Fluvial Landforms: Erosional and Depositional

• Fluvial landforms are the landforms carved (erosional) or built (depositional) by running water, as overland flow and as stream flow; the word comes from the Latin fluvius, a river.
• Running water is the most widespread exogenic agent on Earth, so fluvial landforms dominate most landscapes outside the ice-covered, arid and karst lands.
  • Their scale runs from rills to continental river basins and deltas.
• A river does three-phase work: erosion, transportation and deposition; their balance shifts downstream and through time, and each leaves its own landforms.

Fluvial Processes

Running Water, Streams and Drainage Basins

• Surface runoff begins when rain spreads over the ground as a thin sheet; it concentrates into streams as it flows downslope under gravity.
• Streams are grouped by the permanence of flow:
  • Perennial: flow all year, fed by groundwater, snow or ice (the Ganga, Brahmaputra).
  • Seasonal (non-perennial): flow mainly in the wet season (most Peninsular rivers).
  • Intermittent: flow only when the water table is high.
  • Ephemeral: flow only during and just after rain (desert washes such as the streams of western Rajasthan).
• The first stream to form on an initial slope is the consequent stream; it is soon joined by tributaries from either side.
• The drainage basin (catchment, watershed) is the basic landscape unit of fluvial geomorphology: the area drained by a trunk river and its tributaries, funnelling water and sediment to one outlet.
  • Drainage divides separate adjacent basins; they are sharp ridges in youthful terrain (the Himalaya) and faint in plains and old landscapes.
  • Exorheic basins drain to the sea; endorheic (closed) basins drain to an inland sink, which may be a lake (the Caspian, Dead Sea, Aral Sea), a shrinking lake (Lake Chad) or a swallow hole in karst.
  • Roughly one-sixth of the world’s land drains to inland basins; in India, Pangong Tso in Ladakh and the Luni, which ends in the Rann of Kachchh, are examples of inland drainage.

• The drainage pattern is the plan form of a stream network; it is set by slope, rock resistance and structure.

Pattern	Form	Setting	Example
Dendritic	Tree-like branching	Uniform rocks, gentle slope	Rivers of the Ganga plain
Trellis	Parallel mains, tributaries at right angles	Folded belts of alternating hard and soft rock	Appalachians; Singhbhum
Rectangular	Right-angled bends and junctions	Two sets of joints at right angles	Jointed Vindhyan sandstone tracts
Angular	Junctions at acute or obtuse angles	Joints and faults at oblique angles	Foothill zones
Parallel	Streams side by side	Uniform, steep slope	West-flowing rivers of the Western Ghats; Lesser Himalaya
Radial	Outward from a centre	Domes, volcanoes	Amarkantak; Girnar
Centripetal	Inward to a centre	Basins, depressions	Streams into Loktak Lake
Annular	Ring-like along weak rock	Eroded domes	Black Hills, South Dakota

Fluvial Erosion

• Erosion (Latin erodere, to gnaw) is the removal and carrying away of rock waste by a moving agent.
  • Weathering prepares rock for erosion and speeds it, but is not a prerequisite: a river can erode fresh rock directly.

Controls on River Erosion

• Gradient, discharge and velocity: velocity rises with channel gradient and water volume, and erosive power rises steeply with velocity (roughly with its square, since kinetic energy varies as the square of velocity).
• River load, the tools of erosion: the size, amount and angularity (calibre) of boulders and pebbles decide how hard the bed is attacked.
  • Fine suspended load is passive; coarse, angular bedload drills and scours.
  • Underloaded rivers lack tools and erode little; overloaded rivers spend their energy on transport and deposit; erosion is greatest when load matches transporting capacity.
  • This is the law of erosion: erosion increases as a river approaches the balance between capacity and load, and declines once that balance is reached.
• Rock type and structure: jointing, bedding, hardness and solubility decide where valleys deepen fastest.

Processes of Erosion

• Mechanical erosion works through hydraulic action, abrasion and attrition; chemical erosion through solution (corrosion).

