This document covers rivers from the CSEC Geography syllabus. It covers the water cycle, drainage basin, drainage density, drainage patterns, river processes, characteristics of rivers and river landforms

1. CSEC GEOGRAPHY
RIVERS/FLUVIAL MORPHOLOGY
Prepared by Oral Johnson
THE WATER CYCLE
Over 97% of the world’s water is stored in oceans and seas. These water bodies make up about
70% of the surface of the Earth. The remaining stores of water are:
 2.1% as ice and snow ( most of this is Antarctica and Greenland)
 0.6% as ground water (held in rocks)
 0.1% in rivers and lakes
 0.001% held in the atmosphere as water vapor and clouds (water droplets). This amounts
to about 10 days’ supply of average rainfall around the world. If evaporation and
transpiration from the Earth’s surface suddenly stopped the world would run short of
water very quickly!
The three main processes in the water cycle are evaporation, condensation and precipitation.
 Evaporation is the process in which liquid water is changed into water vapor which
is a gas. Evaporation takes place mainly from surface water. Energy is needed for it to
occur. The energy comes from the sun’s heat and from wind. Look how quickly water
evaporates from concrete or tarmac on a very hot day compared with a cooler day!
Evaporation is also faster on a windy day compared with a calm day. Evaporation
from water surfaces on land would not be enough to keep rivers and lakes full and
provide the human population with drinking water. Fortunately, large amounts of
water evaporated from the seas and oceans are carried by air masses onto land where
condensation and precipitation take place.
 Condensation is the process by which water vapour changes into water droplets. It
happens when water vapour is cooled to a level known as the dew point.
Condensation forms cloud and can also occur at the surface as fog.

2.  Precipitation occurs when water in any form falls from the atmosphere to the surface.
This is mainly as rain, snow, sleet, and hail. Thus, water is constantly recycled
between the sea, air and land.
The Drainage Basin System
When precipitation reaches the surface it can follow a number of different pathways.

3. A drainage basin is an area of land drained by a river and its tributaries. A drainage basin may
be described as an open system and it forms part of the hydrological or water cycle. If a drainage
basin is viewed as a system then its characteristics are:
 Inputs: how water is introduced into the drainage basin system. This is known as
precipitation.
 Stores: How water is stored or held for a period of time within the drainage basin system-
interception ( by vegetation), soil moisture, surface storage (lakes) and groundwater
 Transfers/flows : a process or flow of water from one place to another in the drainage
basin system- surface run-off/ overland flow, infiltration, percolation, through-flow,
throughfall and groundwater flow
 Outputs: How the water is released either back to the sea or back into the atmosphere-
evapotranspiration and river carrying water to the sea.
Elements of the drainage basin system

4. Precipitation forms the major input into the system. Precipitation occurs when water in any form
falls from the atmosphere to the surface. This is mainly as rain snow, sleet and hail. When
precipitation reaches the surface it can follow a number a different pathways. A small amount
falls directly into rivers as direct channel precipitation. The rest falls onto vegetation or the
ground. If heavy rain has fallen previously and all the air pockets in the soil (pore spaces) are full
of water, the soil is said to be saturated. Because the soil unable to take in any more water, the
rain flows on the surface under the influence of gravity. This is called surface runoff or
overland flow. Overland flow is common in urban areas where the surface is made up of
impermeable materials such as tarmac and concrete.
Rainwater can be intercepted by vegetation. Interception is the precipitation that is collected and
stored by vegetation. Interception is greatest in summer when trees and plants have most leaves.
Some rainwater is stored on leaves and then evaporated directly into the atmosphere. The
remaining intercepted water either drips to the ground from leaves and branches ( throughfall)
or trickles down tree trunks or plant stems (stemflow) to reach the ground.
The water that reaches the ground may then enter the soil as infiltration. Infiltration is the
passage of water into the soil. The maximum rate at which water can pass through the soil is
called its infiltration capacity and is expressed in mm/hr. Some of the water will flow laterally
through the soil (roughly parallel to the surface) as through flow.
If the soil is not saturated, rainwater will soak into it. If the rock below the soil is permeable
(allows water into it), the water continues to soak down deeper into the rock. This continuous
downward vertical movement of water into the rock is called percolation. The water eventually
comes to an impermeable rock (which does not allow water into it). The underground water level
builds up towards the surface from here to create a groundwater storage. Here all the pore spaces
are filled with water and is sometimes called a zone of saturation. The underground water does
not remain stationary but flows downslope under gravity (laterally). The upper boundary of
underground water or the upper level of the saturated material is known as the water table.
Water contained in rocks is known as ground water. The groundwater may then be slowly
transferred laterally as groundwater flow or base flow. The flow of groundwater is much slower
than runoff with speeds usually measured in centimeters per day, meters per year or centimeters
per year. Rock that holds ground water is known as an aquifer.

