Running water: depositional landforms

Detail

The basic rule: why a river drops its load

• Deposition means a river setting down the sediment (its load) that it was carrying.
• What makes a river drop its load
• The river's speed falls. This happens when the slope becomes gentle, when the river enters still water (a lake or the sea), or when floodwater spills out of the channel.
• Slower water has less energy, so it can carry less.

• The heaviest load (boulders, gravel, coarse sand) settles first. Fine silt and clay are carried further.

• This sorting by grain size explains every landform below. Fans are coarse, levees are coarse near the bank, flood deposits are fine, and deltas are layered from coarse to fine.

Alluvial fans

• An alluvial fan is a cone of coarse load. It forms where a fast mountain stream suddenly reaches a gentle plain at the foot of the slope.
• How a fan forms
• In the mountains the stream is steep and fast, so it carries a heavy, coarse load.
• At the break of slope (the point where the steep mountain slope meets the flat plain) its speed suddenly drops.

• It dumps its load in a low cone that spreads out from the mouth of the valley.

• Distributary: a branch channel that flows away from the main river and spreads water and sediment across a fan or delta.

• On a fan the main channel does not stay in one place. It keeps getting choked by its own load, so it breaks into many distributaries that keep shifting.

• Fan shape depends on climate

Feature	Humid areas	Arid (dry) areas
Height and slope	Low, gentle cone	High, steep cone
Reason	Plenty of water spreads the load thinly and far	Little water; the stream dies out quickly and dumps its load close to the hills

• NCERT example (Fig. 6.3): an alluvial fan deposited by a hill stream on the way to Amarnath, Jammu and Kashmir.
• Bhabar belt: a narrow belt of pebbles and boulders along the foot of the Siwaliks (the outermost, lowest Himalayan range). It is a chain of joined (coalescing) fans.
• The soil is so porous (full of gaps) that small streams sink into it and flow underground.

• Further south, this water comes back to the surface in the marshy Terai belt.

• Kosi megafan (north Bihar): the best Indian example of a giant fan

• Course: the Kosi (Sapta Kosi) is formed in Nepal by the Sun Kosi, Arun and Tamur. It leaves the Himalaya, enters Bihar and joins the Ganga near Kursela (Katihar district).
• Size: it is one of the largest alluvial cones built by any river in the world. It covers about 10,351 sq km, with a maximum length of about 155 km and width of about 115 km [2].
• Shifting course: over the last 200 years the Kosi has moved about 112 km westward. It left behind and buried its old channels and cut new ones [2].
• Below the Siwaliks it breaks into more than 12 separate channels, and all of them shift during floods [2].
• How the shifting happens
  • The river leaves the steep Himalaya and enters a very flat plain, so it drops millions of tonnes of silt every year [2].
  • Its bed rises higher than the land around it.
  • During a flood it breaks out sideways into lower ground and takes a new course. This is called avulsion (a sudden switch of the river to a new channel) [2]. This is why the Kosi is called the "Sorrow of Bihar".

Deltas

• A delta is a low cone of sediment built where a river enters the sea (or a lake). It forms when the river drops its load faster than waves and tides can remove it.
• How a delta forms
• The river meets still sea water and suddenly slows down.
• Coarse material settles first, near the mouth. Fine silt and clay are carried further out.
• Layer is laid on layer, so deltas are well sorted (graded by grain size) and stratified (laid down in clear layers).

• As the delta grows towards the sea, its distributaries become longer (NCERT Fig. 6.4: Krishna delta).

• How a delta differs from a fan: a fan forms on land at a break of slope and its load is coarse and poorly sorted. A delta forms at a river mouth and its load is finer, well sorted and layered.

• Types of delta (by shape)

Type	Shape and cause	Examples
Arcuate	Fan- or arc-shaped with a curved seaward edge; many distributaries; waves and river are fairly balanced	Nile (Egypt), Ganga
Bird's-foot	Long distributaries stick out like a bird's claws; strong river flow, weak waves	Mississippi (USA)
Cuspate	Tooth-shaped; usually one main channel; strong waves hit it head-on and push sediment to both sides	Tiber (Italy)

• Ganga-Brahmaputra delta (Sundarbans)
• It is the world's largest delta, about 80,000 sq km. It is built from sediment brought by three rivers, the Ganga, Brahmaputra and Meghna, which meet in the Bengal Basin [3].
• It holds the Sundarbans, the world's largest mangrove forest. Mangroves are salt-tolerant trees that grow in tidal mud. The forest is about 10,000 sq km, with 40% in India and 60% in Bangladesh [4].
• The Indian Sundarban, in West Bengal, lies at the south-western end of the delta. It holds over 60% of India's mangrove forest area and 90% of Indian mangrove species [4].
• It is the only mangrove forest in the world with a large tiger population [4].

