Here's a fact that surprises most students: only 2.5% of all the water on Earth is freshwater — the rest (97.5%) is salty ocean water. And of that tiny 2.5% slice, most of it isn't even sitting in rivers where you could drink it — 68.7% is locked up in glaciers and ice sheets, and another 30% is groundwater hidden underground. That leaves just 1.3% of all freshwater in rivers, lakes, soil moisture and the atmosphere — the water actually available and moving around the surface of the planet.

Think of it like a giant bank account: Earth's total water is your total savings, but 97.5% is locked in an account you can't touch (the sea). Of what's left, most is frozen in a long-term deposit (glaciers) or buried underground. What's actually "in your wallet" — usable, visible, everyday water — is a tiny sliver.

The hydrological cycle is called a closed system because no water enters or leaves Earth — it just gets recycled endlessly between different stores (places water sits for a while) and flows/transfers (ways water moves between stores).

Stores — places where water is held for some time:

• Atmosphere (water vapour, cloud droplets)
• Surface stores — puddles, lakes, rivers, reservoirs
• Interception — water caught on leaves/branches before it reaches the ground
• Aquifers — permeable rocks (e.g. limestone, sandstone) that hold water underground
• Ice and snow
• Seas and oceans

Flows (transfers) — the eight you need to know cold:

A drainage basin is the entire area of land that drains water into one river system. Unlike the hydrological cycle (a closed system), a drainage basin is an open system — water enters from precipitation and leaves via the river mouth or evapotranspiration.

Every drainage basin has the following key features — these come up constantly in labelling questions:

There are exactly four erosion processes examiners expect you to name and explain. A great way to remember them: H.A.A.C.

Four processes carry material downstream, sorted roughly by particle size from heaviest to lightest:

When a river doesn't have enough energy to keep carrying its load, it drops (deposits) material. This happens for several reasons:

• Reduced discharge — less water due to lack of rain, or water being extracted (abstraction) upstream
• Decreased gradient — the river is flowing on flatter land, so it has less energy
• Slower flow zones — e.g. the inside of a river bend, or shallower stretches
• Reaching the mouth — when the river enters the sea, ocean, or a lake, it suddenly loses velocity

The heaviest material is dropped first — this is the bedload. Lighter material (gravel, sand, silt) is called alluvium and travels further downstream before settling. Dissolved material just gets carried all the way out to sea.

This is one of the most commonly tested diagrams in the whole topic. It shows how various characteristics of the river change as you travel from the source (upstream) to the mouth (downstream).

The long profile shows how the river's height/gradient changes from source to mouth — it's normally concave (steep near the source, flattening out near the mouth), like the curve of a skateboard ramp.

The cross profile is a cross-section slice of the valley/channel at any given point. These look very different depending on which course you're in:

Waterfalls & Gorges

Waterfalls form where the river bed suddenly drops in level — usually because a layer of hard rock sits on top of soft rock. Here's the step-by-step formation sequence, and you MUST know this for the exam:

1. Hydraulic action and abrasion erode the softer rock faster than the hard rock, creating undercutting beneath the hard rock layer.
2. This undercutting carves out a plunge pool at the base of the falls.
3. The hard rock layer is left jutting out with nothing supporting it — an overhang.
4. Eventually, gravity wins and the overhang collapses into the plunge pool below.
5. The fallen rock chunks increase abrasion in the plunge pool, making it even deeper.
6. This cycle repeats again and again, and the waterfall gradually retreats upstream, leaving behind a long, steep-sided gorge — essentially the "trail" of retreat.

V-Shaped Valleys

Vertical erosion dominates in the upper course, cutting straight down into the riverbed and deepening the channel. Meanwhile, weathering (rock breaking down in place) and mass movement (material sliding/falling downhill) cause loose rock from the valley sides to tumble into the river. Combined, this carves out a steep-sided, V-shaped valley — picture a knife slicing straight down through the landscape while the edges crumble inward.

Interlocking Spurs

In the upper course, the river channel already starts to wind slightly (meander), even in hilly terrain. As the river's vertical erosion carves its path, it has to bend around areas of harder, more resistant rock rather than through them (since it lacks the lateral erosion power to cut sideways yet). This leaves ridges of land — spurs — sticking out from alternate sides, which interlock like the teeth of a zipper when viewed from above.

Potholes

Potholes are small, round depressions worn into the riverbed, formed by abrasion. Where there's already a small dip in the bed, the swirling flow of the river causes trapped sediment (pebbles) to spin around inside it. This spinning action grinds the dip into a deeper, rounder hollow. As the pothole gets bigger, it traps even larger material, which speeds up further erosion — a self-reinforcing cycle.