5. Water is lost from the system by evaporation and transpiration. Vegetation takes moisture
through its root system. It loses some of this into the air by transpiration. Surface water is also
lost by evaporation. The combination of the two is known as evapotranspiration. Once in the
river, water flows toward the sea and is lost from the drainage basin system.
Drainage basins
A drainage basin (or catchment area) is the area drained by a river and its tributaries. The
boundary of a drainage basin. The boundary of a drainage basin is called a watershed. This is a
ridge of high land that separates one drainage basin from another. The point where a river begins
is its source. A river reaches the sea at its mouth. A tributary joins the main river at a confluence.
A main river and all its tributaries form a river system. For example, the Mississippi and its
tributaries drain over one-third of the USA. Watershed in the Caribbean islands are typically
‘pear-shaped’: they are broad along the upstream divide and relatively narrow near the sea. In the
volcanic Windward Islands, watershed are steep and deeply dissected.
When small stream begin to flow they act under gravity, following the fastest route downslope.
Along the way water is added to them from tributaries, groundwater flow, throughflow and
overland flow.

6. The source of a river
The starting point of a river may be:
 An upland lake

7.  A melting glacier
 A spring in a boggyy upland area where the soil is saturated that recognizable surface
flow begins
 A spring at the boundary between permeable and impermeable rocks
Drainage Density
Some big rivers have a large number of tributaries so that no place in the drainage basin is very
far from a river or stream. Such an area is said to have a high drainage density. Where a main
river has few tributaries the drainage density is low. High drainage densities occur where:
 The bedrock is impermeable
 The soils are easily saturated
 Precipitation is high
 Slopes are steep
 Interception by vegetation is limited
Where drainage density is high, water reaches streams quickly. It moves rapidly through the
basin. Therefore the flood risk is high compared with basins with low drainage densities. In the
Windward Islands, for example, drainage density is relatively high due to the steep slopes and
the volcanic nature of the islands. However, in the coralline/limestone Leeward Islands, slopes
within watersheds tend to be gentle with relatively low drainage densities. In these watersheds on
limestone there is significant percolation into rocks which builds up groundwater reserves.

8. Drainage patterns
River systems often form a distinct pattern which is due to the structure of the rocks in the
drainage basin. The point at which one river or stream flows into another is known as the
confluence. Three distinctive patterns can be recognized, dendritic, trellis and radial.
Dendritic
This pattern looks like tree branches. This pattern develops in gently sloping basins with fairly
uniform rock type. The tributaries flow into the river at random forming a pattern like the veins
of a leaf. Examples of dendritic drainage are in the Caroni River in Trinidad and the Bruce Vale
river basin in Barbados. This type of drainage pattern is the most common in the Caribbean
region.
Trellis
This drainage pattern has an appearance of a rectangular grid. Rivers and their tributaries flows
almost perpendicular to each other with confluence of almost 90o. Trellis drainage takes place
where there is an alternate band of hard and soft rock at right angles to the main direction of the
slope. The main river has the power to cut though the hard rock while the tributaries cut though

9. the softer rock at more or less right angles. This pattern can be seen in some areas of western
Barbados and is also present in the Northern range of Trinidad.
The principal river which flows down the slope is called a consequent river (C) next the
tributaries which cut out the vales and which do not flow down the main slope are called
subsequent rivers (S).
Radial
Radial drainage patters happens on a dome or volcanic cone. This pattern resembles the spokes
of a wheel. The river radiates outwards in all directions from a high central point or dome. The
volcanic islands in the eastern Caribbean have radial drainage pattern. The southern half of St.
Lucia and Nevis are good examples of where radial drainage takes place.