• It is cut by a maze of tidal creeks and channels. The larger channels are often over 1.5 km (a mile) wide and run north to south [3][4].

• Peninsular India: deltas in the east, estuaries in the west

River	Origin	Mouth	Feature at mouth
Mahanadi	Sihawa, Chhattisgarh	Bay of Bengal, Odisha	Delta
Godavari	Trimbakeshwar, Nashik, Maharashtra	Bay of Bengal, Andhra Pradesh	Delta
Krishna	Mahabaleshwar, Maharashtra	Bay of Bengal, Andhra Pradesh	Delta
Kaveri	Talakaveri, Brahmagiri hills, Karnataka	Bay of Bengal, Tamil Nadu	Delta
Narmada	Amarkantak, Madhya Pradesh	Gulf of Khambhat, Arabian Sea	Estuary
Tapi	Multai, Betul, Madhya Pradesh	Gulf of Khambhat near Surat	Estuary

• An estuary is a funnel-shaped river mouth where the tide flows in and out. The tides carry the sediment out to sea, so no delta builds up.
• Why the Narmada and Tapi form estuaries
• They flow west through rift valleys (long troughs where land has sunk between faults). The Narmada flows between the Vindhya and Satpura ranges. Their beds are hard rock.
• They flow through hard rock and carry less fine load to their mouths.
• The Gulf of Khambhat has very strong tides, which wash the sediment out to sea.

• The east-flowing rivers have long courses over gentle slopes and carry a heavy load into the calmer Bay of Bengal, so they build deltas.

• Human use (Class 9 NCERT): delta plains are very fertile. They grow rice and jute, and many people live on them. But they flood often and are hit by cyclones and storm surges.

Floodplains

• A floodplain is flat land beside a river, built from the sediment left behind by floods.
• Active floodplain: the river bed itself, which the river reworks every year.

• Inactive floodplain: land above the banks, flooded only during high floods. It holds two kinds of deposit:

  • Coarse channel deposits: sand and gravel that fill old, abandoned river courses.
  • Fine flood deposits: silt and clay that settle from still floodwater.

• In deltas, floodplains are called delta plains.

• Life on floodplains (Class 6 NCERT): the Ganga plain is a vast floodplain. Its fertile soil, flat land and water support farming and very dense settlement. Floods renew the soil but also destroy crops and homes.

Natural levees

• A natural levee is a low, long ridge of coarse deposits running along both banks of a large river.
• How a levee forms
• In a flood, water spills over the bank.
• Outside the channel it suddenly slows down.
• It drops its heaviest (coarsest) load right at the bank. Finer silt and clay travel further onto the floodplain.

• Flood after flood, the bank grows into a ridge that is higher than the land behind it.

• Levees are often cut by breaks. Floodwater rushes through these breaks onto the plain.

• Rivers with high levees can flow above the level of the surrounding land, so a single break can flood a large area. The Kosi is an example.

Point bars

• A point bar (or meander bar) is a deposit of mixed sediment (sand, gravel, silt) along the inner bank of a meander bend.
• How it forms: on the inner side of a bend, water flows slowly, so it drops part of its load there.
• Watch out: NCERT (Class 11) says point bars form on the "concave side". This is wrong. Point bars form on the convex (slip-off) bank, which is the inner side of the bend. The concave bank is the outer side, where erosion happens.
• NCERT Fig. 6.5 shows natural levees and point bars on the Krishna delta.

Meanders

• A meander is a loop-like bend in a river channel on a very gentle slope. NCERT says it is a channel pattern (the way a channel bends), not a landform.
• Why rivers meander
• On flat land the river cannot cut downward much, so its energy works sideways. This is called lateral erosion.
• The banks are made of loose, unconsolidated alluvium (river sand and silt that is not packed into rock), so they wear away easily.
• Coriolis force helps. It is the effect of the earth's spin, which turns moving water and air to one side (to the right in the northern hemisphere).

• Any small bend makes water hit the outer bank harder. That bank wears back, so the bend grows into a loop.

• Slip-off slope and undercut bank: the two sides of a meander bend are opposite in shape.

Bank	Shape	What happens	Result
Outer (concave) bank	Steep	Fast water erodes it	Undercut bank or cut-off bank, a steep scarp
Inner (convex) bank	Long, gentle	Slow water deposits on it	Slip-off slope with a point bar

• Incised (entrenched) meanders, which are deep gorges cut into rock, are erosional landforms. Do not mix them up with the depositional meanders of plains.

Oxbow lakes

• An oxbow lake is a curved lake formed when a meander loop is cut off from the main river.
• How it forms
• Erosion on two outer banks brings the two ends of a loop closer, and the neck between them becomes narrow.
• In a flood, the river cuts straight across the neck.

