Geomorphic Processes and Landforms

Geomorphic Processes

A geomorphic process is any natural process or force that changes the Earth’s surface. Such processes may build landforms or wear them down. Together, they help the Earth maintain a broad state of equilibrium through isostatic adjustment (also described in the notes as homeostasis).

The forces and materials that produce these changes are called geomorphic agents. Examples include radioactive decay, mantle convection, gravity, temperature changes, rainfall, rivers, wind, glaciers, coastal water, and groundwater.

Geomorphic processes are broadly divided into two groups.

Exogenic processes

Exogenic processes include:

• Weathering: in-situ disintegration and decomposition of rocks.
• Mass wasting: downslope movement of material under gravity.
• Erosion: removal of weathered material.
• Transportation: movement of eroded material.
• Deposition: accumulation of transported material.

Important exogenic agents and the landform systems associated with them include:

• Rivers: fluvial landforms
• Wind, especially in deserts: aeolian landforms
• Ice caps and glaciers: glacial landforms
• Waves and coastal water: coastal landforms
• Groundwater: karst landforms

Endogenic Geomorphic Processes

Endogenic processes originate inside the Earth and modify its surface. Based on their intensity and duration, they may be divided into diastrophic and catastrophic processes.

Diastrophism

Diastrophism is the movement and deformation of the Earth’s crust, producing warps, folds, and faults. It is primarily constructive because it builds and reshapes large landforms.

Based on the main direction of movement, diastrophic forces are classified as:

• Epeirogenic forces: mainly vertical or radial movements that build or deform continents.
• Orogenic forces: mainly horizontal or tangential movements that build mountains.

Epeirogenic movements

Epeirogeny refers to broad vertical movement of the crust. It includes upwarping or uplift and downwarping or subsidence, caused mainly by large-scale mantle movements and isostatic adjustment.

Upwarping and uplift

Rising mantle currents may produce a broad bulge over a large area. Such uplift contributes to the elevation of continental masses. Radial forces may also act locally and produce local uplift.

Evidence and examples noted include:

• Coringa in Andhra Pradesh, and Kaveripattinam and Korkai in Tamil Nadu, were flourishing ancient ports but are now located farther inland. This is cited as evidence of coastal uplift.
• Raised beaches occur above or away from the present shoreline and indicate uplift.
• Fossil-bearing marine beds found above the present sea level may also indicate uplift.

Downwarping and subsidence

Downwarping is the broad sinking or caving-in of a region. Large depressed regions formed in this way are called geosynclines in older geomorphological terminology and may contribute to the formation of seas and ocean basins. When sinking is local, it is called subsidence.

Examples noted include:

• Subsidence between the Chota Nagpur and Meghalaya plateaus helped form the Malda Gap. Sediments deposited by the Ganga and Brahmaputra contributed to the formation of the Sundarbans delta.
• The Andaman and Nicobar Islands are regarded as a continuation of the Arakan Yoma mountain system of Myanmar, partly submerged beneath the Bay of Bengal.
• Parts of the western flank of the Western Ghats, the Rann of Kutch, and the area associated with the ancient city of Dwarka are cited as examples of local subsidence along India’s western coast.

Orogenic Movements

Orogeny is the natural geological process of mountain building. Plate tectonic theory explains these horizontal movements in terms of interactions between lithospheric plates.

Orogenic forces are of two main types:

1. Compressional forces
2. Tensional forces

Compressional forces

Compressional forces push rock strata together from one or both sides. They are associated with convergent or destructive plate margins. Compression bends the crust, producing folds and fold mountains. In some cases it produces reverse or thrust faults.

Mountain building by plate type

Continent-continent convergence forms major fold-mountain systems, including:

• Himalayas
• Hindu Kush and Kunlun ranges
• Arakan Yoma
• Alps
• Zagros Mountains

Compression involving ancient, stable continental cores called cratons can also form old fold mountains. Examples include:

• Aravalli Range, associated with the Aravalli and Singhbhum cratonic regions
• Ural Mountains
• Appalachian Mountains

Continent-ocean convergence commonly forms coastal mountain chains, such as:

• Rocky Mountains of North America
• Andes of South America

Ocean-ocean convergence forms volcanic island arcs, such as:

• Indonesian islands
• Philippine islands
• Aleutian Islands

Folds

Compression causes rock layers near convergent zones to bend or fold.