10. Rivers: energy and processes
Energy is needed for transfers to occur. Around 95% of rivers energy is used to overcome
friction. The remaining 5 percent or so is used to erode the river channel and transport material
downstream. The amount of energy in a river is determined by:
 The amount of water in the river
 The speed at which it is flowing
Near the source, river channels are shallow and narrow. Also the bed is often strewn with
boulders and very uneven. High levels of friction upstream can cause considerable turbulence.
The water flows more slowly here than further downstream where the channel is wider, deeper
and less uneven. Although the river is unique, most show similar changes from source to mouth.
Three sections can be recognized along rivers: the upper course, middle course and the lower
course. Figure 3.3 show these sections which combine to form the long profile of the river. In
each section the main process taking place and the shape of the valley are different

11. From source to mouth the rivers
 Gradient decreases
 Depth increases
 Width increases
 Volume increases
 Velocity increases
 Discharge increases
The volume is the amount of water in the river. The velocity is the speed of the water. The
discharge is the volume times the velocity. Discharge is defines as the amount of water passing a
specific point at a given time. It is measured in cubic meters per second m3/ sec. The discharge
rate can also show big variations between dry and wet seasons. The Amazon has the world’s
highest discharge at around 219000m3/sec
Erosion
There are four processes of erosion
Hydraulic action

12. The pressure of water breaks away rock particles from the river bed and banks. The force of the
water hits river banks, and then pushes water into cracks. Air becomes compressed, pressure
increases and the riverbank may, in time collapse. Where velocity is high e.g. the outer bend of
meaner, hydraulic action can remove material from the banks which may lead to undercutting
and river bank collapse
Corrasion (or abrasion)
This is the wearing away of the bed and banks by the rivers load. This is the main type of erosion
in most rivers. Where depressions exist in the channel floor the river can cause pebbles to spin
around and turn hollows into potholes.
Attrition:
When pieces of rocks are broken away from the bed and banks the edges are usually sharp.
However, in swirling water rocks and stones collide with each other and with the bed and banks.
Over time the sharp edges become smooth and the pieces of rock become smaller in size.
Corrosion or Solution
Some rocks, such as limestone, dissolve slowly in river water which contains dissolved carbon
dioxide from the air. This process is common where carbonate rocks such as limestone and chalk
are evident in a channel

13. Most erosion occurs when discharge is high and rivers are said to be in flood. Erosion acts on the
landscape in three ways:
 Near its source a river cuts down into its bed, deepening the valley. This is vertical
erosion
 In the middle and lower courses sideward or lateral erosion is most important. This
widens the valley
 Headward erosion takes place at the source. it causes the valley to grow very slowly
upstream.
Transportation
The load is the total amount of material being carried by the river. There are four processes by
which a river can transport its load: traction, saltation, suspension and solution.
Traction
Traction occurs when the largest cobbles and boulders roll or slide along the bed of the river. The
largest of these may only be moved during times of extreme flood.
Saltation
Saltation occurs when pebbles, sand and gravel are temporarily lifted up by the current and
bounced along the bed in a hopping motion. They are too heavy to carry in suspension
Suspension
Suspension is when material made up of very fine particles such as clay and silt is lifted as the
result of turbulence and transported by the river. Faster-flowing, turbulent rivers carry more
suspended material. The material held in suspension usually forms the greatest part of the total
load; it increases in amount towards the river’s mouth, giving the water its brown or black color.
Solution

14. Solution is when dissolved material is carried by a river. Water flowing within a river channel
contains acids (e.g. carbonic acid from precipitation). If the bedrock is soluble, like limestone, it
is constantly dissolved in the running water and removed in solution.
Deposition
When the velocity of a river begins to fall, it has less energy and so no longer has the
competence or capacity to carry all its load.
Deposition occurs when:
 Discharge is reduced following a period of low precipitation
 Velocity is lessened on entering the sea or a lake (resulting in a delta)
 The gradient decreases significantly
 The current slows on the inside of a meander
 The river overflow its banks so that the velocity outside the channel is reduced.
When a river loses energy the first part of the load to be deposited is the large, heavy material
known as the bedload. Lighter material is carried further. The gravel, sand and silt deposited is
called alluvium. This is spread over the flood plain. The solution load- the lightest suspended
particles which include clay- is carried out to sea. Some rivers get their name from the colour of
the silt that they carry, for example the Yellow River in China.
THE UPPER COURSE
The many features/landforms in the upper course of a river are:

15.  V-Shaped Valleys
 Interlocking spurs
 Potholes
 Rapids
 Waterfalls and Gorges
Rapids
Sometimes very thin alternating bands of hard and soft rock cross the course of a river. The
softer rocks wear away/erodes faster than the harder rocks. This is known as differential erosion.
The softer rocks are then on a lower level compared to the harder rocks. This creates an uneven
river bed and the river falling in a series of steps along the bands of the hard rock to form a zone
of turbulent water known as rapids

16. Potholes
Where the bed is very uneven, pebbles carried by fast, swirling water can become temporarily
trapped by obstacles in the bed. The swirling currents cause the pebbles to rotate in a circular
movement, eroding circular depressions in the bed (abrasion). These are potholes. They general
increase in size only very slowly.
Interlocking Spurs
The river wind its way (meanders) around obstacles of hard rock. Erosion is concentrated on the
outside banks of these small meanders. This eventually creates spurs which alternate on each
side of the river, so they interlock. A spur is a ridge of high land which project towards a river at
right angles, decreasing in height towards the river.

17. Waterfalls
Waterfalls are the most spectacular feature of the upper course, but they can also be found in the
middle course. This occur when there is a sudden change in the course of the river. This may be
due differences in rock hardness along the valley or for several other reasons:
 A steep drop at the edge of a plateau has been formed by uplift of the land
 A lava flow crosses the path of the river which pours over its edge as a waterfall
Waterfalls can form when the rock is horizontal, vertical or dipping upstream. The lower softer
rock is eroded more quickly causing the hard rock to overhang. The undercutting is caused by
corrosion and hydraulic action, with water swirling around in the plunge pool and spray hitting
the soft rock as the water plunges over the waterfall. The overhang steadily becomes larger until
finally it collapses. The rocks that crash down into the plunge pool are swirled around by the

18. currents. This increases erosion and makes the plunge pool deeper. The rocks in the plunge pool
are eroded mainly by attrition.

19. This process, beginning with the collapse of a layer of hard rock, is repeated multiple times. As a
result the waterfall retreats upstream, leaving a steep-sided gorge.

21. V-shaped valleys
In the upper course much of the rivers energy is needed to overcome friction. The rest is used to
transport the load. The river in this section contains large boulders which can erode the bed
rapidly when the river is in flood. This results in the river cutting downwards into its bed, a
process known as vertical erosion. It forms steep V-shaped valley. Soil and loose rock on the
valley sides are washed down the steep slopes into the river. This adds to the load

22. THE MIDDLE AND LOWER COURSES
In the middle course of the river profile the gradient is much less than in the upper course. The
volume of the water increases, with more tributaries joining the main river. More water is added
by through flow and, if the rock is permeable, by groundwater flow. Lateral erosion takes over
from vertical erosion as the most important process. Channel is much wider.
The lower course is nearest to the sea. The gradient is gentler. This section is characterized by
an even greater volume of water and higher velocity. Deposition is now much more important
than erosion. Meanders are more pronounced. The valley has the shape of an Open V in cross
section.
River cliffs and Point bars
Meanders occur in the middle course and are the result of erosion and deposition processes
operating in the river. The current is fastest and most powerful on the outside of the meander.
Within the river the fastest current is on the outside of the bend and the slowest current on the
inside of the bend. The concave or outside bend is much deeper so less friction and a higher
velocity.
Erosion is relatively rapid and the outside bank (concave bank) is undercut. Eventually the bank
collapses and retreats, causing the meander to spread across the valley. If the meander has
already reached the side of the valley, erosion on the outside bend may create a very steep slope
or river cliff. The current on the inside (convex bank) of the meander is much slower. As the
river slows it drops some of its load and deposition occurs. This builds up to form a gently
sloping slip-off slope, or point bar. Thus the water is shallow on the inside of the meander and
deep on the outside.
Diagram showing processes operationg on the inside and outside banks of a meander

23. Diagram showing erosional and depositional actions of a river as it flows around a
meander

24. Diagram showing a cross section of a meander
Meander migration
Because of the power of lateral erosion in the middle course, meanders slowly change their shape
and position. As they push sideways they widen the valley. But they also move or migrate
downstream. This erodes the interlocking spurs, giving a much more open valley compared with
that in the upper course.
Diagram showing how meanders migrate downstream

