
• After "prolonged flooding in 1947", the Kissimmee River, which "once meandered for 103 miles", was cut into a 30-feet-deep straight channel in the 1960s.
• It "destroyed much of a floodplain-dependent ecosystem". Restoration construction was completed in July 2021.

Frequency: recurrence intervals

• Recurrence interval (RI) = (n + 1) ÷ m: n = years of record; m = the flood's rank (1 = largest).
• A "100-year flood" has a 1% chance of being equalled or exceeded in any year, so two can happen in successive years.
• Over a 30-year mortgage, a home on that floodplain has a 26% chance of flooding at least once.
• Estimates depend on the length of the record and change with land use and climate.

Mission 6: In 1995, more than 250,000 people in the Netherlands had to leave their homes as the rivers rose. The Dutch response was not simply higher dikes. What did they do?

Flood prediction: weather forecasting

• Forecast skill 3–10 days ahead has increased by "about one day per decade": "today's 6-day forecast is as accurate as the 5-day forecast ten years ago".
• Ensemble forecasts run a model many times from slightly different starting conditions, giving a probability rather than one answer.
• Flood forecasts feed rainfall forecasts into river models, so better weather forecasts give earlier flood warnings.

Flood warning technology

• Where rivers have few gauges, warnings are hardest to give.
• An AI model forecast extreme river floods in ungauged basins up to five days ahead as reliably as, or better than, an existing global system's same-day estimates.
• By 2024 it was running in an early warning system in over 80 countries.

Uncertainty in climate modelling

• IPCC: heavy rainfall has become more frequent and intense since the 1950s over most land areas with enough data to judge (high confidence). Extreme daily rainfall is projected to intensify by about 7% for each 1 °C of warming.
• Three sources of uncertainty in regional rainfall projections: internal variability (dominant in the first decade), model uncertainty (generally dominant later) and scenario uncertainty (found to be small).
• So planners use ranges and confidence levels, not single numbers.

Structural measures

• Dams store floodwater in reservoirs and release it slowly.
• Afforestation increases interception and infiltration upstream; most effective for smaller floods.
• Channel modification: straightening, widening, deepening or adding relief channels.
• Levee strengthening: raising and reinforcing embankments.

Planning measures

• Personal insurance spreads the cost of damage.
• Flood preparation: household plans, flood-resistant fittings, evacuation routes and drills.
• Flood warning technology: gauges, radar, satellites, forecast models and phone alerts (screens 6.1–6.2).
• Land-use planning: keep new building off the floodplain.

Insurance in the UK: Flood Re

• Flood Re is a reinsurance scheme, "a joint initiative between the Government and insurers", to make flood cover in household policies more affordable.
• It replaced an earlier arrangement in 2016, and covers homes "built before 1st January 2009", so it does not reward new building on floodplains.
• It runs until 2039, when insurers should price policies on actual risk.
• Exam move: insurance spreads losses, but it does not reduce the hazard.

Example 1: the Rhine, Netherlands — why act?

• High water in 1993 and in 1995, when more than 250,000 residents had to leave, was "the direct reason" for the Room for the River programme.
• Design flow: a Rhine discharge of 16,000 m³/s at Lobith, on the German border, with a 1-in-1,250 chance each year.
• Approach: "give rivers space again" rather than only raising dikes.

Room for the River: what was done

• Measures at 34 places along the Rhine branches: the IJssel, Waal, Nederrijn and Lek. Total cost: €2.3 billion.
• Planning 2001–2007; building 2007–2017. The programme was officially completed when the Reevediep high-water channel opened in early 2019.
• Measures included dike relocation, floodplain lowering, high-water channels, depoldering (returning reclaimed land to the river) and removing obstacles.
• The Meuse had a separate programme, the Meuse Works.

Nijmegen: moving a dike back

• At Lent, the dike was moved 350 m inland to make room for a side channel 3 km long and 150–200 m wide.
• Flood levels fell by 35 cm, beyond the initial 27 cm target. Cost: about €351 million; completed 2016. The new island is "a unique urban river park".
• Cost to residents: about 50 houses in Lent had to be demolished and their families moved, according to Rijkswaterstaat's account of the programme.
• Stakeholders' "doubts and opposition were largely addressed" through newsletters, meetings and workshops.

