2. There are three main types of processes that occur in a river. These are:
Erosion
Transportation
Deposition.
All three depend on the amount of energy there is in a river.
Which of this three processes are at any time predominant depends on the
local conditions.
The river always tries to adjust on the existing conditions.
By these processes rivers are an important agent in wearing down the
landscapes in all parts of the world.
4. Erosion starts when the flow energy of the water exceeds the resistance of
the material of the river bed and banks.
Flow energy depends on depth of water and gradient and thus of stream
velocity.
The point in time when material is set in motion is regarded as critical
state.
The corresponding stress is termed ‘critical shear stress’.
Erosion involves the wearing away of rock and soil found along the river
bed and banks.
Erosion also involves the breaking down of the rock particles being carried
downstream by the river.
5. River bed of Adirako showing severe bank erosion in alluvial and
colluvial infills; watershed of Adirako Dam in Ethiopia
6. The energy in a river causes erosion. The bed and banks can be
eroded making it wider, deeper and longer.
Headward erosion makes a river longer. This erosion happens near its
source. Surface run-off and throughflow cause erosion at the point
where the water enters the valley head.
Vertical erosion makes a river channel deeper. This happens more in
the upper stages of a river (the V of vertical erosion should help you
remember the V-shaped valleys that are created in the upper stages).
Lateral erosion refers to the widening of a stream channel or valley.
When a stream is high above its base level, downcutting will take
place faster than lateral erosion; but as the level of the stream
approaches its base level, the rate of lateral erosion increases.
7. There are four main processes of erosion that occur in rivers. These are:
Hydraulic action: 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.
Abrasion / Corrasion: The sediment carried by a river scours the bed and banks. Where
depressions exist in the channel floor the river can cause pebbles to spin around and turn
hollows into potholes.
Attrition: Eroded rocks collide and break into smaller fragments. The edges of these rocks
become smoother and more rounded. Attrition makes the particles of rock smaller. It does
not erode the bed and bank. Pieces of river sediment become smaller and more rounded as
they move downstream.
Corrosion: Carbon dioxide dissolves in the river to form a weak acid. This dissolves rock by
chemical processes. This process is common where carbonate rocks such as limestone and
chalk are evident in a channel.
8.
9. Transportation of material in a river begins when friction is overcome. Material that
has been loosened by erosion may be then transported along the river.
During the transport the particles are gradually reduced in size and rounded
(attrition).
As erosion on hill slopes is the main source of the transported material the transport
rate can be considered to be a geomorphological measure for the total rate of soil
loss of the watershed above the measuring point. Thus, the transport rate is a
measure for the fluvial morpho dynamic within a watershed.
The average loss of soil in the watershed above this measuring point you can
estimate by division of the transport rate through the area of the watershed.
Rivers need energy to transport material, and levels of energy change as the river
moves from source to mouth.
When energy levels are very high, large rocks and boulders can be transported.
Energy levels are usually higher near a river's source, when its course is steep and its
valley narrow. Energy levels rise even higher in times of flood.
When energy levels are low, only small particles can be transported (if any). Energy
levels are lowest when velocity drops as a river enters a lake or sea (at the mouth).
11. The four different river transport processes:
Solution – When dissolved material is carried by a river. This often happens in areas
where the geology is limestone and is dissolved in slightly acidic water.
Suspension - 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. This is why river appear muddy as
they are approaching bankfull discharge and towards the mouth of the river (where
velocity is greater as is the occurrence of finer sediment).
Saltation - Small pebbles and stones are bounced along the river bed. When material
such as pebbles and gravel that is too heavy to be carried in suspension is bounced
along the river by the force of the water.
Traction - Large boulders and rocks are rolled along the river bed.
13. Deposition is the process of the eroded material being dropped.
When a river loses energy, it will drop or deposit some of the material
it is carrying.
Deposition may take place when a river enters an area of shallow
water or when the volume of water decreases - for example, after a
flood or during times of drought.
Deposition is common towards the end of a river's journey, at the
mouth.
Deposition at the mouth of a river can form deltas - for example, the
Mississippi Delta.
14. Alluvial cone in miniature in the
highlands of Ethiopia
The Mississippi Delta
15. Aeolian processes, also spelled eolian or æolian, pertain to wind activity in
the study of geology and weather and specifically to the wind's ability to
shape the surface of the Earth (or other planets).
Winds may erode, transport, and deposit materials and are effective agents
in regions with sparse vegetation, a lack of soil moisture and a large supply of
unconsolidated sediments.
Although water is a much more powerful eroding force than wind, aeolian
processes are important in arid environments such as deserts.
16. Wind erodes the Earth's surface by deflation (the removal of loose, fine-grained particles
by the turbulent action of the wind) and by abrasion (the wearing down of surfaces by the
grinding action and sandblasting by windborne particles).
Regions which experience intense and sustained erosion are called deflation zones. Most
aeolian deflation zones are composed of desert pavement, a sheet-like surface of rock
fragments that remains after wind and water have removed the fine particles. The rock
mantle in desert pavements protects the underlying material from deflation.
Wind-driven grains abrade landforms. In parts of Antarctica wind-blown snowflakes that
are technically sediments have also caused abrasion of exposed rocks. Grinding by
particles carried in the wind creates grooves or small depressions. Ventifacts are rocks
which have been cut, and sometimes polished, by the abrasive action of wind.
Sculpted landforms, called yardangs, are up to tens of meters high and kilometers long
and are forms that have been streamlined by desert winds. The famous Great Sphinx of
Giza in Egypt may be a modified yardang.
17. Wind erosion of soil at the foot
of Chimborazo, Ecuador.
A rock sculpted by wind erosion in
the Altiplano region of Bolivia
18. Particles are transported by winds through suspension, saltation (skipping or bouncing) and
creeping (rolling or sliding) along the ground.
