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RIVERS
1 Water cycle, drainage basins, energy and processes

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THE WATER CYCLE

3. THE WATER CYCLE
 http://www.epa.gov/ogwdw/kids/flash/flash_watercy
cle.html
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4. THE WATER CYCLE
 The Water Cycle (also known as the hydrologic
cycle) is the journey water takes as it circulates
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from the land to the sky and back again
 The Sun's heat provides energy to evaporate water
from the Earth's surface (oceans, lakes, etc.).
 Plants also lose water to the air (this is called
transpiration).
 The water vapor eventually condenses, forming tiny
droplets in clouds.
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5. THE WATER CYCLE
 When the clouds meet cool air over
land, precipitation (rain, sleet, or snow) is
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triggered, and water returns to the land (or sea).
Some of the precipitation soaks into the ground.
 Some of the underground water is trapped between
rock or clay layers; this is called groundwater. But
most of the water flows downhill as runoff (above
ground or underground), eventually returning to the
seas as slightly salty water.
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6. THE WATER CYCLE
 More than 97% of the world’s water is stored in
oceans and seas.
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 These water bodies make up about 70% of the
surface of the earth.
 The rest of the water is stored as:
 2.1% as ice and snow
 0.6% as groundwater
 0.1% in rivers and lake
 0.001% in the atmosphere as water vapor and clouds
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THE WATER CYCLE

8. PROCESSES IN THE WATER CYCLE
 Evaporation
 Condensation
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 Precipitation
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EVAPORATION

10. EVAPORATION
 This is the process by which water changes from a
liquid into water vapor which is a gas
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 Approximately 80% of all evaporation is from the
oceans, with the remaining 20% coming from inland
water and vegetation.
 Winds transport the evaporated water around the
globe, influencing the humidity of the air throughout
the world
 The energy to carryout this process comes from the
sun’s heat and the wind.
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CONDENSATION

12. CONDENSATION
 Condensation is the change of water from its
gaseous form (water vapor) into liquid water.
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 Condensation generally occurs in the atmosphere
when warm air rises, cools and looses its capacity
to hold water vapor.
 Excess water vapor condenses to form cloud
droplets.
 The upward motions that generate clouds can be
produced by convection in unstable air.
 The level at which water vapor cools and changes
to water is called the dew point.
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PRECIPITATION

