River Basin Processes
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Transcript of River Basin Processes
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River Basins
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The Hydrological (Water) Cycle97% of the worlds water iscontained in the oceans.
2% of the worlds water is stored
as ice and snow.That leaves just 1% is either freshwater on land or water vapour
As the amount of fresh water islimited, it has to be recycled over andover again. This is known as thehydrological cycle.
As no water is added or lost from
the hydrological cycle it is called aclosed system.
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The Drainage Basin System
The drainage basin forms part of the hydrologicalcycle, but unlike the hydrological cycle it is anopen system. This is because and open system has:
INPUTS where water enters the system throughprecipitation (rain and snow) OUTPUTS where water is lost to the system either by
rivers carrying it to the sea or throughevapotranspiration. (Evapotranspiration is the loss ofmoisture directly from rivers or lakes evaporation, or
from vegetation transpiration.
Definition:A drainage basin is the land area or catchmentarea drained by a single river and its tributaries. An
imaginary line called the watershed separates onedrainage basin from another. The watershed usuallyfollows a ridge of high land; any water falling on the otherside flows into an ad acent draina e basin.
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The Drainage Basin System
Within the system are storesand transfers(flows)
STORES are places where water is held, eg: in lakes, on the surface, in soil
and in rocks underground. TRANSFERS are processes by which water flows or moves through the
system, eg: infiltration, surface runoff, throughflow, percolation.
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Diagram of a typical drainage system.
Source River Basin
Confluence
Tributary
WatershedRiver
Channel Mouthof river Sea or Lake
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From its source, a river flows downhill. This is because of the Earth's gravity. Small trickles of
water join up to form a stream. The amount of water in the stream increases steadily as morestreams, called tributaries, join it. Eventually the stream becomes big enough to be a river.
A drainage basin is all the land that supplies ariver and its tributaries with water. If you couldlook down on a river from above, you would seethat it branches. This is called a drainagepattern. The shape of the pattern depends onrocks, soil, climate and the changes made to theriver. Radial drainage happens when streams
flow down from a central high point, such as amountain top. Other rivers, such as the Amazon,form a pattern like the branches of a tree. This
is called dendritic drainage.
Rivers and their tributaries can form many
other types of drainage patterns. Paralledrainagehappens when streams flow in valleysthat are parallel to one another. This mightbe.because movements in the Earth, millionsof years ago, made the rocks 'fold' intoparallel lines. Trellis drainage is commonwhere massive layers of sedimentary rockhave 'slipped'. Streams flow in channels thatare parallel to each other, and tributaries join
the streams at right angles.
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The force of a river depends on 2 things - the energy and the volume of the river.Energy = Speed + volumeThe steeper it is the faster it flows.Speed can be reduced by friction and turbulence, so a lot depends on the natureof the river bed. This is related to two things:
the material it is made ofthe cross sectional shape of the river bed.
The most efficient shape is where the depth is equal to half the width W = 2D
http://library.thinkquest.org/28022/erosion/index.html -
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A wide shallow channelmeans little erosion, butoften plenty of deposition
because of increasedfriction which slows downthe flow.
A narrow, deep channelincreases the speed ofthe water and so theamount of erosionincreases anddeposition decreases.
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Corrasion, otherwise known as abrasion, isthe grinding of rock fragments carried by
the river against the bed and banks of the
river. This action both widens and deepens
the channel. The power of the grinding is
especially powerful in floods when large
fragments of rock are carried along theriver bed eroding the bed and the banks.
Attrition is the knocking of rock
fragments in the water against one
another. The fragments are
broken into smaller pieces and
become smoother along the
process.
Solution, also known as corrosion, is the
process by which river water reacts
chemically with soluble minerals in therocks and dissolves them.
For example, when a river stream flows
over an area of limestone (calcium
carbonate), it erodes the limestone by
reacting chemically with it and dissolves it.
Hydraulic action is the breaking
down of rocks and dragging them
away from the bed and banks bythe force of the running water
itself. When water from a fast
moving stream enters cracks in a
rock, the force breaks up the
force into pieces.