Example 2: the Yangtze, China — why act?

• China's 1998 floods affected 186 million people and caused 4,150 deaths.
• The Yangtze, Songhua, Nen, Min and Pearl rivers were all hit.
• The response combined forest protection, returning land to lakes, dikes and dams.

Yangtze: room for water and trees

• Afforestation: forest protection and reforestation, and the Grain for Green Program (1999–2020), converting cropland and wasteland on hillslopes to forest and grassland.
• From 1998 to 2002, 1,461 polders (embanked farmland) were removed and 2.4 million people relocated in the Yangtze basin.
• This added 2,900 km² of inland water area and a storage capacity of 13 billion m³.

Yangtze: dams and dikes

• The Three Gorges Dam was completed in 2006. Downstream at Jingzhou, the protection standard rose from a 10-year to a 100-year flood.
• Across China, river banks protected by well-maintained dikes grew from 76,532 km (1998) to 201,124 km (2016).
• August 2020: the Three Gorges reservoir had its highest inflow since it was built, 74,600 m³/s. During the 2020 floods the cascade of reservoirs, working together, held back 19.0 billion m³ of flood water. The study's authors include staff of the dam's operating company.

Evaluating the two (AO3)

• China: annual flood deaths fell from 3,909 in the 1990s to 1,454 in the 2000s. But flood damage "reached a new peak during the 2010s".
• Fewer deaths may also reflect better warnings and evacuation, not only dams and dikes. The authors call for approaches such as risk-based urban planning "in addition to traditional structural protection".
• Costs of each: in China, 2.4 million people relocated from polders; in the Netherlands, €2.3 billion and homes demolished at Lent.

Mission 7: In January 2018 Cape Town, a city of just over 4 million people, warned that its taps could be turned off in April: "Day Zero". What pushed it to the edge, and how did it pull back?

Quantity and quality

• Water quantity: how much water is available. Water quality: whether it is safe and fit for use, free of pathogens, harmful chemicals and excess salt.
• Water security: reliable access to enough water of acceptable quality.
• 2.1 billion people still lack safely managed drinking water, including 106 million who drink untreated surface water (WHO/UNICEF, 2025).

Physical and economic water scarcity

• Physical scarcity: more than 75% of river flows are withdrawn for agriculture, industry and households.
• Economic scarcity: "human, institutional, and financial capital limit access to water even though water in nature is available locally"; less than 25% is withdrawn, but malnutrition exists.
• Pattern (IWMI, 2007): arid regions are most often physically scarce; "much of Sub-Saharan Africa" is economically scarce. Overcommitted basins such as the Indus and Yellow River are physically scarce.

Drought: types and causes

• Drought: a long period of below-normal water availability. Meteorological drought (low rainfall) leads to agricultural drought (dry soils, failing crops) and hydrological drought (low rivers, reservoirs and aquifers).
• Causes: persistent high pressure and shifted storm tracks; heat increases evapotranspiration.
• IPCC: human-induced climate change "has contributed to increases in agricultural and ecological droughts in some regions" through higher evapotranspiration (medium confidence).

Cape Town: the causes

• 2015–2017: "three consecutive years of below average rainfall", with "a return time of more than a hundred years".
• Attribution: climate change made such a drought about three times as likely (95% range 1.5–6).
• Storage in the six largest dams fell from 100% in 2014 to 71%, 60% and 38% in the following years, measured each 1 November, at the start of the dry season.
• Demand had grown, including from "increasing numbers of people living in backyard shacks".

Cape Town: the impacts

• Farms: irrigation restrictions rose to 30% (1 March 2017), then 50% (October 2017) and 60% (December 2017), with lost work "particularly for seasonal labourers".
• Businesses: commercial and industrial use fell about 20%; non-domestic tariffs more than doubled.
• Households: from 1 February 2018, a limit of 50 litres per person per day (Level 6B).
• Day Zero meant turning off taps in neighbourhoods if the dams fell to 13.5% full.

The stakeholders and the response

• The supply system is managed by the national Department of Water and Sanitation "in cooperation with the City".
• City: restrictions, tariffs and campaigns. National government: allocations, including to farms. Farmers: cuts. Residents and businesses: saving water.
• Daily use fell from over 1,000 Ml (million litres) in early 2016 to "just below 500Ml a few times", "about 50% of the pre-drought usage".
• On 7 March 2018, Day Zero was deferred; winter rain then "filled the dams to above 75%". Critique: some officials said the campaign "and the panic it caused, came at a high price".