• The sloping sides of a fold are called its limbs.
• The imaginary line running through the crest or trough of a fold is its fold axis.

Types of folds

Tensional Forces and Block Topography

Tensional forces act when crustal blocks or plates move away from one another. The crust may fracture into fissures and faults, and the block between parallel faults may subside.

• A raised or remaining block is called a horst or block mountain.
• A down-dropped block is called a graben or rift valley.
• A landscape of alternating raised and down-dropped blocks is called horst-and-graben topography.

In fault terminology, the block above an inclined fault plane is the hanging wall, while the block below it is the footwall.

Faults

A fault is a fracture in the Earth’s crust along which displacement has occurred. Faults may form under either tensional or compressional stress.

Normal fault

In a normal fault, tensional forces cause the hanging wall to move downward relative to the footwall. The formation of the Narmada Rift Valley between the Vindhya and Satpura highlands is given as an example.

Reverse or thrust fault

In a reverse fault, compressional forces cause the hanging wall to move upward relative to the footwall. A low-angle reverse fault is called a thrust fault. The Satpura region, with the Narmada and Tapi rivers on either side, is cited in the notes as an example of land uplift associated with faulting.

Transform or strike-slip fault

In a transform fault, crustal blocks slide horizontally past each other. Examples include:

• San Andreas Fault in California, United States
• Dead Sea Transform between the Arabian and African plates
• North Anatolian and East Anatolian fault systems associated with seismic activity in Türkiye

Earthquakes

An earthquake is the shaking of the Earth caused by a sudden release of energy below the surface, commonly due to movement along faults or plate boundaries. The released energy travels as seismic waves; this activity is also called seismicity.

A weak release may be felt only as a tremor, whereas a larger release may produce severe ground shaking and destruction.

• The focus or hypocentre is the point within the Earth where the energy is released.
• The epicentre is the point on the surface directly above the focus.
• Magnitude measures the energy released.
• Intensity describes the observed effects and damage at a particular place.

Magnitude and intensity are related but not identical. A high-magnitude earthquake in an uninhabited desert or polar region may cause little damage, while a lower-magnitude earthquake in a densely populated or ecologically fragile region may have high intensity and severe effects.

Measuring earthquakes

Magnitude

• The Richter scale was traditionally used to express earthquake magnitude.
• Modern earthquakes are generally reported using the moment magnitude scale (Mw), which provides a more reliable estimate of total energy released.
• Magnitude scales are logarithmic; they are not bounded in practice by a fixed maximum of 10.

Intensity

• The Modified Mercalli Intensity Scale measures observed shaking and damage.
• It uses twelve levels, from I (not felt) to XII (extreme destruction).

Types of Seismic Waves

Seismic waves are divided into body waves and surface waves.

Body waves

Body waves travel through the Earth’s interior and are generally faster and less destructive than surface waves.

• P-waves: the fastest waves; compressional and able to travel through solids, liquids, and gases.
• S-waves: slower than P-waves; shear waves that travel only through solids.

Surface waves

Surface waves travel along the Earth’s surface and generally cause the greatest destruction.

• Love waves: produce mainly horizontal, side-to-side ground motion. They are usually faster than Rayleigh waves.
• Rayleigh waves: create a rolling motion with both vertical and horizontal displacement. They are generally the slowest major seismic waves.

Classification of Earthquakes

By cause

Natural earthquakes

• Tectonic earthquakes: caused by movement along faults and plate boundaries, including earthquakes along subduction-zone Benioff zones.
• Volcanic earthquakes: associated with the movement of magma and volcanic eruptions.
• Collapse earthquakes: caused by the collapse of underground cavities, often in mining areas.

Human-induced earthquakes

• Reservoir-induced seismicity: associated with the filling and operation of large reservoirs and dams.
• Explosion earthquakes: caused by major underground explosions, including nuclear tests.