14. HOW WATER FLOWS ON REACHING THE
SURFACE
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15. WATER ON THE SURFACE
 When water reaches the surface it is not stagnant.
It follows a number of paths that eventually ends up
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in the sea.
 A small amount goes directly into rivers. The rest
falls on the vegetation or the ground.
 When it rains and the ground becomes over
saturated the water falls on the surface and run off;
this is called surface runoff or overland flow.
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16. WATER ON THE SURFACE
 If the soil is not soaked, the rain water will soak
through.
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 If the rock below the ground is permeable it will
allow water to soak through until it reaches
impermeable rock.
 After that it begins to build up towards the surface
after which gravity causes it to flow down slope.
 The upper level of the groundwater is called the
water table. The water in the rocks is know as
groundwater while the water moving through the
rocks is called groundwater flow.
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17. WATER ON THE SURFACE
 Water flowing through the soil is called infiltration.
 Percolation is the downward movement of water
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within a soil or rock.
 Rainwater can be intercepted by vegetation
 Vegetation takes in water through the roots.
 It looses some of it through transpiration. Surface
water is also lost by evaporation. The two
processes together is known as evapotranspiration.
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18. WATER ON THE SURFACE
 Where water enters the system these are known as
Inputs.
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 Where water is held are called stores.
 Where water flows through the system is known as
transfers.
 Where water is lost to the system are called
outputs.
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19. WHAT IS A DRAINAGE BASIN
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20. DRAINAGE BASIN
 This is the area of land drained by a river system (a
river and its tributaries). It includes the surface run-
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off in the water cycle, as well as the water table.
 Drainage basins are separated by watersheds.
 The drainage basin system is demarked by a
watershed, this is the upper limit around the
drainage basin that separates one drainage basin
from another.
 This is a ridge of island that separates drainage
basins.
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21. DRAINAGE BASIN
 A tributary joins the main river a t a confluence
 A main river and all its tributaries form a river
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system
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22. DRAINAGE BASIN
 The Mississippi drainage
basin is one of the largest in
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the world. It drains over
1/3 of the USA.
 Watersheds in the
Caribbean are pear-shaped;
broad along the upstream
divide and relatively narrow
near the sea.
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24. SOURCE OF A RIVER
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25. SOURCE OF A RIVER
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26. SOURCE OF A RIVER
 The source of a river or stream is the original point
from which the river flows.
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 The source of a river or stream may be a lake, a
marsh, a spring or a glacier. This is where the
stream starts.
 The farthest stream is called the headstream or
headwater. The source is the farthest point of the
river stream from its estuary or its confluence with
another river or stream.
 Where a river is fed by more than one source, it is
usual to regard the highest as its source. The
source is where a river begins, and the mouth is
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27. DRAINAGE DENSITY
 Drainage density is defined as the total length of
the streams in a drainage basin divided by the area
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of the basin.
 Climate affects drainage density both directly and
indirectly. The amount and the type of precipitation
influence directly the quantity and character of
runoff
 When a river has few tributaries the density is low
 When a river has a lot of tributaries the density is
high.
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28. HIGH DRAINAGE DENSITY
 High drainage densities occur where:
 The bedrock is impermeable
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 The soils are easily saturated
 Precipitation is high
 Slopes are steep
 Interception by vegetation is limited
 In high drainage density water moves through the
system rapidly and reaches the stream quickly
 The risk of flood is very high
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29. DRAINAGE PATTERNS
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30. DRAINAGE PATTERNS
 Distinctive patterns are acquired by stream
networks in consequence of adjustment to geologic
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structure.
 River streams forms a distinctive pattern depending
on the type of rock on which it flows.
 Drainage basins are commonly viewed by scientists
as being open systems
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31. DENDRITIC DRAINAGE PATTERN
 Dendritic resembles the branches of a tree.
 This pattern develops on gentle slopes that has a
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uniformed rock type.
 Streams flow into each other almost at random.
 This is the most common in
the Caribbean, e.g. Caroni
river in northern Trinidad
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32. TRELLISED DRAINAGE PATTERN
 Trellised drainage pattern resembles a rectangular
grid.
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 This occurs when there are
alternate bands of hard and soft
rock at right angles to the main
direction of slope.
 The main river can cut through
both the hard rock and soft rock
however the tributaries mainly
form in the soft rock and joins
the river at right angles.
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 E.g. Western Barbados

33. RADIAL DRAINAGE PATTERN
 Radial drainage pattern resembles the spokes of a
wheel
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 The river radiates outwards in all directions from a
high central point.
 E.g. the volcanic islands in the eastern Caribbean
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35. RIVER PROCESSES
 Energy is needed for transfers to occur. 95% of the
rivers energy is used to overcome friction. The
remaining 5 is used to erode the river channel, transport
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and deposit the load.
 Near the source the river channels are shallow and
narrow.
 The bed is usually strewn and uneven with boulders
 The water flows more slowly upstream than
downstream.
 Processes of the river:
 Erosion
 Transportation
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 Deposition

36. RIVER PROCESSES
 Every river is unique but most show similar
changes from source to mouth.
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 The river has three distinct sections:
 The upper course
 The middle course
 Lower course
 From source to mouth the rivers;
 Gradient decreases
 Depth increases
 Width increases
 Volume increases
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 Discharge increase