Mission 8: Every summer, parts of Chesapeake Bay, USA, become "dead zones" with too little oxygen for fish and shellfish. Much of the cause starts on land, in a watershed spanning six states. How can a farm field suffocate a bay?

Eutrophication

• Fertiliser and manure wash nitrogen and phosphorus off fields into rivers, lakes and seas.
• The nutrients "can fuel the growth of algae blooms" that shade underwater plants.
• Dead algae sink and "are decomposed by bacteria that consume oxygen until there is little or none left".
• Water with less than 2 mg/L of dissolved oxygen is low-oxygen (hypoxia): a dead zone.

Chesapeake Bay: the sources

• Agriculture is "the single largest source of nitrogen pollution entering the Chesapeake Bay".
• 2024 modelling: agriculture supplied about 46% of nitrogen and 29% of phosphorus loads.
• By the Program's 2024 model data, the largest source of phosphorus and sediment is the "natural" sector, such as stream beds and banks. (The same page also calls agriculture "the single largest source of nutrient and sediment pollution".) The watershed spans six states and the District of Columbia.

Chesapeake Bay: the stakeholders

• Federal government: on 29 December 2010 the EPA set a "pollution diet" (a Total Maximum Daily Load), with measures to be in place by 2025.
• States: each plans how to cut its share.
• Farmers: best management practices such as cover crops, forest buffers, fencing livestock out of streams and nutrient management plans.

Chesapeake Bay: the results so far

• By 2023, practices in place had achieved 57% of the nitrogen and 67% of the phosphorus reductions needed.
• Sediment goals were met Bay-wide.
• The 2025 goal "will not be met on time".

Salinization

• In hot, dry regions, irrigation water evaporates and leaves its salts in the soil.
• Over-irrigation and poor drainage raise the water table; salty groundwater then rises by capillary action towards the surface.
• Remedies include drains to control the water table and mulching to "reduce evaporation losses and thus decrease or prevent soil salinization".
• FAO's 2021 global map: more than 424 million ha of topsoil and 833 million ha of subsoil are salt-affected, from natural and human causes (118 countries).

Mission 9: The Aral Sea was once the fourth-largest lake in the world. In 2014 the eastern part of its southern basin disappeared completely. What drained it?

The Aral Sea: a lake drained by economic growth

• In the 1960s the Soviet Union diverted the Amu Darya and Syr Darya to turn desert into farms "for cotton and other crops".
• The Aral Sea, once "the fourth largest in the world", shrank. In 2014 the eastern lobe of the South Aral Sea completely disappeared.
• Fisheries collapsed. Salty dust from the exposed bed settled on fields, "degrading the soil", so croplands "had to be flushed with larger and larger volumes of river water": salinization feeding back on demand.

Aral: a partial recovery (synthesis)

• Kazakhstan's Kok-Aral dike and dam, finished in 2005, separates the North Aral from the South Aral.
• North Aral water levels "rebounded significantly" in 2005–2006 and its fisheries recovered, but flow into the South Aral was limited.
• One part of the lake gained at another's expense: a spatial interaction.

Aquifers under pressure: the High Plains, USA

• The High Plains aquifer underlies parts of eight states, from South Dakota and Wyoming to Texas.
• Water levels began to fall soon after large-scale groundwater irrigation began, around 1950.
• From before development to 2015: an area-weighted average decline of 15.8 feet (about 4.8 m), and about 273.2 million acre-feet less recoverable water in storage.

Migration and an aquifer: Las Vegas Valley, USA

• A new period of growth began in 1932 with the building of Boulder (Hoover) Dam, which "brought workers to Las Vegas from throughout America".
• A "seemingly unlimited supply of water and power" then attracted industry, the military and gambling in the 1940s and 1950s.
• In 1999 it was "the fastest growing metropolitan area in the United States".

