IGCSE
Geography
River
processes and landforms
Introduction
This
essay is about how river
processes shape
landforms at different stages as
the river moves from its source to its mouth. A river transports,
erodes, or deposits sediment (load).
River
processes – erosion, transportation and deposition
1)
Erosion
involves the wearing away of rock and soil from the river bed and
also banks. This way the river can get deeper and/or wider.
There
exist four main types of Erosion. Corrasion
(abrasion)
describes the process when a river picks up material (sand and
pebbles) and then rubs it against its bed and banks wearing it away.
Second
is called attrition.
Here
boulders collide
with one another moving down the river and break into smaller pieces.
Over time rocks become more round. In
the third process, called
hydraulic
action,
the power of water hits river banks, and then pushes water into
cracks. Air becomes compressed; pressure increase and the riverbank
may collapse. The
last process includes solution.
This continual process is a result of the chemical composition of the
water.
Fig.
5.1
2)
Transportation
The
eroded material becomes than the river’s load which will be
transported by the river
energy (also called
velocity).
The faster the river is flowing the more and heavier load can be
transported.
There
are a three different ways of transportation.
Suspension
means that,
tiny particles are carried in suspension down the river stream.
Solution
describes
invisible
acidic chemicals, which are dissolved in the
water. This solution can react with the bed rock. Easy dissolving bed
rocks are called
soluble or
soft rocks.
There exist also hard
rocks which can not
be dissolved. By
saltation,
pebbles
and stones are lifted up by the current and drawn in a skipping
motion along the bed. If
the load is very large (boulders)
and
can not be picked up by the rivers flow it
will roll or slide along the river bed. This is called traction.
Additionally the
more volume
(water) is in the river, the greater the load.
Fig.
5.2
3)
Deposition
occurs when a river
loses energy. Than it is forced to deposit its load. The bedload
involves the heavy material which is carried along the bottom of the
river. There are several reasons why deposition happens: Decrease in
gradient, decrease in river flow (water drains away after rain), meet
of river and sea/ lake or a slower flowing on the inside of bends.
Landforms
- A River’s Course
1)
In
the
upper stage
the
river is close to its source. A source is the place, where a river
starts its course. Here the gradient of the river is steep, the river
is high above sea level, the valley floor is narrow and the valley
sides are steep. Here often occur rapids,
because of the rocky, irregular river bed, which make the water
rough. The steep gradient gives the river enough gravitational
potential energy to carry out vertical erosion. This kind of erosion
leads to the formation of gorges, canyons and potholes. Potholes
are smooth and round holes in the river bed caused of eddies in the
water swirl, which drill stones down into the rock and form those
holes. The narrow V-shaped upland, forces the river to adopt and to
wind its course around interlocking
spurs,
because only after storms large boulders can be moved.
Fig.5.9
Small
waterfall in the upper course of the Cinca, in Valle
de Pineta,
near its source in Aragon,
Spain.
A
different type of the rivers upper stage can be a waterfall
or gorge.
Waterfalls are formed where a layer of hard rock lies over a layer of
soft rock in a river valley. The soft rock erodes more quickly and a
plunge pool
develops, which undercut together with splashing water and eddy
currents the rock. Then the formed overhang collapses, the process
repeats and gorges
are created.
Fig.
5.11
Border
of Ontario,
Canada & New
York,
United States
Niagara
Falls
Through
the scenery of waterfalls economic benefits develop such as tourism
or use for hydraulic power. However they cause problems with
navigation.
2)
In the middle
stage
the gradient begins
to level out resulting new river shaping’s. The valley and the
valley floor widen, the slopes become gentler and the gradient
lessens. That’s why now lateral
(sideways) erosions
and deposition
take over. There
exist three different main river
channel forms. A
few bends mean straight
and when the river splits in many channels and sometimes re-join
describes a braided
shape. In contrast if the river flows in series of bends it is called
meandering.
Table
5.1
Meanders
have their sweeping shape because of the helical
flow inside the
water, which sends the energy to the sides (laterally). That leads to
the formation of a river
cliff (erosion) and
slip-off-slope/point
bar (deposition).
Fig.
5.15
Meanders
of the Rio
Cauto
at Guamo Embarcadero, Cuba.
Continued
erosion creates a narrow neck, which can be cutted through. Than the
river form a new channel and the old meander become an oxbow
lake.