Meanders

In lowland areas, lateral erosion takes over as the dominant process, and meanders (large winding bends) grow bigger over time. The key concept here is the thalweg — the line of fastest flow within the channel.

• On the outside of a bend, the water flows fastest (thalweg is here) → this causes erosion, undercutting the bank to form a steep river cliff.
• On the inside of a bend, the water flows slowest → this causes deposition, building up a gently sloping slip-off slope.

Because one side erodes while the other builds up, the whole meander gradually migrates (shifts sideways and downstream) across the valley floor over time — this is exactly what widens the valley and helps build the floodplain.

Ox-Bow Lakes

As meanders migrate and grow bigger over distance, the neck of the meander (the narrow strip of land between two loops) gets progressively thinner from erosion on both outside bends. Here's the formation sequence:

1. Continued erosion at the outside bends narrows the neck of the meander.
2. During a flood, the river has enough power to cut straight through the neck, creating a new, straighter channel.
3. Water now prefers this shorter, straighter route — flow at the entrance and exit of the old meander loop slows down.
4. This slower flow causes deposition at both ends of the loop, sealing it off from the main channel.
5. The abandoned loop becomes a crescent-shaped ox-bow lake, separate from the river.

Floodplains & Levees

A floodplain is the flat land either side of the river channel, built up over time by the sideways migration of meanders across the valley (which flattens and widens it) and by repeated flood deposits.

When discharge is very high, the river overflows its banks. As soon as the water spreads out across the floodplain, it meets far more surface area, causing increased friction, which slows the water down and triggers deposition. The heaviest material is dropped closest to the channel (because it loses energy fastest), building up natural embankments called levees along the riverbanks. Finer material spreads further across the floodplain. Each flood adds another layer, gradually raising the levees higher and higher.

Deltas

Deltas form where a river meets a standing body of water (a sea, ocean, or lake) and rapidly loses velocity, forcing it to dump its sediment load all at once. Two special processes help this along:

• Flocculation — salt in seawater causes fine clay/silt particles to stick together into bigger clumps, which are heavy enough to sink and deposit.
• Bioconstruction — vegetation (like mangroves) slows the water down further, encouraging even more deposition.

There are three classic delta shapes you should recognise:

Rivers flood when water reaches bankfull discharge and spills over onto the floodplain. There are two main natural rainfall-related causes:

• Heavy, torrential rain — falls too fast for the ground to infiltrate it, so it becomes overland flow.
• Prolonged, steady rain — over time, the ground becomes fully saturated (soil pores are full), so even gentle rain can no longer infiltrate and instead runs off.

Other natural causes include landslides blocking channels, rapid snow/ice melt, and storm surges pushing seawater back up a river channel.

While precipitation is the root cause, human activity increases the risk:

• Urbanisation — impermeable surfaces (concrete, tarmac) stop infiltration
• Deforestation — removes interception and root uptake of water
• Building bridges/dams — can restrict flow or fail catastrophically
• Climate change — may increase storm frequency/intensity
• Agriculture — ploughed, bare soil increases overland flow

• Disease spread — floodwater can breed disease-carrying insects (e.g. mosquitoes) or become contaminated with bacteria (e.g. cholera)
• Deaths and injuries — floodplains are often densely populated (due to their fertile soil), putting many people at risk
• Infrastructure damage — bridges and transport routes destroyed
• Land loss — erosion of riverbanks destroys farmland, housing and roads
• Economic impact — destroyed crops, higher insurance costs, falling house prices

It's easy to only think of rivers as dangerous — but the exam often wants balance. Rivers also bring huge benefits:

• Fertile soil — silt deposited during flooding is rich in nutrients, ideal for farming
• Food source — fishing
• Flat land — floodplains are easy to build transport networks and settlements on
• Irrigation — water for farmland
• Leisure & tourism
• Hydroelectric power (HEP) generation
• Transport — moving goods and people by boat

A flood hydrograph is a graph combining rainfall (usually as bars) with river discharge (as a line) over a short period, typically 24-72 hours after a storm. You need to be able to label and interpret every part of it:

Human factors that INCREASE flood risk (all work by increasing overland flow, thereby shortening lag time):

• Deforestation — less interception and infiltration
• Urbanisation — impermeable concrete/tarmac + drains carry water to the river fast
• Agriculture — bare soil and ploughing increase overland flow
• Climate change — potentially more frequent/intense storms

Physical factors that INCREASE flood risk:

• Relief — steep slopes reduce infiltration time
• Rock type — impermeable rock reduces percolation
• Soil — frozen, saturated, compacted, or clay soils all reduce infiltration
• Weather — heavy/prolonged rain, or rapid snowmelt
• Seasonal variation — e.g. monsoon seasons, spring snowmelt
• High drainage density — many tributaries deliver water to the main channel quickly
• Lack of vegetation — reduced interception

Hard engineering = building physical structures or changing the channel directly. Soft engineering = working with natural processes rather than against them — increasingly popular because it's cheaper, more sustainable, and less environmentally damaging. Soft engineering is a form of mitigation — it aims to reduce damage rather than prevent flooding altogether.