Las Vegas: pumping and its effects

• By about 1955, wells were pumping nearly 40,000 acre-feet a year, more than the estimated natural recharge of 25,000–35,000 acre-feet.
• By 1990, water levels had fallen more than 300 feet in places; the land had subsided nearly 6 feet since 1935.
• Responses: water imported from Lake Mead and, from 1988, treated water injected into the aquifer.

Lake Chad: check the evidence (AO3)

• A 2020 satellite study reports that Lake Chad shrank to less than 2,000 km² in the 1980s, a fall of more than 90% in area, citing earlier work.
• For 2001–2018 it found the southern pool "not shrinking but, on the contrary, slightly increasing"; groundwater contributes about 70% of the lake's annual change in water storage.
• Its conclusion: "for now Lake Chad is not disappearing".
• Lesson: a dramatic figure may describe one period. Check its date and method.

Mission 10: About 85% of the Nile's water starts in Ethiopia, yet a 1959 agreement shared it between Egypt and Sudan. Who decides how it is used now, and what would make a deal work?

Why shared rivers cause conflict

• Upstream states control flow and timing; downstream states depend on what arrives.
• Pressure grows with population, irrigation, hydropower and drought.
• Stakeholders: national governments, farmers, cities, power companies, basin organisations and mediators.
• Responses range from treaties and basin commissions to diplomatic disputes.

The Nile basin

• Natural flow at Aswan averaged 86.5 billion m³ a year from 1900 to 2018, ranging from 45.6 to 120 billion.
• The Blue Nile contributes about 55%, the White Nile 32% and the Atbara 13%.
• "85% of Nile waters originate in Ethiopia", but "nearly all consumptive use occurs downstream in Egypt and Sudan".
• The Blue Nile and Atbara give 68%, so the 85% presumably includes Ethiopian water reaching the White Nile through the Sobat (Baro–Akobo); the paper does not break it down.

The 1959 agreement

• Egypt and Sudan's agreement, signed in Cairo on 8 November 1959, assumed a mean flow at Aswan of about 84 billion m³ a year.
• Once the High Aswan Dam (162 billion m³ of storage) was working, it gave 55.5 billion m³ a year to Egypt and 18.5 billion to Sudan, with storage losses estimated at 10 billion.
• "Not being signatories, Ethiopia and other upstream states do not recognize this agreement".

The Grand Ethiopian Renaissance Dam (GERD)

• Ethiopia began building the GERD on the Blue Nile in 2011. Installed capacity: 5,150 MW, expected to generate about 16 TWh a year.
• Reservoir: 74 billion m³ in total, 59 billion of it active; the active storage alone is about 1.2 times the Blue Nile's mean annual flow at the dam site.
• Prime Minister Abiy Ahmed inaugurated the dam on 9 September 2025 (Ethiopian News Agency).

The stakeholders' interests (Ev, Sy)

• Ethiopia: electricity demand projected to rise from 11.1 TWh (2020) to 27.3 TWh (2030); the GERD's 5,150 MW adds to 4,400 MW of existing capacity.
• Egypt: on the day of the inauguration, its foreign minister wrote to the UN Security Council that Egypt is "entirely dependent on the Nile River for its survival", with water per person "approaching the absolute water scarcity threshold of 500 cubic metres".
• Sudan: smoother Blue Nile flows could eliminate flood losses, raise hydropower and add summer irrigation water.
• Each country's interest depends on where it sits in the basin: a spatial interaction.

Attempts at a resolution

• The Nile Basin Initiative, founded on 22 February 1999, links 10 basin countries.
• Its Cooperative Framework Agreement (CFA) sets out "the principle of equitable and reasonable utilization" and "the obligation not to cause significant harm".
• Opened for signature in 2010 (Egypt and Sudan rejected that step), it entered into force on 13 October 2024, after South Sudan acceded as the sixth party. Egypt and Sudan are not parties.
• In 2020 a study described the GERD talks between Egypt, Ethiopia and Sudan as "difficult, ongoing negotiations".

What the evidence suggests (AO3)

• Modelling: once filled, the GERD "will benefit Ethiopia and Sudan without significantly affecting water users in Egypt".
• But "management of multi-year droughts will require careful coordination", because both reservoirs could be low at once.
• So the critical issue is less the dam itself than agreed rules for droughts and data sharing.