Process	Mechanism	Main effect
Hydraulic action	Force of water alone; water surges into cracks, compresses air and loosens blocks	Bank collapse, plucking of jointed beds
Abrasion (corrasion)	Load grinds and scrapes the bed and banks	Valley deepening and widening; potholes
Attrition	Load particles collide and wear each other down	Rounder, finer sediment downstream
Solution (corrosion)	Soluble minerals dissolved, largely by carbonation	Dissolved load; strong on limestone
Cavitation	Vapour bubbles collapse in very fast, turbulent flow	Pitting of beds at falls and rapids

Directions of Erosion

• Vertical erosion (downcutting) deepens the valley; it dominates in youthful, steep channels.
• Lateral erosion undercuts the banks and widens the valley; it dominates once gradient falls.
• Headward erosion cuts back at the channel head, lengthening the stream upslope; it drives gully growth and river capture.

Transportation

• A river’s competence is the largest particle it can move; its capacity is the total load it can carry.
  • By the sixth-power law, the weight of the largest particle a stream can move varies with the sixth power of velocity, so doubling velocity raises competence about 64 times. Floods therefore move boulders that normal flow cannot stir.
• Filip Hjulström (1935) plotted the velocities at which grains are eroded, transported and deposited: fine sand is the easiest to lift, while clay and silt need higher velocities because of cohesion, although once lifted they stay in suspension at very low velocities.
• Load moves in four ways:

Mode	Load	Movement
Traction	Boulders, cobbles, gravel (bedload)	Rolled or slid along the bed
Saltation	Sand and small pebbles (bedload)	Short hops off the bed
Suspension	Silt and clay	Held up by turbulence; carried far
Solution	Dissolved salts	Invisible; carried to the sea

• The bedload stays in contact with the bed; the suspended and dissolved loads travel with the water. The huge suspended load of Himalayan rivers feeds the world’s largest delta.

Deposition

• A river deposits when velocity falls or load rises beyond what it can carry.
  • Velocity falls with a drop in gradient (at mountain fronts, on subsiding or tilted land, in growing deltas), when water spreads over a floodplain, behind obstructions (landslide debris, logs, sand bars), and when discharge falls (drier climate, evaporation, seepage, canal diversion, river capture).
  • Load rises with deforestation and soil erosion in the catchment, glacial meltwater debris, tributary inputs and gully erosion.
• Deposition is sorted: coarse material drops first, finer silt and clay travel farthest, so floodplains, fans and deltas grade in texture away from the source.

Base Level of Erosion

• Base level is the lowest level to which a river can cut its bed; below it, running water has no gradient and cannot erode.
• John Wesley Powell (1875), from his survey of the Colorado River, gave the concept; sea level is the ultimate base level for rivers that reach the sea.
  • It is best pictured as an imaginary curve, rising and concave upstream, to which the sea surface is a tangent at the river mouth.

Types of Base Level

• Ultimate (grand, permanent) base level: sea level; it shifts whenever sea level or the land moves.
• Temporary base levels: lakes, reservoirs and resistant rock bands across the course; each controls the reach upstream until it is cut through or silted up.
• Local base levels: the level of a tributary’s confluence with its main stream; the tributary grades itself to that junction.
• Rivers flowing to inland basins have the basin floor as their base level (the Dead Sea, below sea level, is an extreme case).

Changes in Base Level

• Base level moves with relative sea level: through eustatic (global, e.g. glacial–interglacial) changes and local tectonic movements of land or sea floor.

Change	Cause	Effect on the fluvial cycle	Landforms
Positive (rise)	Subsidence of coastal land, rise of sea floor, deglaciation	Cycle advanced (shortened); valleys aggraded	Rias, estuaries, buried valleys, wider floodplains, filled lowlands, coastal islands
Negative (fall)	Uplift of land, sinking of sea floor, glaciation	Cycle pushed back (lengthened); rejuvenation	Knickpoints and knickpoint falls, paired terraces, valley-in-valley forms, incised meanders, polycyclic relief

River Course and Stages

• A river’s course and landscape pass through youth, maturity and old age, the stages of the normal cycle of erosion of William Morris Davis (1899); along one river the same sequence appears as upper, middle and lower courses.