Small particles may be held in the atmosphere in suspension. Upward currents of air
support the weight of suspended particles and hold them indefinitely in the surrounding air.
Typical winds near Earth's surface suspend particles less than 0.2 millimeters in diameter
and scatter them aloft as dust or haze.
Saltation is downwind movement of particles in a series of jumps or skips. Saltation
normally lifts sand-size particles no more than one centimeter above the ground and
proceeds at one-half to one-third the speed of the wind. A saltating grain may hit other
grains that jump up to continue the saltation. The grain may also hit larger grains that are
too heavy to hop, but that slowly creep forward as they are pushed by saltating grains.
Aeolian transport from deserts plays an important role in ecosystems globally, e.g. by
transport of minerals from the Sahara to the Amazon basin. Saharan dust is also responsible
for forming red clay soils in southern Europe. Aeolian processes are affected by human
activity, such as the use of 4x4 vehicles.
19. (a) Wind transport is by suspension, saltation, and creep (bed load). (b) In a sandstorm, sand is
usually within a meter of the ground. A dust storm's smaller particles can travel higher. A dust
storm as it approaches Al Asad, Iraq
(a) (b)
20. Wind-deposited materials hold clues to past as well as to present wind directions and
intensities. These features help us understand the present climate and the forces that
molded it. Wind-deposited sand bodies occur as sand sheets, ripples, and dunes.
Wind blowing on a sand surface ripples the surface into crests and troughs whose long
axes are perpendicular to the wind direction. The average length of jumps during
saltation corresponds to the wavelength, or distance between adjacent crests, of the
ripples. In ripples, the coarsest materials collect at the crests causing inverse grading.
This distinguishes small ripples from dunes, where the coarsest materials are generally
in the troughs.
Accumulations of sediment blown by the wind into a mound or ridge, dunes have gentle
upwind slopes on the windward side. The downwind portion of the dune, the lee slope,
is commonly a steep avalanche slope referred to as a slipface.
Wind-blown sand moves up the gentle upwind side of the dune by saltation or creep.
Sand accumulates at the brink, the top of the slipface. When the buildup of sand at the
brink exceeds the angle of repose, a small avalanche of grains slides down the slipface.
Grain by grain, the dune moves downwind.
21. Cross-bedding of sandstone near Mount
Carmel road, Zion National Park,
indicating wind action and sand dune
formation prior to formation of rock
Mesquite Flat Dunes in Death Valley looking
toward the Cottonwood Mountains from the
north west arm of Star Dune (2003)
Aeolian deposition near
Addeha, Kola Tembien,
Ethiopia. (2019)
22. A glacier is a persistent body of dense ice that is constantly moving under its own
weight. A glacier forms where the accumulation of snow exceeds its ablation
(melting and sublimation) over many years, often centuries.
Glaciers slowly deform and flow due to stresses induced by their weight, creating
crevasses, seracs, and other distinguishing features. They also abrade rock and
debris from their substrate to create landforms such as cirques and moraines.
Glaciers form only on land and are distinct from the much thinner sea ice and
lake ice that form on the surface of bodies of water.
Glaciers shape the land through processes of erosion, weathering, transportation
and deposition, creating distinct landforms.
23. Abrasion - as the glacier moves downhill, rocks that have been frozen into
the base and sides of the glacier scrape the rock beneath. The rocks scrape
the bedrock like sandpaper, leaving scratches called striations behind.
Plucking - rocks become frozen into the bottom and sides of the glacier. As
the glacier moves downhill it 'plucks' the rocks frozen into the glacier from
the ground.
Freeze-thaw weathering - During the day when temperatures are higher,
the snow melts and water enters the cracks in the rock. When the
temperature drops below 0°C the water in the crack freezes and expands
by about 9 per cent. This makes the crack larger. As this process is
repeated through continual thawing and freezing the crack gets larger over
time. Eventually pieces of rock break off.
24.
25. Glaciers move very slowly. As they move, they transport material from one place to another:
As freeze-thaw weathering occurs along the edge of the glacier pieces of rock, which
break off larger rocks, fall onto the glacier and are transported.
Rocks plucked from the bottom and sides of the glacier are moved downhill with the ice.
Bulldozing is when rocks and debris, found in front of the glacier, are pushed downhill by
the sheer force of the moving ice.
Rotational slip is the circular movement of the ice in the corrie.
Any material carried or moved by a glacier is called moraine. There are three different types
of moraine:
Lateral moraine - material deposited along both sides of the glacier. This moraine is usually
made up of weathered material that has fallen from the valley sides above the glacier.
Medial moraine - material deposited in the middle of the glacier. This is caused by the
lateral moraines of two glaciers when they meet.
Terminal moraine - material deposited at the end of the glacier.
26.
27. As glaciers flow over many years, all sorts of debris falls onto the glacier through
mechanical weathering of the valley walls. Glaciers are solid ice, so unlike water, they
can carry pieces of rock of any size.
Glaciers move boulders as large as a house as easily as the smallest particles of sand and
silt. These pieces of rock are carried by the glacier for many kilometers and are only
deposited as the ice melts. When you think of a glacier, you may think of white ice and
snow, but actually glaciers have lots of rocky bits all over them.
Each of these different deposits has its own name based on where it forms, but as a
group they are called moraines. A long pile of rocky material at the edge of a glacier is
called a lateral moraine and one in the middle of the glacier is called a medial moraine.
Lateral moraines form at the edges of the glacier as material drops onto the glacier from
erosion of the valley walls. Medial moraines form where two glaciers join together. In
this case, the lateral moraines from the edges of each glacier meet in the middle to
form the medial moraine
28. These long, dark lines on the Aletsch glacier in Switzerland are examples of
medial and lateral moraines.