By depth of focus

Shallow-focus earthquakes are often the most destructive because their energy is released close to the surface. Continent-continent collision zones are dominated by shallow-focus earthquakes, whereas subduction zones may experience shallow, intermediate, and deep-focus earthquakes.

By casualties

The source notes use the following informal, casualty-based classification:

• Moderate: fewer than 50,000 deaths
• Highly hazardous: 50,000 to 100,000 deaths
• Most hazardous: more than 100,000 deaths

Note: This is not a standard seismological classification and should not be confused with magnitude or intensity scales.

Global Distribution of Earthquakes

Earthquakes are concentrated mainly in three broad zones.

Circum-Pacific Belt or Pacific Ring of Fire

• Accounts for roughly 70% of global earthquakes.
• Contains shallow-, intermediate-, and deep-focus earthquakes.
• Seismicity is caused mainly by the subduction of the Pacific Plate and neighbouring oceanic plates beneath other plates.

Mediterranean-Himalayan Belt

• Accounts for roughly 20% of global earthquakes.
• Extends from the Mediterranean region through West Asia to the Himalayas.
• Results mainly from the collision of the African, Arabian, and Indian plates with the Eurasian Plate.
• Shallow-focus earthquakes dominate much of this belt, especially in continental collision zones.

Other regions

The remaining earthquakes are scattered across other parts of the world and are caused by local tectonic activity or human activity.

Seismic Zones of India

The Bureau of Indian Standards (BIS) classifies India into seismic zones according to tectonic activity and earthquake hazard. The current scheme uses Zones II, III, IV, and V; there is no Zone I, meaning that all of India has at least some seismic risk.

Zones IV and V: high to very high risk

The notes identify the following broad areas:

• Much of the Himalayan belt
• Much of Northeast India
• Northern and western parts of Punjab and Haryana, Delhi, and parts of Uttar Pradesh
• Parts of Gujarat, especially the Rann of Kutch
• Andaman and Nicobar Islands

Zone III: moderate risk

• Remaining parts of the Great Northern Plains
• Parts of the western coastal plains and Western Ghats

Zone II: lower risk

• Much of the remaining Peninsular Plateau
• Lakshadweep Islands

Note: Exact seismic-zone boundaries should be checked against the latest BIS map; the descriptions above preserve the broad regional grouping in the source notes.

Tsunami

A tsunami is a series of very long sea waves generated by the sudden vertical displacement of a large volume of water, most commonly during an undersea earthquake. As the waves enter shallow coastal water, their height increases and they may strike the coast with destructive force.

Exogenic Processes in Detail

Exogenic processes operate at or near the Earth’s surface. Their most important controls are temperature and precipitation, assisted by agents such as rivers, wind, glaciers, waves, and groundwater. Collectively, the wearing down of the landscape is called denudation.

The principal exogenic processes are weathering, mass wasting, erosion, transportation, and deposition.

Weathering

Weathering is the physical disintegration or chemical decomposition of rocks, soil, and minerals at or near the place where they occur. It is an in-situ process, involving little or no transport of the weathered fragments.

Based on the dominant agent and process, weathering is classified as physical, chemical, or biological.

Physical or mechanical weathering

Physical weathering breaks rock into smaller pieces without changing its chemical composition. Important agents include temperature changes, pressure release, water, ice, and salts.

Exfoliation

Exfoliation is the peeling or separation of successive outer layers of rock.

Pressure release or unloading: Intrusive rocks form under the weight of overlying material. When erosion removes this material, pressure is released. The exposed rock expands, joints develop parallel to the surface, and outer sheets break away.

Thermal stress: Repeated heating and cooling make the outer layers of rock expand and contract more than the inner layers. The resulting stress causes the surface layers to peel away. This process is common where daily or seasonal temperature ranges are large. Bhongir Fort in Telangana is cited in the notes as an example of an exfoliated rock mass.

Granular disintegration

Rocks commonly contain minerals of different colours and thermal properties. Dark minerals generally heat and expand more rapidly than light minerals. Repeated differential expansion and contraction loosen the grains and eventually disintegrate the rock grain by grain.

Salt weathering

1. Saline water enters pores, joints, and cracks in a rock.
2. Water evaporates and leaves salt crystals behind.
3. Crystal growth and repeated hydration cause pressure within the pore spaces.
4. The rock gradually flakes or breaks apart.