Feature	Youth (upper course)	Maturity (middle course)	Old age (lower course)
Streams	Few, poorly integrated, on original slopes	Many, well integrated	Few, gentle tributaries
Dominant erosion	Vertical	Lateral replacing vertical	Lateral only; deposition dominant
Valley	Deep, narrow V	Wide V, floodplain begins	Broad, shallow, very wide floodplain
Divides	Broad, flat, marshy	Sharp	Low, broad, marshy again
Typical forms	Gorges, canyons, falls, rapids, potholes, capture	Meanders, slip-off slopes; falls vanish	Floodplains, ox-bows, levees, deltas, peneplain

Erosional Landforms

• Fluvial erosion produces valleys of many forms, falls and rapids, potholes, structural benches, terraces, incised meanders and, at the end of the cycle, peneplains.

Slope-Wash Forms: Sheets, Rills, Gullies and Ravines

• Splash erosion: raindrops strike bare soil like tiny bombs, detaching particles and breaking soil structure; it is the first step of water erosion.
• Sheet erosion: a thin, even sheet of overland flow removes a uniform layer of topsoil, often unnoticed until fertility falls.
• Rills: small, shallow channels a few centimetres deep, easily smoothed by ploughing.
• Gullies: rills enlarged until ploughing cannot remove them; they grow by headward erosion.
• Ravines: networks of deep gullies, larger than gullies but smaller than valleys, cut into soft alluvium along river bluffs.
  • Indian example: the Chambal ravines of Madhya Pradesh, Rajasthan and Uttar Pradesh, and the ravines of the Yamuna and Mahi.
• A nala (rivulet) is a small stream fed by such channels, the first link in the network of permanent valleys.

River Valleys

• A river valley is the elongated trough in which a stream flows; rills deepen into gullies and gullies into valleys.
• Valley form changes with the stage of erosion:
  • Youth: deep, narrow V-shaped valleys with steep convex sides; the water touches both walls.
  • Maturity: lateral erosion widens the valley into a broad, flat floor with straight (rectilinear) sides.
  • Old age: very broad, shallow valleys with gentle concave sides.
• Rock type modifies the form: hard, massive rocks keep walls steep; alternating hard and soft beds give stepped sides; a fall in base level cuts a new valley inside the old one (multi-storeyed or valley-in-valley forms).

V-Shaped Valleys and Interlocking Spurs

• Where a swift stream on a steep slope cuts mainly downward, weathering and slope wash open the sides into a V.
• The stream swings around spurs of high ground projecting alternately from each side, which interlock like the teeth of a zip when seen upstream.
  • If a glacier later occupies the valley, it cuts them off as truncated spurs.

Gorges

• A gorge is a very deep, narrow valley with near-vertical walls, almost as wide at the top as at the bottom (an I-shaped valley).
• Gorges form where downcutting far outpaces widening: in hard rock, in rising mountains, and below retreating waterfalls; pothole drilling is the main deepening mechanism.
• Indian examples:
  • Himalayan gorges of the Indus past Nanga Parbat, the Sutlej, the Alaknanda and the Siang (Brahmaputra); the Indus, Sutlej and Brahmaputra are antecedent rivers that kept cutting down as the ranges rose.
  • Narmada at Bhedaghat near Jabalpur, cut through marble below the Dhuandhar Falls.
  • Recession gorges below the plateau falls of Chotanagpur (Hundru on the Subarnarekha, Dasam on the Kanchi) and of the Rewa plateau (Chachai, Keoti).

Canyons

• A canyon is an extended form of the gorge: very deep and long, but wider at the top than at the bottom, with step-like sides.
• Canyons form best in horizontally bedded sedimentary rocks in arid or semi-arid climates, where the river cuts down while weak side-wash leaves resistant beds as cliffs and soft beds as benches.
• Examples: the Grand Canyon of the Colorado, Arizona, about 446 km long and more than 1.6 km deep; in India, the Gandikota gorge of the Penneru in Andhra Pradesh, called the “Grand Canyon of India”.