37. RIVER PROCESSES
 The volume is the amount of water in the river
 The velocity is the speed of the water
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 The discharge is the volume times the velocity
 The discharge is the amount of water passing a
specific point at a given time
 The Caribbean rivers are small and does not carry
a heavy discharge
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38. EROSION
 Rivers are usually the main causes of erosion and
deposition
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 A river may erode in any of the following ways
 Attrition
 Corrasion
 Corrosion (solution)
 Hydraulic action
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39. ATTRITION
 This occurs when boulders and rocks are knocked
against each other and gradually reduce this stage.
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40. CORRASION
 Corrasion occurs when smaller material gets
thrown against the riverbanks and erode them.
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41. CORROSION
 Corrosion occurs when acids in the river dissolve
rocks, such as limestone.
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42. HYDRAULIC ACTION
 Hydraulic action is when the sheer force of the river
dislodges rocks from the riverbanks and bed
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43. EROSION
 Most erosions occurs when discharge is high and
rivers are in flood.
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 Erosion acts on the landscape in three ways:
 Vertical erosion
 Lateral erosion
 Headward erosion
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44. TRANSPORTATION
 The river transport what is called its load
 There are four processes by which a river can
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transport its load:
 Traction
 Saltation
 Suspension
 Solution
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46. DEPOSITION
 A river deposits its load when the speed is checked
due to its gentle gradient such as in a flat or gently
sloping plain :
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 A river has three courses/sections/stages
 Upper (youthful stage)
 Middle (mature stage)
 Lower (old age stage)
 The gradient decreases significantly
 Discharge falls during a dry period
 The current slows down on the inside of a meander
 The river enters a lake or the sea.
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47. DEPOSITION
 When a river loses energy the first part of the load
that is deposited is the large, heavy material called
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the bedload.
 The lighter material is carried farther
 The gravel, sand and silt deposited is called
alluvium.
 The lightest material is carried in suspension which
is clay
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48. RIVER LANDFORMS
 The upper course
 V-shaped valley
 Steep gradient
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 Interlocking spurs
 Potholes
 Waterfalls, rapids and gorges
 The middle course
 River cliffs
 Slip-off slopes
 Confluence
 Flood plains and levees
 Lower course
 Meanders
 Braiding
 Ox-bow lakes 48
 Deltas

49. V-SHAPED VALLEY AND STEEP GRADIENT
 Most of the energy in the upper course is used to
overcome friction
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 There are large boulders
in this section which erode
the bed rapidly during flood.
 The river eventually cuts
downward in the bed in
vertical erosion
 This creates the
V-shaped valley
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50. INTERLOCKING SPURS
 This occurs when the river winds its way around
obstacles of hard rock.
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 Erosion is concentrated on the outside banks. This
creates spurs which alternate on each side of the
river, so they interlock. A spur is a ridge or high
land.
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51. POTHOLES
 Potholes are created in areas where the river bed is
uneven. Pebbles may get stuck temporarily; the
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swirling currents cause the stone to move in a
circular motion which creates the potholes.
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52. WATERFALLS
 A waterfall is found in the upper course but can also
be found in the middle course.
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 It occurs when there is a sudden change in the
course of the river usually caused by differences in
rock hardness. Other causes may be:
 A fault line has created an escarpment over which the
river flows
 Glaciation has left a tributary valley hanging high above
the main valley
 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 52
over its edge as a waterfall.

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54. RAPIDS
 Rapids are created alternate bands of hard rock
and soft rock cross the course of a river.
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 This makes the bed uneven and cause zones of
turbulence.
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55. GORGES
 A gorge is formed after a plunge pool is created
and the waterfall retreats upstream. The steep
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sided section that remains behind is called a
gorge.
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56. RIVER CLIFFS
 A river cliff is created when a bank undercut
collapse and retreats. Then erosion on the outside
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bend creates a very steep slope.
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57. SLIP OFF SLOPES
 When a river cliff is created the current on the other
side of the meander is slower.
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 When it slows it drops its load which eventually
builds up to form a gently sloping slip-off.
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58. CONFLUENCE
 A confluence of a river is a meeting of two or more
bodies of water; A flowing together of two or more
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streams.
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59. FLOOD PLAINS AND LEVEES
o A flood plain is the area of almost flat land on both sides
of the river.
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o When the river deposits the coarse material it forms
natural embankments, these are called levees.
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60. MEANDERS
 A meander is a curve or bend in a river’s course
swinging from side to side in wide loops.
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61. OX BOW LAKE
 This is formed when the neck of a meander is cut
across by the river which then takes a straight path
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leaving a cut-off.
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62. DELTAS
 A delta is formed where the mouth of a river drowns
and forms an estuary salt water mixes with fresh
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water. The river channel widens and forms a V-
shaped pattern and silt accumulates at the mouth.
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