This document defines key terms related to rivers and their drainage basins, including drainage basin, watershed, source, mouth, tributary, and confluence. It describes the hydrological cycle within drainage basins and factors that influence river flow, such as inputs, outputs, evapotranspiration, interception, surface runoff, infiltration, throughflow, percolation, and groundwater flow. It also discusses flood hydrographs, methods of erosion and transportation, changes that occur from the upper to lower course, and causes and impacts of flooding.

1. EVERYTHING YOU NEED TO
KNOW ABOUT RIVERS

2. ESSENTIAL DEFINITIONS (1)
• DRAINAGE BASIN AREA OF LAND DRAINED BY A RIVER AND ITS TRIBUTARIES.
• WATERSHED BOUNDARY BETWEEN TWO DRAINAGE BASINS.
• SOURCE WHERE A RIVER STARTS.
• MOUTH WHERE A RIVER ENDS (USUALLY AT THE SEA OR A LAKE).
• TRIBUTARY A SMALLER STREAM/RIVER WHICH FLOWS INTO A LARGER ONE.
• CONFLUENCE THE POINT WHERE TWO RIVERS MEET.

3. ESSENTIAL DEFINITIONS (2)
• INPUTS WAYS IN WHICH WATER ENTERS THE SYSTEM THROUGH PRECIPITATION (RAIN,
SNOW ETC.)
• OUTPUTS WAYS IN WHICH WATER IS LOST TO THE SYSTEM, EITHER WHEN THE RIVERS
CARRY IT TO THE SEA OR THROUGH EVAPOTRANSPIRATION.
• EVAPOTRANSPIRATION THE LOSS OF MOISTURE DIRECTLY FROM WATER SURFACES SUCH
AS RIVERS AND LAKES (EVAPORATION) OR FROM VEGETATION (TRANSPIRATION).
• INTERCEPTION WHEN TREES, OTHER PLANTS ETC. ‘INTERRUPT’ THE FLOW OF WATER TO
THE GROUND.

4. ESSENTIAL DEFINITIONS (3)
• SURFACE RUNOFF WATER FLOWING DIRECTLY OVERLAND TO THE RIVER (SOMETIMES CALLED OVERLAND
FLOW).
• INFILTRATION WATER PASSING THROUGH THE EARTH SURFACE INTHE DRAINAGE BASIN INTO THE SOIL LAYER.
• THROUGHFLOW THE MOVEMENT OF WATER THROUGH THE SOIL TOWARDS THE RIVER CHANNEL.
• PERCOLATION THE MOVEMENT OF WATER FROM THE SOIL LAYER TO THE ROCK LAYER.
• GROUNDWATER FLOW (SOMETIMES CALLED BASE FLOW) THE MOVEMENT OF WATER THROUGH THE ROCK
LAYER TOWARDS THE RIVER CHANNEL.

5. THE DRAINAGE BASIN HYDROLOGICAL CYCLE

6. FLOOD HYDROGRAPHS (1)
• A FLOOD OR STORM HYDROGRAPH SHOWS HOW A RIVER RESPONDS TO ONE PARTICULAR PERIOD OF HEAVY RAINFALL.
• ‘LAG TIME’ IS THE TIME BETWEEN THE PEAK RAINFALL AND THE PEAK DISCHARGE OF THE RIVER.
• LAG TIMES CAN VARY DEPENDING ON THE RELIEF OF THE DRAINAGE BASIN, THE UNDERLYING ROCK TYPE, THE
VEGETATION, THE LAND USE AND THE DRAINAGE DENSITY.
• A RIVER REGIME SHOWS HOW THE DISCHARGE OF A RIVER VARIES OVER A LONGER PERIOD OF TIME — USUALLY A YEAR.
• BE AWARE OF THE FACTORS THAT CHANGE THE SPEED, AND AMOUNT, OF PRECIPITATION THAT REACHES A RIVER. THINK
OF A RIVER AS SYSTEM WITH:
• INPUTS
• STORES
• TRANSFERS
• OUTPUTS

7. FLOOD
HYDROGRAPHS (2)
THIS IS A ‘FLASHY’ HYDROGRAPH
WITH A SHORT LAG TIME SHOWING
THAT THE RIVER HAS RISEN QUICKLY
IN RESPONSE TO HEAVY RAINFALL.
ON OTHER OCCASIONS, THE LAG
TIME MAY BE LONGER, RESULTING IN
A ‘SUBDUED HYDROGRAPH’

8. METHODS OF EROSION
• HYDRAULIC ACTION OCCURS WHEN THE SHEER FORCE OF THE WATER DISLODGES
PARTICLES FROM THE RIVER BEDS AND BANKS.
• ABRASION (ALSO KNOWN AS CORRASION) OCCURS WHEN SMALLER MATERIAL,
CARRIED IN SUSPENSION, RUBS AGAINST THE BANKS OF THE RIVER, WEARING THEM
AWAY WITH A SAND-PAPERING ACTION.
• ATTRITION OCCURS WHEN BOULDERS AND OTHER MATERIALS BEING TRANSPORTED
BY THE RIVER COLLIDE AND BREAK UP INTO SMOOTHER, SMALLER PIECES.
• CORROSION (ALSO KNOWN AS SOLUTION) OCCURS WHEN ACIDS IN THE WATER
DISSOLVE ROCKS SUCH AS LIMESTONE, WHICH FORM THE BANKS AND BED OF A
RIVER.