Waterfalls and Rapids

• A waterfall is a sudden, near-vertical drop in a river’s long profile; a rapid is a stretch of steep, broken, fast water over a smaller break, often just upstream of a fall.
• Falls mark ungraded long profiles, arising from differences in rock resistance, relief breaks, faulting, uplift, rejuvenation and damming.
• A fall line is a line of falls along a structural or relief boundary.
  • In the USA it runs along the junction of the Piedmont and the Atlantic coastal plain.
  • In India, a fall line runs along the northern edge of the Peninsular foreland, from the Tons (Purwa) Falls in Rewa district to Sasaram in Bihar, where rivers from the Kaimur and Rewa–Rohtas plateaus tumble onto the Ganga plain in falls of about 15–180 m (Chachai on the Bihar, Keoti on the Mahana, Odda on the Odda, Kuaridah on the Ausane).

Classification of Waterfalls

Group	Type	Origin	Example
Lithological (normal)	Caprock falls	Hard bed over soft beds; undercutting	Niagara; Rewa and Rohtas plateau falls
Lithological (normal)	Barrier falls	Vertical hard beds or dykes across the channel	Great Falls of the Yellowstone; small falls of the Ranchi “patlands”
Lithological (normal)	Plateau (scarp) falls	River leaves a plateau over its edge	Jog (Sharavati), Shivanasamudra (Kaveri), Livingstone (Congo)
Lithological (normal)	Step falls	Alternating horizontal hard and soft beds	Cascades and rapids
Tectonic	Fault and fracture falls	Fault scarp or fracture zone across the valley	Victoria (Zambezi)
Tectonic	Uplift falls	Local uplift of the course	Patam and Datam falls, Palamau
Change of valley level	Hanging-valley falls	Tributary hangs above the main valley	Rajrappa (Bhairavi–Damodar)
Change of valley level	Glacial hanging-valley falls	Glacier over-deepened the main valley	Fjords of Norway
Change of valley level	Coastal hanging-valley falls	River drops over a sea cliff	Cliffed coasts
Change of valley level	River-capture falls	Captured stream drops to the lower captor	Parts of the Himalaya
Change of valley level	Knickpoint falls	Break of gradient from rejuvenation	Hundru, Dasam, Dhuandhar
Damming	Lava, landslide, moraine falls	Barrier thrown across the valley	Mountain and volcanic valleys

• Disposition of beds decides the form of lithological falls:
  • Beds dipping upstream with a hard cap: the soft rock below is undercut and the cap collapses in blocks, giving high falls that retreat fast.
  • Beds dipping downstream: the hard bed slopes with the river, giving rapids rather than falls.
  • Horizontal beds: a hard cap over weak shale gives the largest caprock falls.
    • At Niagara, dolomitic limestone caps weak shales; the falls have retreated about 11 km since they formed some 12,500 years ago, at 1–1.5 m a year for centuries, now slowed to about 0.3 m a year by flow diversion for hydropower.
    • The falls of the Rewa and Rohtas plateaus have Vindhyan sandstone over weaker shale.
  • Vertical beds or dykes: resistant bands left standing across the channel make barrier falls.
• Indian knickpoint falls: the Chotanagpur rivers fall over the edges of stepped erosion surfaces raised in stages during the Tertiary: Hundru Falls (about 98 m) on the Subarnarekha, Dasam on the Kanchi, Jonha (Gautamdhara), where the Gunga hangs above the Raru; also Dhuandhar (about 30 m) on the Narmada.
• Jog (Gersoppa) Falls on the Sharavati, about 253 m, drops over the Western Ghats scarp and is often cited as an uplift-related scarp fall.

Cataracts and Cascades

• A cataract is a large, powerful fall or series of falls on a big river, where the whole flow drops steeply (the cataracts of the Nile).
• A cascade is a small, step-like fall over alternating hard and soft beds.

Recession of Waterfalls

• Falls are temporary: a river works to remove every break in its profile as it grades towards base level.
• They disappear by headward retreat (backwasting), as the caprock is undercut and collapses, and by lowering of the lip (downwasting); a recession gorge marks the path of retreat.