Salt weathering is important in arid regions and along saline coasts.

Frost weathering

Water enters rock joints and freezes when temperatures fall below 0°C. Because water expands on freezing, repeated freeze-thaw cycles enlarge the cracks.

• Frost wedging or block separation: freeze-thaw action in large joints separates the rock into blocks.
• Frost shattering: repeated freezing in porous or highly fractured rock produces numerous sharp, angular fragments.

Chemical weathering

Chemical weathering alters or decomposes minerals through reactions with water, dissolved gases, acids, and oxygen.

Dissolution

Water dissolves soluble minerals and carries them away in solution, leaving a chemically weathered residue.

Solution

Minerals dissolve in water containing acids or other reactive solutes. The acidity or alkalinity of the solution affects the rate and type of reaction.

Carbonation

Atmospheric carbon dioxide dissolves in rainwater and forms weak carbonic acid. This reacts readily with carbonate rocks such as limestone and dolomite. Carbonation is fundamental to the development of karst landscapes and limestone caves.

Hydrolysis

Hydrogen and hydroxyl ions from water react with rock-forming minerals and produce new minerals. Feldspar, for example, may alter into clay minerals.

Hydration

Some minerals absorb water into their crystal structure, increase in volume, and weaken the surrounding rock through repeated expansion and contraction.

Oxidation and reduction

In oxygen-rich conditions, minerals-especially those containing iron-react with oxygen to form oxides and hydroxides, as in rusting. In waterlogged or oxygen-poor conditions, reduction reactions may reverse or alter these compounds, often producing grey, green, or bluish colours.

Biological weathering

Biological weathering is the removal or disruption of minerals by living organisms.

• Growing roots enter cracks, exert pressure, and widen them.
• Burrowing animals disturb and fragment soil and rock.
• Rodents, termites, and earthworms expose fresh material to physical and chemical weathering.
• Organic acids produced by roots and microorganisms may also dissolve minerals.

Mass Wasting

Mass wasting or mass movement is the downslope movement of soil, rock, or weathered material under the direct influence of gravity. Weathering often prepares material for movement but is not a prerequisite.

Mass movements are classified by their speed, slope, material, and type of motion.

Slow movements

Creep

Creep is the extremely slow downslope movement of soil or debris on gentle to moderate slopes. It is usually noticeable only through long observation. Bent tree trunks, tilted fences, and leaning electric poles are common evidence.

Solifluction

Solifluction is a water-assisted form of slow downslope flow. Waterlogged surface material moves over an impermeable or frozen layer, especially in cold environments. The original notes describe it as a variant of creep lubricated by soil moisture or rainwater.

Rapid movements

The terms flow, slide, and fall describe how the material moves:

• Flow: material behaves like a fluid, usually because it contains abundant water.
• Slide: a relatively coherent mass moves along a distinct surface.
• Fall: material detaches from a steep face and descends rapidly through the air.

The name of a movement also reflects its material:

• Earth flow, slide, or fall: dominated by soil and fine earth material.
• Debris flow or slide: contains a mixture of soil and coarse fragments.
• Rockslide or rockfall: dominated by large rock fragments; rock generally slides or falls rather than flows.
• Mudflow: rapid flow of water-saturated clay, silt, and fine sediment.
• Landslide: sudden movement of a large mass of material on a moderate to steep slope.
• Avalanche: rapid downslope movement of snow and ice, often mixed with rock debris.
• Slump: rotational slide in which material moves backward along a curved slip surface before descending downslope.

Major Landforms

Endogenic and exogenic processes together create relief features called landforms. Based mainly on elevation and slope, the three major continental landforms are mountains, plateaus, and plains.

Mountains

Mountains are high, extensive landforms with relatively steep slopes and prominent summits. The notes estimate that mountains occupy about 27% of the Earth’s land surface.

Geological time hierarchy

Geological time units are arranged from largest to smallest as:

Eon -> Era -> Period -> Epoch -> Age

The source notes also mention the Anthropocene, a proposed epoch intended to recognise the major human influence on the Earth system. It has not been formally adopted as an epoch in the international geological time scale.