9. METHODS OF TRANSPORTATION
• RIVERS PICK UP AND CARRY MATERIAL AS THEY FLOW DOWNSTREAM. A RIVER MAY
TRANSPORT MATERIAL IN
• FOUR DIFFERENT WAYS:
• TRACTION LARGE BOULDERS AND ROCKS ARE ROLLED ALONG THE RIVER BED.
• SALTATION SMALL PEBBLES AND STONES ARE BOUNCED ALONG THE RIVER BED.
• SUSPENSION FINE LIGHT MATERIAL IS CARRIED ALONG IN THE WATER.
• SOLUTION MINERALS ARE DISSOLVED IN THE WATER.
• DEPOSITION OCCURS WHEN A RIVER LACKS THE ENERGY TO CARRY ITS LOAD —
PERHAPS AFTER A DRY
• SPELL OR ON THE INSIDE OF A MEANDER WHERE VELOCITY IS LOWER OR WHERE THE
RIVER ENTERS THE SEA.

11. Logarithmic
scale (each
cycle is a
ten-fold
increase)
Fine particles are
cohesive and
difficult to entrain
Critical erosion
velocity (CEV). At
this speed river
will entrain
particles
Sand is the first
size to be
entrained
Critical settling velocity. At
this speed the river begins to
deposit grains of differing
sizes, coarsest first.Very fine-grained
clays and muds
are suspended in
virtually still water
Logarithmic
scale
showing
wide range
of particle
size

12. AS YOU GO DOWNSTREAM...
• THERE IS AN INCREASE IN:
• VELOCITY
• DISCHARGE
• LOAD AMOUNT
• CROSS-SECTIONAL AREA
• EFFICIENCY.
• THERE IS A DECREASE IN:
• GRADIENT
• ROUGHNESS
• FRICTION
• TURBULENCE
• LOAD SIZE

13. THE UPPER COURSE (1)

14. THE UPPER COURSE (2)
IN THIS STAGE:
• THE RIVER IS HIGH ABOVE SEA LEVEL AND HAS LOTS OF POTENTIAL ENERGY WHICH
IT USES LARGELY IN VERTICAL EROSION.
• THE RIVER VALLEY IS OFTEN V-SHAPED WITH INTERLOCKING SPURS.
• THE CHANNEL IS NARROW AND SHALLOW WITH A LARGE, ANGULAR BEDLOAD.
• THE CHANNEL HAS A STEEP GRADIENT, ESPECIALLY AT RAPIDS AND WATERFALLS,
WHERE THE VELOCITY OF THE WATER IS RELATIVELY HIGH.
• HOWEVER, THE OVERALL VELOCITY IS LOW AS SO MUCH ENERGY (UP TO 95%) IS
LOST DUE TO FRICTION WITH THE BANKS AND BEDS.
• THE WATER IS OFTEN VERY CLEAR AS THERE HAS BEEN LITTLE ABRASION AND
ATTRITION — SO THE SUSPENDED LOAD IS VERY SMALL.

15. THE MIDDLE COURSE (1)

16. THE MIDDLE COURSE (2)
• AS A RIVER FLOWS DOWNSTREAM, THE GRADIENT BECOMES LESS STEEP AND
LATERAL (SIDEWAYS) EROSION BECOMES MORE IMPORTANT. THE RIVER THEN
STARTS TO MEANDER.
• THE FLOW IS ALWAYS FASTER ON THE OUTSIDE BEND OF A MEANDER. THIS
MEANS THAT THE WATER HAS MORE POWER TO ERODE ITS BED AND SO IT IS
ALSO DEEPER HERE.
• MEANDER MIGRATION STARTS.
• THE WATER WILL ALSO ERODE/UNDERCUT THE RIVER BANKS TO FORM A STEEP-
SIDED RIVER CLIFF.
• ON THE INSIDE BEND, THE WATER FLOWS MORE SLOWLY; THE WATER IS

17. THE LOWER COURSE
• IN THE LOWER COURSE, THE RIVER BECOMES WIDER AND DEEPER.
• THE VELOCITY ALSO INCREASES BECAUSE THERE IS LESS FRICTION WITH THE
BANKS AND BED.
• THE BEDLOAD IS SMALLER AND MORE ROUNDED AS A RESULT OF THE PROCESS
OF ATTRITION.
• AT THIS STAGE, THE RIVER WILL BE CARRYING A LARGE LOAD OF SUSPENDED
MATERIAL (BROUGHT FROM FURTHER UPSTREAM) AND SO DEPOSITION BECOMES
THE MOST IMPORTANT PROCESS.

18. THE LOWER COURSE

20. CAUSES OF FLOODING
Causes of
flooding
Human
Physical
Urban
growth
Deforestati
on
Poor
agricultur
al
practices
Population
growth
Saturated
ground
Impermeabl
e rock
Snowmelt
Prolonged
rainfall
Intense
rainfallAll linked to case
studies

22. IMPACTS OF FLOODING
Impacts of
flooding
Human,
social and
economic
Physical/
environmental
Disease
Damage to
property/
dispossession
Casualties
Recharge
groundwate
r stores
Scale of
flood
Meander
cutoff/
levee
breach
Depositio
n of silt
All linked to
case studies
Loss of
crops/farm
animals, food
shortages
Overall
cost/
Insurance
Infrastructure/
business
damage