Potholes and Plunge Pools

• Potholes are cylindrical, kettle-like holes drilled into rocky stream beds, usually deeper than they are wide, from a few centimetres to several metres across.
  • Mechanism (pothole drilling): pebbles and boulders caught in eddies swirl round and grind the rock like a drill; the hole widens and deepens as the tools are renewed.
  • Adjacent potholes merge, so pothole drilling is the main way a rocky bed is lowered.
  • They form best in coarse, jointed rocks such as sandstone, granite and basalt.
• Plunge pools are very large potholes at the foot of waterfalls, scoured by falling water and swirling boulders; their undercutting helps the fall retreat.
• Indian examples: the rejuvenated rivers of the Chotanagpur plateau; the Narmada at Bhedaghat; the basalt bed of the Gaur near Jabalpur, studded with potholes.

Structural Benches

• Structural benches are step-like flats on valley sides made by differential erosion of alternating hard and soft horizontal beds: soft beds are cut back, hard beds stand as ledges.
• They reflect lithological control, not changes of base level, which separates them from true river terraces.
• Examples: the stepped walls of the Grand Canyon; the benched valley sides of the Vindhyan scarplands of the Kaimur.

River Terraces

• River terraces are narrow, flat surfaces flanking a valley above the present floodplain, the remnants of former valley floors or floodplains.
• Origin: a graded river builds a wide floodplain; a fall in base level or uplift rejuvenates it; the river cuts a new, narrower valley into its own floodplain, leaving the old floor as a terrace. Repeated rejuvenation leaves a flight of terraces at different heights.
• Types by symmetry:
  • Paired terraces: at the same height on both sides, from rapid downcutting.
  • Unpaired terraces: at different heights on the two sides, from downcutting combined with lateral shifting of the channel.
• Types by material:
  • Rock (strath) terraces: a bedrock platform with a thin alluvial cover.
  • Fill (aggradational) terraces: cut in thick valley fill; common where climate or glaciers once overloaded a river.
• Indian examples: flights of terraces on the Alaknanda at Srinagar (Garhwal), on the Teesta and the Beas in Kullu, and paired terraces on the Narmada; many Himalayan towns and fields stand on them.

Meanders

• Meanders are loop-like bends of a river channel, named after the winding Maeander (now the Büyük Menderes) of western Turkey.
• Each bend has an outer concave bank, attacked by the current and cut into a steep cut bank (river cliff), and an inner convex bank, with a gentle slip-off slope where sand and gravel collect.
• Helical flow: in a bend, water piles up against the outer bank; this sets up a corkscrew secondary current that runs down the outer bank, across the bed and up the inner bank, first explained by James Thomson (1876).
  • It scours the outer bank and deepens the pool there, and carries bedload to the inner bank, building the point bar.
  • The result is erosion on the outside and deposition on the inside, so bends grow, migrate downstream and sharpen until they are cut off.
• Sinuosity index = channel length ÷ straight-line (valley) length; a single-channel river with a sinuosity of about 1.5 or more is called meandering (lower values are straight or sinuous).
• Meander wavelength is typically 10–14 times the bankfull channel width, a regularity that holds from small streams to large rivers.
• Conditions: meanders are best developed on gentle slopes, in alluvium, with steady discharge; youthful rivers are too busy downcutting and overloaded rivers too busy depositing.
• Indian examples: the Gomati, with a gradient of only a few centimetres per kilometre near Lucknow; the Ganga between Prayagraj and Varanasi; the Ramganga, Ghaghara, Rapti, Burhi Gandak and Kosi.

Types of Meanders

Basis	Type	Features
Form	Wavy	Open bends, wide necks (major Himalayan rivers)
Form	Horseshoe	Tight bends, narrow necks
Form	Ox-bow (bracelet)	Near-circular loops about to be cut off
Origin	Simple (free) meanders	Lateral erosion in the first cycle, on alluvium or rock
Origin	Incised meanders	Rejuvenated downcutting into bedrock
Origin	Misfit meanders	Small bends inside large old ones after discharge falls

• Incised meanders are meanders cut deep into bedrock after uplift or a fall in base level: the river keeps the looping course it had on a gentle surface and entrenches it, recording the original land surface.
  • Entrenched (intrenched) meanders: rapid downcutting, symmetrical valley sides (the Goosenecks of the San Juan, Utah).
  • Ingrown meanders: slower downcutting with lateral shift, asymmetrical valleys, with an undercut cliff on the outside and a gentle slip-off slope on the inside.
  • Indian examples: the Damodar at Rajrappa and the Narmada at Bhedaghat.
• Misfit meanders form when a river’s discharge falls sharply (climatic change, capture): the shrunken stream winds in small meanders within a valley shaped by far larger ones.