Key Facts

• The Ganges is 2,510 km long, flowing through India and Bangladesh (becoming the River Padma), joining the Brahmaputra. Drainage basin = 1.2 million km², home to over 650 million people.
• The Brahmaputra is 3,969 km long, flowing through Tibet, India and Bangladesh. Drainage basin = 651,334 km².
• Both rivers have their source in the Himalayan Mountains and enter the sea at the Bay of Bengal.

Opportunities

• Water supply for major cities like New Delhi and Kolkata
• Agriculture & fishing — regular flooding deposits nutrient-rich alluvium, ideal for growing rice and jute; fishing provides food/jobs
• Culture — the Ganges is sacred to Hindus, worshipped as the goddess Ganga
• Tourism — holy pilgrimage sites (Haridwar, Allahabad, Varanasi) attract millions annually; rafting and river cruises
• Flat land for construction — extremely high population density on the floodplains
• Energy — several dams including the Tehri Dam (India's biggest hydroelectric plant)

Hazards

• Regular severe flooding, most recently May 2022
• In 1998: 75% of Bangladesh flooded, over 30 million people made homeless, over 1,000 deaths, 700,000 hectares of crops destroyed
• "Normal" flooding is actually vital for fertile soil and irrigation — but increasingly, flooding is unpredictable and extreme

Human Causes of Flooding

• Deforestation in upland areas → less interception/infiltration
• Climate change → increased Himalayan snow/ice melt, possibly more frequent/severe cyclones
• Urbanisation → rural-urban migration increases overland flow via impermeable surfaces
• Agriculture → increases overland flow and soil erosion, reducing river capacity

Natural Causes of Flooding

• Low-lying land in the Ganges delta — at/just above sea level
• Monsoon climate — heavy, prolonged rains for months
• Tropical cyclones bringing heavy rainfall
• Melting snow/ice from the Himalayas in spring

Management: The Flood Action Plan (FAP)

Bangladesh is an LEDC and lacks the money for large-scale flood defence on its own, so the Flood Action Plan was funded by the World Bank and MEDCs. It proposed: monitoring flood levels, building levees/embankments, 5,000 flood shelters, floodwater storage systems, better warning systems, dams, and reduced deforestation.

Why the FAP was NOT considered a success:

• Many parts (dams, storage areas) were never completed due to poor funding and corruption
• Realisation that some flooding is actually necessary for agriculture
• 8 million people were forced to move to make way for construction
• Changing the channel upstream made flooding worse downstream
• Government cannot afford ongoing maintenance costs

New suggestions focus on cheaper, less environmentally damaging soft-engineering-style solutions: better flood forecasting/warning systems and more well-stocked flood shelters.

4 Erosion Processes

Hydraulic action · Abrasion · Attrition · Corrosion (solution)

4 Transportation Processes

Traction · Saltation · Suspension · Solution

Waterfall Formation Sequence

Hard rock over soft rock → undercutting → plunge pool → overhang → collapse → abrasion increases → retreat upstream → gorge left behind

Ox-Bow Lake Formation Sequence

Erosion on outer bends narrows the neck → river breaks through the neck during a flood → deposition at entrance/exit slows flow → loop cut off → ox-bow lake formed

Hydrograph Golden Rule

Short lag time + steep rising limb = HIGH flood risk
Long lag time + gentle rising limb = LOW flood risk

Bradshaw Model — increases downstream

Discharge, channel width, channel depth, average velocity, load quantity

Bradshaw Model — decreases downstream

Load particle size, channel bed roughness, slope angle (gradient)

Case Study Numbers — Ganges/Brahmaputra 1998 Flood

75% of Bangladesh flooded · 30+ million made homeless · 1,000+ deaths · 700,000 hectares of crops destroyed

What Examiners Actually Look For

• Command words matter — "describe" wants what you see/what happens; "explain" wants the reasons/mechanisms why; "using Fig X" restricts your evidence source.
• Sequence questions (like waterfall or ox-bow lake formation) need to be in the correct chronological order — examiners follow a step-by-step mark scheme.
• Case study specifics — always try to include real numbers, place names, and dates from the Ganges/Brahmaputra case study rather than vague generalisations.
• Balance — questions on rivers often want both hazards AND opportunities, or both hard AND soft engineering — don't answer one-sided unless the question specifically asks for one side.