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Some rivers have greater volume than others. There are four factorsthat influence volume.
Vegetation
The presence of vegetation affects the rate at which
water is intercepted and is able to infiltrate the ground.
If an area is covered with vegetation, the vegetation will
intercept the rain and allows some of it to infiltrate into
the ground. As a result, there will be less water as surface
runoff into the river and so the volume of water into the
river wil l be lowered. On the other hand, in an area with
no vegetation, there will be no interception and little
infiltration into the ground. This will result in more water
as surface runoff into the river and so the volume of the
water is higher than that of a vegetated area.
Permeability of rocks
The permeability of rocks is determined by the size of the por
between the rock particles. If the rock has small pores, water cann
easily infiltrate into the rock and this means that the rock i
impermeable. On the other hand, if the rock has large pores, water c
easily infiltrate and thus the rock is permeable. When water flow
through an area of impermeable rock, little water infiltrates th
ground, as a result there is high surface runoff and leads to a hig
volume of flow of water.
Drainage basin size
A larger drainage basin will have a greater volume ofwater. This is because there is a greater amount of
tributaries bringing water to the river resulting in a
higher surface runoff.
Drainage Basin refers to the area drained by a river and
its tributaries.
Climate
The amount of precipitation and temperature affects the volume oriver. During dry season, a river channel may be almost emp
therefore the volume is low. During the wet season, the volume will b
high and water may overflow the channel and flood the surroundin
land.
Temperature also plays a part in influencing the volume of rivers. Whe
the temperature is high, there is more evaporation and transpiratio
which reduces the amount of water in the river.
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The three factors that influence the velocity of a river are the river
gradient,channel roughness and channel shape.Gradient
A river flowing down a
steep slope or
gradient has higher
velocity than one
which flows down a
gentler gradient. For
example, the speed
of flow in a river that
plunges down a steepslope in the form of a
waterfall is much
higher than the speed
of flow in a river that
winds down a gentler
slope.
Channel Roughness
In figure A, the channel
is smooth while that in
figure B is rough or
uneven with boulders on
the river bed as well as
rocks that protrude out
from the bank. A river
that flows through such a
river has to overcomesuch obstacles and
therefore there will be
more friction and the
velocity of the river is
reduced.
Channel shape
The shape of the
channel or its cross
section affects the
wetted perimeter. The
wetted perimeter
refers to the extent to
which water is in
contact with its channel.
The greater the wettedperimeter, the greater
the friction between
the water and the banks
and the bed of the
channel, and the slower
the flow of river.
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Diag A. Diag B.
In figure A, the channel is smooth while that in figure B is rough
or uneven with boulders on the river bed as well as rocks thatprotrude out from the bank. A river that flows through such a riverhas to overcome such obstacles and therefore there will be morefriction and the velocity of the river is reduced.
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Wetted perimeter is calculating byadding the length and the breadth of
the channel in contact with water.
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A river uses its energy to carry or transport eroded
materials such as mud, sand, boulders and dissolvedmaterials. These materials are called its load. Rivertransport their load by four processes. Theseprocesses are:
Saltation | Traction | Solution | Suspension |
http://library.thinkquest.org/28022/transport/saltation.html -
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A river uses its energy to carry or transport eroded materials such as mud, sand,boulders and dissolved materials. These materials are called its load. River
transport their load by four processes. These processes are traction, saltation,suspension and solution.
Larger particles like pebbles and boulders roll and slide along the river bed.
Dissolved materials containing minerals like calcium and sodium are carried in thewater. Trying to look out for these type of load in a river? Too bad, they cannot be seenby a naked eye.
Smaller particles such as clay, silt and fine sand are carried along without contact withthe river bed. Materials carried in suspension usually forms the greatest part of the
total river bed.
http://library.thinkquest.org/28022/transport/suspension.htmlhttp://library.thinkquest.org/28022/transport/solution.htmlhttp://library.thinkquest.org/28022/transport/traction.htmlhttp://library.thinkquest.org/28022/transport/saltation.html