Ox-Bow Lakes

• As bends tighten, the neck between two loops narrows until a flood cuts through it; the river takes the short course and the abandoned loop, sealed by deposits at both ends, becomes an ox-bow (horseshoe) lake.
  • The process joins erosion (cutting the neck) and deposition (plugging the ends).
• Floods then silt the lake up into a swamp and, in time, a meander scar.
• Indian example: the southward shift of the Ganga in Uttar Pradesh has left palaeochannels and ox-bow lakes north of its present course (as in Pratapgarh district); ox-bow lakes (mauns) and backswamp depressions (chaurs) dot the north Bihar plain.

Peneplains

• A peneplain (“almost a plain”) is the low, gently undulating surface that ends a long fluvial cycle under crustal stability, with residual hills (monadnocks) of resistant rock.
• Other planation end-forms are the endrumpf of Walther Penck, the panplain of Colin Hayter Crickmay, the pediplain of Lester Charles King and the etchplain.

Depositional Landforms

• Depositional (constructional) landforms are built where rivers lose velocity or gain load: alluvial fans and cones at mountain fronts, floodplains, levees and bars along the lower course, and deltas at the mouth.

Alluvial Fans and Cones

• An alluvial fan is a fan-shaped spread of sediment at a mountain front where a stream leaves a narrow valley for a plain.
  • Origin: the abrupt drop in gradient cuts velocity and transporting capacity; the overloaded stream dumps its load and its channel shifts repeatedly (avulses) across the fan, spreading it into an arc.
• Morphology:
  • In plan a sector of a circle, in three dimensions part of a cone, with its apex at the valley mouth.
  • Longitudinal profile concave, transverse profile convex.
  • Sediment fines outward from the coarse apex to the distal margin.
• Fans and cones differ mainly in slope and water supply:

Feature	Alluvial fan	Alluvial cone
Slope	Gentle: under 1° on large fans, up to about 5° on small ones	Steep: about 15°
Water and debris	More water than debris	More debris, little water
Material	Finer, well spread	Coarser
Mountain front	Gentler	Steeper

• Fans grow outward and upward; as they thicken and steepen, a fan can turn into a cone. Coalescing fans form a compound fan (bajada) and, on a large scale, a piedmont alluvial plain.
• Indian examples:
  • The Bhabar, a belt of coalescing gravel fans along the Siwalik foot, where small streams sink into porous deposits.
  • The fans of the Chandigarh and Dehra Dun valleys and of the Himalayan piedmont north of the Brahmaputra.
  • The Kosi megafan in north Bihar and Nepal, over 10,000 km² with an extremely low gradient, built as the Kosi migrated westward by more than 100 km over about two centuries.
    • On 18 August 2008 the Kosi breached its eastern embankment at Kusaha in Nepal and avulsed about 120 km eastward into an old course, flooding north Bihar and affecting some 2.7 million people.

Floodplains

• A floodplain is the flat valley floor built of alluvium that a river floods in high water; deposition makes floodplains as erosion makes valleys.
• Building: in floods, water spills over the banks and slows, dropping silt and clay over the plain in thin layers; channel deposits (sand and gravel) are left as the channel shifts, and abandoned courses fill with coarse material.
• Parts:
  • Active floodplain: the river bed and the low land flooded almost every year (the khadar of the Ganga plain).
  • Inactive (older) floodplain: higher ground above normal floods, of flood and channel deposits (the bhangar).
  • Delta plains: the floodplains of a delta.
• Floodplain surfaces carry levees, backswamps, point bars, meander scars and ox-bow lakes; the bluff is the steep valley side marking the floodplain’s edge.
• Indian examples: the Ganga, Yamuna and Brahmaputra plains.