An
oxbow lake is forming. The
Nowitna
River,
Alaska.
Fig.
5.15
3)
In the lower
stage the river
loses its velocity and the vertical erosion stops. Levées
form as the river floods. When the river overflows the bank, it loses
energy to transport bedload. Consequently, the river deposits its
load and the bank and bed, which become raised, forming gentle
sloping banks towards the floodplain.
The floodplain is the flat, marshy and poorly drained land next to a
river and is build up through depositions of point bars and load.
Fig.
5.17 & 5.18
A
Delta is
a low and flat area where
a river enters a lake/sea and because of loosing its velocity
deposits there its load. The river channel divides into
distributaries
(small rivers
joining big ones) which re-join to form braided
drainage pattern.
Fig.
5.20
Aerial
view of the Lower Prana Delta, north of Buenos
Aires
Long
and cross profile of a river valley
The
long profile shows how a river’s gradient changes as it flow from
its source to its mouth and the river cross profile shows a
cross-section of a river’s channel and
valley at different stages of the river’s course. The energy
carried out by a river changes from one stage to another because of
the river’s height, gradient and speed. The river starts flowing
high in the mountains but the gradient gradually decrease over the
middle stage to the lower stage. In the upper course the river flows
over series of waterfalls rapids and sometimes lakes. The cross
profile of the valley floor is narrow or non-existent. Erosion cuts
down the river bed and carves out steep-sided V-shaped
valleys.
As the river flow towards the mouth, the slopes become
less steep. Same as the flood plain begins to develop and widen.
Additionally the cross profile is often asymmetrical, because of
river cliffs and slip-off slopes.
Fig
5.3
Hydrological
cycle
After
a precipitation tree leaves catch raindrops, what is called
interception.
Surface run-off
describes water that flow overground in case soil is hard or very
wet. Throughflow
includes water flowing through the soil meanwhile infiltration
happens if the soil is soft and rain soaks into the ground. Permeable
rock
allows water to infiltrate, impermeable
rock
not. Water continues flowing down until finally reaches the water
table
and from the groundwater
and
flow as groundwater
flow.
Groundwater needs hundred of years until it reaches the river/sea,
whereas surface run-off and throughflow water need only weeks.
Fig.
5.22
There
are five main factors affecting discharge.
Antecedent
rainfall
means the soil is already saturated and further rainfall will run-off
towards river. Relief
includes the gradient of the slope. Steeps slopes infiltrate more
badly than gently slopes. Weather conditions can reduce discharge
with evaporation or prevent infiltration with frozen ground.
Permeable
rocks and soils absorb water easily, in contrast impermeable
do not and porous
rocks have hollow space between rock particles so they can observe
water even more quickly. It also depends on how the land is used. In
cities it is regulated by drains where the water gathers, because it
cannot infiltrate and in rural areas the water reaches thorough
hillsides the river even faster. Finally deforestation also affects
the discharge because of less interception the surface run-off
increases.
Hydrograph
A
hydrograph predicts the change of a river after a storm by showing
rainfall and discharge. After rain water flows gradually into the
river and entering the water the discharge increases. This is shown
by the rising limb-line. Lag time is the gap between the peak of
rainfall and peak of discharge. Hydrograph graphs can approximately
predict how high a discharge after a rainfall could be. That means
calculations can show how strong areas might be affected. This way
hydrographs can be used to drainage basin management.
Causes
of flooding
Flooding
tends to be caused by heavy rain. The faster the rainwater reaches
the river channel,
the higher is the likelihood to flood. For example heavy rainfall led
to catastrophic flooding in Australia or monsoon rains in Pakistan.
Other reasons are steep-sided
channels
where around the river channel steep are slopes that cause fast
surface run-off.
Also causes for flooding are lack
of vegetation or woodland
or drainage
basin in an urban area.
To
prevent flooding there are several solutions possible.
Planting
vegetation would provide interception and transpiration.
Straightening and dredging the channel adopt the river artificially
to the environment. Same as artificial levees and bridge designs. By
washing land and reservoirs, actions around the river can be solved.
Conclusion
Most
rain that falls on the land travels to the sea through rivers. Large
numbers op people live near rivers and, which provide great benefits
for them. But rivers also can cause problems and hazards for people.
Bibliography
Complete
Geography for Camridge IGCSE