Mission 11: The Lower Mekong's fisheries land an estimated 2.3 million tonnes of fish a year, worth about US$11 billion. Its hydropower capacity could more than double. Can one river be both a great fishery and a giant power station?

Local participation: why it matters

• SDG target 6.1: "universal and equitable access to safe and affordable drinking water for all" by 2030.
• Target 6.b: "Support and strengthen the participation of local communities in improving water and sanitation management".
• Local users know their sources, can maintain schemes, and are more likely to use water efficiently when they share the decisions.

Participation in different development contexts

• India (lower-middle income): the Jal Jeevan Mission aims to give every rural home a tap connection. It is "based on a community approach", with local contributions "in cash, kind and/ or labour".
• South Africa (upper-middle income): Cape Town's residents and businesses halved the city's water use in a drought (Mission 7). This was mostly compliance with rules, not a share in decisions.
• Australia (high income): the Productivity Commission calls on Murray–Darling governments to "partner and share decision making with First Nations people" (Mission 12).

Efficiency and sustainable use

• Efficiency: drip irrigation, fixing leaks, metering and pricing, reusing treated water.
• Sustainable use: take water no faster than it is replaced (compare Las Vegas and the High Plains, Mission 9).
• Affordability: higher prices cut waste but weigh most on poorer households, so pricing needs to protect basic needs.

Multipurpose dams: costs and benefits

• Benefits: hydropower, flood control, storage for irrigation and supply, navigation.
• Costs: displaced communities, flooded land and habitats, blocked fish migration, trapped sediment and changed flows downstream.
• Benefits and costs often fall on different places and people.

The Mekong and its dams

• The Mekong runs nearly 5,000 km from the Tibetan Plateau through six countries.
• China has 12 operational dams on the upper Mekong's main stream (the Lancang).
• Lao PDR has 98 hydropower projects, by the MRC's count, with 64% of the lower basin's installed capacity, for its own use and export.

The lower-basin pipeline

• Lower Mekong: 88 hydropower projects operating, with over 13,257 MW installed.
• A further 20 are under construction and 14 planned.
• If all are built, capacity could reach 27,302 MW.

Who gains and who loses? (Ev, Sy)

• Gains: energy security, revenue from selling electricity to neighbours, rural electricity, some flood and drought regulation.
• Losses: "Dams trap sediment" that maintains the fertility of the Mekong Delta; disrupted fish migration "threatens the food security and income of millions". About 80% of the nearly 65 million people in the lower basin depend on the river.
• Evidence gap: the MRC says reservoirs in the upper and lower basin "have trapped most of the sediment", but information on the impact in the lower basin "is limited".
• Gains go mostly to power users and governments; losses fall mostly on fishers and delta farmers downstream.

Governance: the Mekong River Commission

• The MRC was set up in 1995 by Cambodia, Lao PDR, Thailand and Viet Nam. China, upstream, is not one of these four members.
• Its procedures for notification and prior consultation "provide independent reviews of individual projects" and share information.
• It also issues design guidance for mainstream dams, to "avoid, minimize, and mitigate" impacts.

Mission 12: Since 1970 the world has lost about 22% of its wetlands. Can a plan for a whole river basin bring a wetland back?

Integrated drainage basin management (IDBM)

• IDBM plans land and water use across a whole basin rather than river by river or sector by sector.
• It balances water for the environment, farms, towns, industry and culture, and involves all the stakeholders.
• It is part of SDG target 6.5: integrated water resources management "at all levels, including through transboundary cooperation".

Case study: the Murray–Darling Basin Plan, Australia

• The basin spans parts of New South Wales, Victoria, Queensland, South Australia and the Australian Capital Territory, each with its own water plans.
• Under the Water Act 2007, the Basin Plan came into effect in November 2012, aiming for "a healthy, working Basin".
• To get there, governments agreed to recover 2,750 GL a year for the environment (about a 20% cut in water for consumptive use) and an additional 450 GL a year.
• Benefits by 2023: recovered water "improved river flows, connectivity, and ecosystem and biodiversity outcomes".

Murray–Darling: shortfalls (Ev)

• The Productivity Commission (2024): the Plan "will not be fully implemented within the original timeframe or budget".
• Only 26 GL a year of the extra 450 GL had been recovered, and less than half of a 605 GL supply-measure offset.
• 10 of 20 New South Wales water resource plans, due in 2019, were still not in place.