Natural Levees, Backswamps and Yazoo Streams

• Natural levees are low ridges of coarse sediment along both banks of large rivers, built when floodwater leaves the channel and drops its coarsest load at once.
  • They are highest at the bank and slope gently away; most are under 10 m high (the Mississippi’s are about 6–8 m).
  • Levees confine normal floods, attract settlement and farming, and are often raised into artificial embankments.
• Rising beds: a river confined between levees aggrades its own bed, which may come to stand above the surrounding plain; a breach then sends water rushing over the plain.
  • The Huang He (Yellow River) earned the name “China’s Sorrow” from such breaches; the Kosi breach of 2008 was an Indian case.
• Backswamps are the low, poorly drained areas behind the levees, where the finest clay settles.
• A yazoo stream is a tributary blocked by the main river’s levees that runs parallel to the main river through the backswamps before joining it downstream (a deferred junction); it is named after the Yazoo River, which follows the Mississippi for about 270 km.
  • Yazoo basins flood repeatedly because they are low, poorly drained and hemmed in by the higher levee ridge; water ponds there when the main river is high.
• Crevasse splays are fans of sediment spread through breaks in a levee.

Point Bars and Cut Banks

• Point bars (meander bars) are crescent-shaped deposits of sand and gravel on the inside (convex bank) of a meander, built by helical flow below the slip-off slope.
  • Successive bars leave ridge-and-swale (scroll) topography, with long narrow hollows between the ridges.
• Cut banks are the steep, eroding outer (concave) banks; point-bar growth on one side and cut-bank retreat on the other make the meander migrate.

Doabs

• A doab (“two waters”) is the interfluve between two converging rivers, a term used in South Asia.
• Examples: the Ganga–Yamuna doab, and the five doabs of the Punjab, such as the Bist doab (Beas–Sutlej) and the Bari doab (Beas–Ravi).

Riffles, Pools and Bluffs

• Gravel-bed channels alternate between riffles, shallow, fast, turbulent reaches over coarse bars, and pools, deep, slow reaches of finer bed, spaced roughly five to seven channel widths apart.
• A bluff is a steep bank or valley-side cliff overlooking a river or floodplain, usually where a meander has cut into the valley wall.

Braided Channels and River Islands

• A braided channel is a river divided into many interlacing threads separated by bars and islands.
• Conditions: heavy, coarse load, highly variable discharge, easily eroded banks and a fairly steep gradient.
• Mechanism: coarse load settles as mid-channel bars, which divert flow against the banks; bank erosion widens the channel, flow becomes shallower, and more bars and lateral bars form, splitting the river into threads.
• Aits (eyots) are small river islands of deposited sediment; vegetation can make them permanent, while floods can erode them and build new ones downstream.
• Indian examples: the Brahmaputra in Assam and the Kosi are classic braided rivers.
  • Majuli, in the Brahmaputra, is the largest inhabited river island in India; bank erosion has shrunk it by roughly a third since the early twentieth century, from about 1,250 km².

Deltas

• A delta is a roughly triangular body of sediment deposited at a river mouth in a sea or lake; the term was first used by the Greek historian Herodotus (5th century BC) for the Nile mouth, which is shaped like the Greek letter Δ.
• In effect a delta is the seaward extension of the floodplain; its deposits are better sorted and stratified than those of an alluvial fan.
• Size ranges from a few square kilometres to the Ganga–Brahmaputra delta, the world’s largest at more than 105,000 km², about two-thirds of it in Bangladesh; sediment beneath large deltas can be hundreds of metres thick.

Conditions for Delta Formation

• A large sediment supply, from a long river draining a rapidly eroding catchment.
• Sediment of medium grade: very fine load is carried out to sea; very coarse load sinks near the mouth.
• A shallow sea or lake at the mouth, so deposits build up rather than vanish into deep water.
• Sheltered, low-energy water: weak tides, waves and currents, and no strong current across the mouth.
• No large lake upstream to trap the load, and a stable coast, free of rapid tectonic uplift or subsidence.

Formation and Structure

• At the mouth the river loses velocity and drops its load on the channel sides, the bed and in front of the mouth; small fans build seaward and coalesce.
• The deposits block the channel, which splits (bifurcates) into distributaries; the delta surface is laced with shifting channels.
• Grove Karl Gilbert (1885), from the deltas of glacial Lake Bonneville, described the three-layer structure of a simple delta:
  • Topset beds: gently sloping, coarser beds of the delta plain, partly above water.
  • Foreset beds: steeply inclined beds built forward at the advancing delta front.
  • Bottomset beds: fine clays spread flat on the floor beyond the front.
• Subsidence: deltas sink through compaction, sediment load and isostatic adjustment, so they keep growing only while supply keeps pace.