• The old loop is left behind as a horseshoe-shaped lake. Silt then seals its ends.

• Over time oxbow lakes fill with silt and turn into swamps.

• NCERT example (Fig. 6.6): the Burhi Gandak near Muzaffarpur, Bihar. The Burhi Gandak is a north-bank tributary of the Ganga.
• In the Ganga plain such lakes are called mauns or chaurs (Bihar terms).

Braided channels

• A braided channel forms when a river carries more sediment than it can move.
• How braiding happens
• The river is overloaded, often with a steep supply of load and a falling gradient.
• It drops sand and gravel in the channel, which forms bars (ridges of sediment) and islands.
• The water splits into many thin channels that join and separate again around the bars. Seen from above, the pattern looks like a braid of hair.

• Braiding also happens where the banks are weak and the channel is wide.

• Brahmaputra in Assam (link: india-drainage-rivers): the classic Indian braided river.

• Majuli is a large river island in the Brahmaputra, in Assam. It is about 80 km long and 10–15 km from north to south, with an area of about 875 sq km, according to India's UNESCO Tentative List entry [5].
• It is a fluvial landform, meaning one made by a river. Small sand islets around it, locally called chaporis, form because the river is braided [5].
• Its shifting channels keep eating away its banks, so the island's area keeps changing.

Human use of rivers: Kallanai

• Kallanai (Grand Anicut) is an ancient stone dam on the Kaveri in Tamil Nadu. It was built by the Chola king Karikala (about 2nd century CE).
• It diverts Kaveri water into the canals of the delta. It is one of the oldest water-control structures in the world that is still in use (Class 9 NCERT).

Prelims Hooks

• Point bar: it forms on the convex (slip-off, inner) bank of a meander, not on the concave bank as NCERT states. The concave (outer) bank is the undercut bank.
• Fan vs climate: fans are high and steep in arid areas and low and gentle in humid areas.
• Kosi megafan: about 10,351 sq km. The river shifted about 112 km westward in about 200 years [2].
• Delta shapes: arcuate is the Nile and Ganga; bird's-foot is the Mississippi; cuspate is the Tiber.
• Deltas vs estuaries: the Mahanadi, Godavari, Krishna and Kaveri build deltas (Bay of Bengal). The Narmada and Tapi form estuaries (Gulf of Khambhat, Arabian Sea).
• Sundarbans: part of the world's largest delta (about 80,000 sq km), built by the Ganga, Brahmaputra and Meghna [3]. It has the largest mangrove forest in the world, about 40% in India and 60% in Bangladesh [4].
• Oxbow lake example: the Burhi Gandak near Muzaffarpur, Bihar. Local names in Bihar are mauns and chaurs.
• Majuli (Assam) is a river island in the braided Brahmaputra, about 875 sq km. Its sand islets are called chaporis [5].
• Bhabar: a chain of coalescing fans along the Siwalik foot. Streams sink into it and come back up in the Terai.
• Trap: NCERT calls the meander a channel pattern, not a landform. Incised meanders are erosional, not depositional.

Mains Points

• River avulsion and flood risk in north Bihar (GS-I/GS-III disasters):
• The Kosi drops heavy silt where it leaves the Himalaya, which raises its bed.
• The river then breaks out sideways and moves to a new course. It has shifted about 112 km west in about 200 years [2].
• Embankments trap the silt inside the channel. The bed rises even faster, so a breach becomes more destructive.

• Managing this needs silt management, early warning and floodplain zoning, not just embankments.

• Deltas: fertile but fragile:

• Delta plains such as the Ganga-Brahmaputra, Godavari-Krishna and Mahanadi support rice and jute, fisheries and dense population.
• They face cyclones, storm surges, rising seas and salt water entering the land.
• Dams upstream trap sediment, so less reaches the delta and the land sinks and wears away.

• The Sundarbans mangroves act as a natural buffer against storms [4].

• Why deltas and estuaries are spread unevenly in Peninsular India:

• The east-flowing rivers have long courses, gentle slopes and a heavy load, and enter the calmer Bay of Bengal, so they build deltas. These are the rice bowls of Andhra Pradesh, Odisha and Tamil Nadu.

• The Narmada and Tapi flow through hard-rock rift valleys and meet strong tides, so they form estuaries. Estuaries suit ports, as in the Surat–Bharuch area.

• Braided rivers and island erosion (GS-I/GS-III):

• The heavy load of the Brahmaputra makes its channels braid and shift.
• This keeps eroding the banks of Majuli, which threatens its people and its satra (Vaishnava monastery) heritage [5].
• The answer needs bank protection, mapping of channel shifts by satellite, and planned resettlement.