Murray–Darling: who bears the costs (Ev)

• Water purchases should be gradual "to avoid driving water market disruption and community adjustment pressures": farming towns bear costs.
• First Nations interests need stronger partnership and shared decision making.
• So the plan's environmental gains depend on who gives up water, and how they are helped.

Wetlands under pressure

• Since 1970, about 411 million ha of wetlands, roughly 22% of the global total, have been lost, and losses continue at 0.52% a year.
• About 25% of the remaining wetlands are in poor condition. Pressures: agricultural expansion, pollution, infrastructure, hydrological disruption and climate change.
• The remaining 1.4 billion ha provide ecosystem services worth up to US$39 trillion a year: they regulate floods, store carbon and purify water.

The Ramsar Convention

• The Convention on Wetlands was adopted at Ramsar, Iran, on 2 February 1971, and came into force in 1975.
• It has 172 Contracting Parties and over 2,530 Wetlands of International Importance, covering about 2.58 million km², roughly 14–17% of the world's wetlands.
• Parties commit to wise use and to maintaining the ecological character of listed sites.
• Limitation: listing a site does not control the water that reaches it from upstream.

Case study: the Everglades, Florida

• Water once "flowed slowly from the Kissimmee River to Florida Bay", forming a "River of Grass".
• The South Florida ecosystem covers about 18,000 square miles and is the primary water source for 9 million residents. Everglades National Park (1947) protects 1.5 million acres.
• Pressures: "an extensive network of canals and levees to drain land for agricultural and residential development", and "flood control and water supply needs for a growing population base".
• Results include phosphorus pollution that spreads cattail, severe wildfires in drier areas that "consume peat soils", and the loss of tree islands in wetter areas.

The Everglades: the restoration plan (CERP)

• The Comprehensive Everglades Restoration Plan (CERP), authorized in 2000, is a federal–state partnership restoring the "quantity, quality, timing, and distribution" of water.
• It has 68 components.
• Example: the EAA Reservoir (240,000 acre-feet of storage) will store and treat releases from Lake Okeechobee.

The Everglades: funding and the future

• Funding: $2 billion in federal CERP funding by September 2021, then $1.098 billion from the November 2021 infrastructure law.
• Planning now includes resilience to sea-level rise.
• The Kissimmee restoration (5.8) is at the head of the same system.

Scales of water management (Sy/Ev)

• Personal: Cape Town's 50 litres a day. Local: India's village tap-water schemes.
• National: Room for the River; the Murray–Darling Basin Plan; CERP.
• International: the Nile CFA; the Mekong River Commission. Global: SDG 6; the Ramsar Convention.
• Exam move: in a "to what extent" answer, judge which scale made the difference, and for whom.

How Option A is examined (Paper 1)

• Paper 1 covers the options: two at SL (1 hour 30 minutes, 40 marks) and three at HL (2 hours 15 minutes, 60 marks). It is 35% of the grade at both levels.
• About 45 minutes per option, and 20 marks per option: a structured question (10 marks) and one extended answer from a choice of two (10 marks).

What the top band rewards (question 2, 9–10 marks)

• "In-depth and question-specific (topic and command term)".
• Examples, statistics and details are integrated and explained, not listed.
• Evaluation weighs other perspectives and the strengths and weaknesses of evidence, as with the forest–flood debate or Lake Chad's "90%".
• The conclusion is balanced and matches the evidence. Maps and diagrams are annotated and support the argument.

The big ideas

• A drainage basin is an open system of stores and flows; groundwater and ice keep rivers flowing (M1).
• Discharge and channel efficiency set velocity; velocity sets erosion, transport and deposition (M2), which build waterfalls, meanders, levees and deltas (M3).
• Geology, seasonality and land use shape hydrographs and flood risk (M4–M5); better forecasts and two contrasting strategies manage it (M6).
• Scarcity can be physical or economic; quality and quantity both matter (Cape Town, Chesapeake, Aral) (M7–M9).
• Shared rivers need agreed rules (Nile); dams, IDBM and wetland plans bring costs and benefits to different people (M10–M12).

Final screen. Score, the mission list ticked, and "Next: test yourself in the Verify stage" (links to the Verify quiz).