Mountains by age

Ancient mountains

• Precambrian mountain systems formed during the earliest geological eras. Most have been deeply denuded and survive as residual uplands. The Laurentian and Algoman uplands of North America are examples cited in the notes.
• Caledonian mountains formed mainly from the Late Ordovician to Early Devonian periods. Their remnants occur in parts of northern Europe, Greenland, and eastern North America. The source notes group the Aravallis and Appalachians with old fold mountains.
• Hercynian or Variscan mountains formed mainly during the Carboniferous and Permian periods. Examples cited include the Urals, Altai, and Vosges ranges.

Young mountains

Young fold mountains formed mainly during the Cenozoic Era and generally retain high, rugged relief. Examples include:

• Rockies
• Andes
• Alps
• Zagros Mountains
• Hindu Kush and Kunlun ranges
• Himalayas
• Arakan Yoma

Mid-oceanic ridges are also geologically young mountain systems, though they form at divergent rather than convergent boundaries.

Mountains by geographical location

• Continental mountains: located within continental interiors or broad continental belts; examples include the Himalayas, Urals, and Aravallis.
• Coastal mountains: lie near continental margins; examples include the Rockies and Andes.
• Oceanic mountains: rise from the ocean floor; the Mid-Atlantic Ridge is an example.

Mountains by mode of formation

Types of volcanic mountains

Shield volcanoes

Low-viscosity basaltic lava spreads over large distances, forming broad mountains with gentle slopes and relatively flat profiles. Examples include Mauna Loa, Mauna Kea, Kīlauea, and Fernandina.

Composite or stratovolcanoes

Alternating layers of lava and pyroclastic material form high, steep-sided cones. Examples include Krakatoa, Pinatubo, Stromboli, and Vesuvius.

Cinder cones

Loose cinders and other pyroclastic fragments accumulate around a vent, forming relatively small, steep cones. The source notes cite examples from Japan and the Philippines, though some handwritten names are unclear.

A caldera is a large volcanic depression formed when a summit collapses after a major eruption or when an explosive eruption removes part of the volcanic edifice. Caldera-forming eruptions can be exceptionally violent.

Plateaus

A plateau is an elevated, relatively flat-topped tableland, often with an elevation around 1,000 m or more. The source notes estimate that plateaus occupy roughly one-third of the Earth’s land surface. Plateaus are often rich in minerals and include some of the world’s major mining regions, such as the Kimberley region of South Africa and the Chota Nagpur Plateau of India.

Intermontane plateaus

These are enclosed or bordered by mountain ranges. Examples include:

• Tibetan Plateau between major Himalayan and related ranges
• Mexican Plateau
• Altiplano or Bolivian Plateau in the Andes

Piedmont plateaus

A piedmont plateau lies at the foot of a mountain range and is bordered on the other side by a plain or the sea. It is often a plateau of denudation.

Examples in the notes include:

• Malwa Plateau near the Vindhyas
• Piedmont Plateau east of the Appalachian Mountains
• Patagonian Plateau east of the Andes

Continental plateaus

Continental plateaus are extensive uplands produced mainly by broad crustal uplift. They are commonly surrounded by plains or water bodies and are sometimes described as plateaus of accumulation. Examples cited include the Chota Nagpur Plateau and the Antarctic Plateau.

Volcanic plateaus

Repeated outpourings of fluid basaltic lava from fissures build broad, layered plateaus. The Deccan Traps formed through extensive fissure eruptions as the Indian Plate passed over the Réunion hotspot.

Dissected plateaus

A dissected plateau has been deeply cut by rivers and erosion, leaving a rugged network of valleys and remnant uplands. The notes cite plateaus along parts of the Western Ghats.

Plains

Plains are broad, nearly level or gently sloping surfaces with low relative relief. The source notes estimate that they occupy about 40% of the Earth’s land surface, making them the most extensive of the three major continental landforms. They are often fertile, agriculturally productive, densely populated, and historically important centres of civilisation.

Erosional plains

Erosional plains form when geomorphic agents reduce pre-existing mountains or plateaus to low-relief surfaces.

