Rivers have distinct features along their upper, middle, and lower courses. In the upper course, the river flows through a V-shaped valley with steep sides, eroding vertically. Meanders begin to form in the middle course as the river erodes more horizontally and widens the valley floor. In the lower course, the river flows through a wide, flat floodplain with features like oxbow lakes formed from abandoned meander loops.
Flowing water has the ability to dissolve the soluble mineral substances available on its way. The processes enacted by streams are called as fluvial processes. The word “fluvius” is derived from the latin word meaning “ river”. The world fluvial is used to denote the running water as streams or rivers. Fluvial processes entail the erosion, transportation, and deposition of earth materials by running water. Fluvial processes and fluvial landforms dominate land surfaces the world over, as opposed to the limited effects of glacial, coastal, and wind processes.
Flowing water has the ability to dissolve the soluble mineral substances available on its way. The processes enacted by streams are called as fluvial processes. The word “fluvius” is derived from the latin word meaning “ river”. The world fluvial is used to denote the running water as streams or rivers. Fluvial processes entail the erosion, transportation, and deposition of earth materials by running water. Fluvial processes and fluvial landforms dominate land surfaces the world over, as opposed to the limited effects of glacial, coastal, and wind processes.
Rivers are powerful geological agents that shape the Earth's surface through a variety of processes. The geologic action of rivers includes erosion, transportation, and deposition, which collectively contribute to the formation and modification of landscapes. Here's a brief overview of these processes:
Erosion:
Abrasion: The river carries sediments (such as rocks and pebbles) that can wear away the riverbed and banks over time.
Hydraulic action: The force of flowing water can dislodge and transport loose particles, further eroding the riverbed.
Transportation:
Traction: Large particles, like boulders and pebbles, are rolled along the riverbed by the force of the water.
Saltation: Smaller particles are bounced or skipped along the riverbed.
Suspension: Fine particles, like silt and clay, are carried in the flow of the water without directly touching the riverbed.
Solution: Dissolved minerals are carried in the water without being visible, contributing to the river's overall load.
Deposition:
When the velocity of the river decreases, it loses the ability to transport certain sizes of particles. As a result, these particles are deposited along the riverbed or banks.
Larger particles are deposited first, closer to the river source, while smaller particles may be transported farther downstream before deposition.
Meandering and Oxbow Lakes:
Over time, rivers can create meanders or bends in their course. As the outer bank of a meander erodes and the inner bank accumulates sediment, the meander may migrate.
Eventually, a meander may become so pronounced that the river cuts through the neck of the meander, forming an oxbow lake.
Delta Formation:
When a river enters a standing body of water, like an ocean or a lake, the reduced velocity causes sediment deposition. This can lead to the formation of a delta, a fan-shaped landform composed of sediment carried by the river.
Canyon Formation:
In areas with resistant rock layers, rivers can erode deep canyons over time. The Colorado River carving out the Grand Canyon is a notable example.
Valley Formation:
Rivers contribute to the formation of valleys through erosion and transportation of sediment, shaping the landscape over millions of years.
The geologic action of rivers is dynamic and continuously shapes the Earth's surface, playing a crucial role in the ongoing process of landscape evolution
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4. Upper Course - Channel features
• River channel is rocky.
• Covered with various shapes and sizes
of boulder.
• Discharge is low.
• Under flood conditions rivers energy is
expended on vertical erosion with
hydraulic action and corrosion processes
at work.
• Potholes may form.
7. Upper Course - Valley features
• Valley sides are steep and form a ‘V’ shaped
cross section.
• Interlocking spurs.
V shaped valley
Steep sides
Zig-zag bends
(interlocking
spurs)
8. V – shaped valley
Form due to a combination of
the following processes:
Vertical erosion by the river
itself.
Physical weathering (eg:
frost action) which provides
debris to move down slope.
Mass movement (inc: soil
creep & landslides) to move
debris down slope.
10. Upper Course – Long Profile
• Generally the gradient is steep and the
profile is uneven, particularly where
waterfalls and rapids form.
11. Waterfall formation
Hard Rock – Lava
Soft Rock – Sandstone or Conglomerates
Soft rock is easy to
erode, but the hard rock
is resistant.
So over time a ledge
develops.
12. Waterfall formation
The water rushes over the
ledge and erodes a plunge
pool by abrasion and
hydraulic action.
Hard Rock – Lava
Soft Rock – Sandstone or Conglomerates
13. Waterfall formation
The ledge collapses into
the plunge pool, where
the debris helps to
speed up the erosion.
Hard Rock – Lava
Soft Rock – Sandstone or Conglomerates
14. Waterfall formation
The process is repeated
and the waterfall
gradually retreats
upstream, carving out a
gorge.
Hard Rock – Lava
Soft Rock – Sandstone or Conglomerates
19. Middle Course - Valley features
• River erosional energy is now increasingly
expended horizontally rather than vertically.
• Lateral erosion by the river’s meanders
broadens the valley floor into a narrow flood
plain.
• Meanders gradually shift their course
downstream.
21. Meanders
A meander starts as a slight bend:
Water flows faster on the outer curve of
the bend (more energy), and slowest on the
inner curve (less energy).
So the outer bank gets eroded while
material is deposited at the inner bank.
Over time the outer bank gets worn away
(river cliff) and the inner one builds up
(river beach). The bend grows into a
meander.
22. Meanders
(refer to previous notes and diagrams)
• Alternating series of irregularities develop
• Pools – deeper stretches of slow moving water
• Riffles – shallower section of faster flow,
flowing above coarser material
• River develops a winding or sinuous course
• Faster flow on outer bend results in erosion
and formation of River Cliff
• Slower flow on inside of bend results in
deposition and formation of Slip-off Slope
23. Meanders
• Meanders develop and
migrate laterally and
downstream
• Turbulance flow further
assists meander
formation and transports
sediment from river cliff to
the slip-off slope on the
inside of the next bend.
26. Lower Course - Channel features
• The channel is now at its broadest and deepest.
• Bedload is carried entirely in suspension and is
solution.
• Deposition now dominates – particularly during
floods.
• Erosion also occurs – in the formation of
meanders
27. Lower Course – Valley features
• Thanks to lateral erosion the valley sides
may now be several kilometres away.
• Typically it may also contain the following
features:
Floodplain & natural levées
Braided channels
Meanders
Oxbow lakes
Estuaries and deltas
29. Gentle valley sides
Flat floodplain
Layers of silt
Deposited during floods
Coarse material
Forms natural
levees
River is actually flowing above
The floodplain !!
Levees is often
artificially
strengthened
32. Braided channels
• Formed by the choking of the main
channel by the deposition of a
considerable amounts of the river load.
• The channel splits into several smaller
channels which flow around fresh ‘islands’
of deposited material before rejoining.