25. Flood plain
A flood plain is the area of almost flat land on both sides of a river. It is formed by the movement
of meanders explained above. Meanders are more pronounced in the lower course. The
floodplain is constantly build up by flooded alluvial deposits. After each flood new layers of
alluvium are formed. This gradually builds up the height of the flood plain. The flood plain is
much more pronounced in the lower course as the river develops a very wide and flat valley
floor.
Diagram below show the flood plain

26. Levees
When discharge is high the river is able to transport a large amount of material in suspension. At
times of exceptionally high discharge the river will overflow its banks and flood the low-lying
land around it. The sudden increase in friction as the river water surges across the flood plain
reduces velocity and causes the material carried in suspension to be deposited on the flood plain.
The heaviest or coarsest material will be dropped nearest to the river. This can form natural
embankments alongside the river called Levees. Levees are sometimes strengthened by engineers
to control flooding.
The lightest material is carried towards the valley sides. Each time there is a flood a new layer of
alluvium is formed. This gradually builds up the height of the flood plain.
Diagram showing the development of a Levee

27. Meander necks and Ox-bow lakes
As a river flows from its middle course to its lower course, meanders become even more
pronounced and the valley becomes wider and flatter. Oxbow lakes are shallow, crescent shaped
lakes found on the flood plain and are the remains of a former course of a river.
An oxbow lake develops when a meander becomes so pronounced that only a narrow neck of
land separates the two ends of the meander. Erosion continues to cut into outside bends of the
meander and a meander neck is form. With continuous erosion the meander neck becomes
narrower and narrower. Eventually, when the river is in flood and discharge is high, it may cut
right across the meander neck following a more direct route and shortening its course. For a
while water will flow along both the old meander route and along the new straight course.
However, because the current slows down at the entry and exit points of the meander, deposition
will occur. After a time the meander will be cut off from the new straight course, leaving behind
an Ox-bow lake. When cut-off occurs the only sources of water for the ox-bow lake will be

28. precipitation and flooding from the river. If evaporation is greater than these additions of water
the ox-bow lake will eventually dry up
Diagram the development of an oxbow lake
Braiding or braided rivers
Braiding is when a river divides for various distances into two or more channels. The channels
are separated by islands of sediment. Braiding occurs when:
 A river carries a very large load, particularly of sand and gravels, in relation to its
velocity.
 The discharge changes rapidly from season to season.
During a dry period or by increase load the river may not be capable of carrying its full load, and
so a great deal of deposition takes place on the bed of the river and the river channels become

29. choked. This give rises to sandbanks and small islands in the bed. The river is forced to split up
into several channels (known as braided and its way through its own deposits. This is known as a
braided river.
Diagram showing a braided channel
Deltas
Deltas are formed by the deposition of sediments at the mouth of a river as it enters a sea or lake.
Deltas only form under certain conditions and most rivers do not end in a delta.
Large rivers in the lower course have the energy to transport a great deal of material in
suspension. As a river enters the sea its speed of flow is reduced, sometimes very suddenly,
causing deposition. The coarsest materials (like sand) is deposited first because of a greater
weight, while finer material (like clay) are carried out further into the sea. Thus layers of
different sediments are built up on the sea floor until they reach the surface. This happens first at

30. the landward end of a delta, extending gradually out to sea. This huge platform of river sediment
is called delta.
A delta is therefore an accumulation of sediments at the mouth of the river which has been
formed by deposition of successive layers of sediments. When a river flows into a delta it has to
flow over its own deposit. This causes the river to braid. Each channel in a delta is called a
distributary.
The two main conditions required for deltas to form are:
 The river must have a large amount of sediment
 Coastal currents and waves must not be so strong as to remove sediment faster than the
river can deposit it- if this happens the sediments are spread over a much wider area of
sea floor beyond the mouth of the river.
There are three main types of delta:
 Fan-shaped or arcuate: This is triangular in shape with a slightly rounded outer margin.
The Nile and Yallahs River in Jamaica are examples
 Bird’s foot or digitate: distributaries flanked by sediment extend out to sea like the
claws of a bird’s foot. The Mississippi delta is a good example.
 Estuarine or cuspate: the delta forms an islands in the river’s mouth. The Amazon, and
Essequibo river in Guiana are examples
Major deltas are not common in the Caribbean. This is because it usually takes a large river to
build out into the sea.
Diagram showing the structure of delta