Growth of Deltas

• Seaward growth is called progradation. It depends on river velocity, sediment supply, waves, tides, currents and the slope of the sea floor.
  • Slow rivers drop their load close to the mouth and grow sluggishly; fast rivers carry it farther, building long, narrow deltas.
  • Strong waves and currents rework or remove sediment and retard growth.
• Density contrasts matter too (Charles C. Bates, 1953): river water denser than sea water spreads along the bed as an elongated submarine delta; equal density builds a lobate delta; lighter river water floats out and builds a bird’s-foot delta.

Classification of Deltas

Basis	Type	Features	Example
Shape	Arcuate (fan-shaped)	Convex seaward front, smoothed by waves; many distributaries	Nile, Ganga, Niger, Mahanadi, Godavari
Shape	Bird’s-foot (finger)	Long distributaries flanked by levees reaching into calm water; fine sediment	Mississippi
Shape	Cuspate	Tooth-shaped; single main channel, waves spread sediment evenly on both sides	Tiber (Italy)
Shape	Estuarine	Fills a drowned mouth; long and narrow, largely submerged	Narmada, Tapi; Seine, Ob
Growth	Growing	Actively prograding	Ganga, Mississippi
Growth	Blocked	Growth halted by waves, currents or reduced supply	Deltas of dammed rivers
Growth	Abandoned	Left unfed when the river shifts its mouth	Old Huang He deltas south of the Shandong peninsula
Growth	Truncated	Eroded and cut back by waves and currents	Wave-attacked deltas

• Current process view: William E. Galloway (1975) classified deltas by the relative power of river, waves and tides:
  • River-dominated: elongate, bird’s-foot (Mississippi).
  • Wave-dominated: smooth arcuate or cuspate fronts (São Francisco, Nile).
  • Tide-dominated: funnel-shaped mouths with tidal channels and islands (Ganga–Brahmaputra).
• This process-based scheme is now the standard frame; the older shape classes remain as descriptive labels.

Deltas of India

• Ganga–Brahmaputra delta: its arc runs about 400 km from the Hugli to the Meghna; the seaward margin is cut by tidal creeks, with marshy islands and the Sundarbans mangroves.
  • The delta subsides by a few millimetres a year as its sediments compact, which adds to sea-level rise.
  • New Moore (South Talpatti) island, which emerged after the 1970 cyclone and was claimed by both India and Bangladesh, was submerged by 2010 through erosion and rising sea level.
• Mahanadi delta: a triple delta of the Mahanadi, Brahmani and Baitarani, fed by rapid erosion of a rugged catchment, with freshwater deltaic lakes.
• Godavari and Krishna deltas: arcuate deltas of the Andhra coast, whose growth is checked by monsoon currents, longshore drift and waves.
• Kaveri delta: the old “granary of the south”, whose sediment supply is much reduced by upstream dams.
• West-coast rivers: the Narmada and Tapi end in estuaries, because they flow through rift valleys over hard rocks and meet strong tides in the Gulf of Khambhat.
• Abandoned (moribund) delta: the western, Bhagirathi–Hugli part of the Ganga delta is India’s example of an abandoned delta.
  • From about the 16th century a tectonic tilt of the Bengal basin shifted the Ganga’s main flow eastward into the Padma; the western distributaries silted up and were left poorly fed, while the active delta moved east into Bangladesh.
  • The Farakka Barrage (1975) now diverts water through a feeder canal into the Hugli to keep the channel to Kolkata port flushed.

Deltas Today

• Across the world, deltas are sinking faster than sea level is rising in many places: dams trap sediment, embankments stop floods from spreading silt, and groundwater and sand extraction speed compaction.
• Indian deltas share the problem: reservoirs on the Krishna, Godavari and Kaveri have cut sediment delivery to their mouths, and coastal erosion now eats into parts of these deltas and the Sundarbans.