• Peneplain: an almost level surface produced by prolonged river erosion. Isolated rounded residual hills are called monadnocks.
• Pediplain: a broad, low-relief surface produced mainly by the coalescence of pediments in arid and semi-arid regions. Isolated residual hills are called inselbergs.
• Glacial erosional plain: a level or gently undulating rock surface scoured by moving ice.
• Coastal erosional plain or wave-cut platform: a flat rock surface cut by wave erosion near the coast.

Depositional plains

Fluvial depositional plains

Rivers deposit sediment from mountain fronts to their mouths, producing several kinds of plains and related features:

• Alluvial cone: a relatively steep, cone-shaped deposit at a mountain foot.
• Alluvial fan: a broad, fan-shaped deposit with a gentler gradient.
• Alluvial plain: an extensive sedimentary plain built by rivers over long periods, often beyond the present channel.
• Floodplain: flat land bordering a river, built by sediment deposited during floods.
• Delta: a depositional plain built at a river mouth where sediment accumulates faster than marine processes remove it.

Loess plain

A loess plain consists of extensive deposits of fine, wind-blown silt, commonly found in arid and semi-arid regions and downwind of deserts or glacial outwash areas.

Glacial depositional plains

• Till plain: an undulating or flat surface of unsorted sediment deposited directly by glacial ice.
• Outwash plain: a broad surface of sorted sand and gravel deposited by meltwater beyond a glacier’s margin.

Coastal depositional plain

Waves and currents deposit sediment along a coast to form beaches and other low coastal plains. The source notes call this a wave-built platform.

Lacustrine plain

A lacustrine plain forms when a lake basin is filled with sediment and the water drains or evaporates. Streams entering a mountain lake gradually deposit sediment on its bed. The Kashmir Valley’s karewa deposits, known for saffron cultivation, are cited as an example.

Structural plains

Structural plains are broad, nearly horizontal surfaces developed on relatively undisturbed rock strata. The southeastern coastal plain of the United States near the Gulf of Mexico is cited as an example.

Abyssal or deep-sea plains

Abyssal plains are exceptionally flat areas of the deep-ocean floor, generally at depths of about 3,000-6,000 m. Fine sediments bury the uneven basaltic crust produced at mid-ocean ridges as it moves away through seafloor spreading. They cover a very large part of the ocean floor.

Volcanic Landforms

The solidification of lava or magma creates both extrusive and intrusive landforms.

Extrusive landforms

Extrusive features form at the surface and include:

• Volcanic mountains, including shield volcanoes, composite volcanoes, and cinder cones
• Lava plateaus built by fissure eruptions
• Broad lava plains

Intrusive landforms

Intrusive features form when magma solidifies below the surface.

Fluvial Landforms

Fluvial landforms are created by rivers, rainfall, and surface runoff.

In the youthful upper course, steep gradients give a river strong vertical erosive power. In the mature and old stages, gradients decrease, lateral erosion becomes more important, and deposition increasingly shapes the channel and floodplain.

Fluvial erosion processes

• Hydraulic action: the force of moving water loosens material from the bed and banks.
• Attrition: transported fragments collide with one another and become smaller and rounder.
• Abrasion or corrasion: sediment carried by the river scrapes and wears the bed and banks.
• Corrosion or solution: river water chemically dissolves soluble minerals.

Fluvial erosional landforms

Valleys

A valley is an elongated low area between uplands, commonly occupied by a river.

• V-shaped valley: forms where vertical erosion dominates in a youthful river with a steep gradient.
• Broad or U-like river valley: develops as lateral erosion widens a valley on gentler slopes. A true U-shaped trough, however, is characteristically glacial.
• Gorge: a very deep, narrow valley with steep or nearly vertical sides and a similar width at top and bottom. Examples include the Marble Rocks gorge on the Narmada and Mekedatu on the Kaveri.
• Canyon: a deep, steep-sided valley that is commonly wider at the top than at the bottom. Examples include the Grand Canyon, Fish River Canyon in Namibia, and Gandikota gorge on the Penna River in Andhra Pradesh.
• River terrace: a step-like bench on one or both sides of a valley, representing an older floodplain left above the present river level. Similar terraces on both sides are paired; unequal terraces are unpaired.

Interlocking spurs

In a youthful V-shaped valley, a river winds around projecting ridges rather than cutting directly through them. The alternating ridges appear to interlock. If later erosion cuts through their ends, the remnants are called truncated spurs.

Potholes

Potholes are cylindrical or rounded depressions drilled into a rocky riverbed by turbulent water and rotating pebbles through hydraulic action and abrasion.

Waterfalls, plunge holes, and plunge pools

Where resistant rock overlies or adjoins softer rock, the softer layer erodes more rapidly and creates a sudden drop called a waterfall. Hydraulic action and abrasion excavate a plunge hole at its base; when filled with water, it forms a plunge pool.

Cascades

A cascade is a stepped series of small waterfalls or rapid drops, often produced by alternating resistant and weak rock layers.

Rapids and cataracts

Partial erosion of alternating hard and soft rock creates an uneven channel and turbulent, broken flow called rapids. Large, powerful rapids or waterfall-like sections are called cataracts.

River capture or piracy

River capture occurs when a lower or more erosive stream extends its valley by headward or lateral erosion and intercepts a neighbouring stream.

• Captor stream: the stream that captures the flow.
• Captured stream: the stream whose upper flow is diverted.
• Misfit stream: the reduced remnant occupying the abandoned lower valley.
• Elbow of capture: the sharp bend where the diverted stream enters the captor.
• Wind gap: the dry or underfit section of the former valley beyond the capture point.

The notes cite a proposed reconstruction in which the Yamuna captured part of the former Saraswati system and the Ghaggar became a misfit stream. This interpretation is debated and should be treated as a hypothesis rather than a settled example.

Peneplains and monadnocks

Prolonged river erosion may reduce a landscape to an almost level peneplain. Resistant, rounded residual hills left above it are called monadnocks.

Meanders

A meander is a looping bend that develops mainly in a river’s mature and old stages as the channel migrates laterally.

• Faster flow erodes the outer, concave bank, producing a cut bank or river cliff.
• Slower flow deposits sediment on the inner, convex bank, producing a point bar, sand bar, or meander bar.

Meanders therefore combine erosional and depositional processes.

Oxbow lakes

As erosion narrows the neck between adjacent meander bends, the river may cut through during a flood and adopt a shorter course. Deposition seals the abandoned loop, forming an oxbow lake. Kanwar Lake or Kabartal in Bihar, associated with the Gandak basin, is cited in the notes.

Frequent meandering and channel migration can intensify flooding. The Kosi is traditionally called the “Sorrow of Bihar,” while the Damodar has been called the “Sorrow of Bengal.” The notes identify the Jhelum as an unusual example of marked meandering in its youthful mountain course.

Fluvial depositional landforms

Point bars

Sand and gravel deposited along the inner bank of a meander form point bars or meander bars.

Alluvial cones and fans

When a stream emerges from a steep mountain valley onto a gentler plain, its velocity and carrying capacity decline. A steep, narrow deposit forms an alluvial cone; a broader and gentler deposit forms an alluvial fan.

Alluvial plains and floodplains

Long-term river deposition creates broad alluvial plains. Sediment laid down beside a channel during floods builds the floodplain.

Delta

A delta forms where a river deposits sediment at its mouth before entering a sea or lake.

Natural levees

During floods, coarse sediment is deposited close to the river channel, gradually building elevated embankments called natural levees. They form a barrier between the normal channel and the floodplain.

Channel bars and riverine islands

When velocity is low, sediment may accumulate within the channel as a channel bar. A large, stable bar that rises above the water and supports vegetation may form a riverine island.

Examples cited in the notes include:

• Majuli on the Brahmaputra in Assam
• Bhavani Island on the Krishna River
• Islands in the Godavari system around Diviseema and Pattiseema

Braided channels

Numerous channel bars and riverine islands divide a sediment-laden river into multiple shallow, interweaving channels, creating a pattern resembling a braid.

Related note: Phumdis are floating masses of vegetation, soil, and organic matter in Loktak Lake, Manipur. They support Keibul Lamjao National Park, often described as the world’s only floating national park. They are lacustrine features rather than fluvial channel bars.