UNSTRATIFIED
OXBOW LAKES
STRATIFIED BEDS
Fluvial Depositional Landforms
Alluvial Fans · Floodplains, Levees & Point Bars · Meanders, Oxbow Lakes & Braided Channels · Deltas (28 Micro-Realms)
From Piedmont Foothill Cones to Coastal Deltas (NCERT Chapter 7)
Whenever stream gradient gentles and velocity drops, running water sheds its sediment load sequentially from coarse to fine: 1) Alluvial Fans at mountain-foot slope breaks (low cones in humid climates, steep high cones in arid climates), 2) Floodplains, Natural Levees, and Point Bars along mature/old valleys, 3) Meanders (a channel pattern, not a landform!), Oxbow Lakes, and Braided Channels, and 4) well-sorted, stratified Deltas at the river mouth.
I. Alluvial Fans, Floodplains, Natural Levees & Point Bars (NCERT Core)
Alluvial Fans (Foothill)
- Mountain streams exit onto foot-slope plains
- Abrupt drop in gradient & velocity
- Dumps coarse load as cone-shaped fan
Fan Distributaries
- Stream choked by its own coarse load
- Shifts position & splits into channels
- Called Distributaries (NCERT)
Humid vs. Arid Fans (NCERT)
- Humid: Low cones with gentle slope
- Arid/Semi-Arid: Steep high cones
- High-Yield UPSC Prelims distinction!
Bajadas & Piedmont Aprons
- Adjacent alluvial fans grow laterally
- Coalesce into continuous piedmont apron
- Bajada (arid) & Bhabar belt (Himalayas)
Active vs. Inactive Floodplain
- Active: Riverbed made of channel deposits
- Inactive: Floodplain above the banks
- Major landform of river deposition (NCERT)
Channel vs. Spill Deposits
- Inactive floodplain has 2 deposit types:
- 1) Fine overbank flood spill (silt/clay)
- 2) Coarse channel deposits in old cut-offs
Natural Levees (NCERT)
- Low, linear & parallel ridges
- Of coarse deposits along banks of large rivers
- Often cut into individual mounds
Point Bars (Meander Bars)
- Also known as Meander Bars (NCERT)
- Found on CONVEX side of meanders
- Linear mixed-size sediments of uniform profile
Backswamps & Yazoo Streams
- High natural levees trap floodwaters behind
- Creates poorly drained Backswamps
- Yazoo stream flows parallel before joining
II. Meanders (Channel Pattern), Oxbow Lakes & Braided Channels (NCERT)
Meander ≠ A Landform!
IT IS A CHANNEL PATTERN
- Verbatim NCERT Prelims statement!
- Meander is NOT a landform
- It is only a type of channel pattern
3 Reasons Why Rivers Meander
- 1) Propensity of water on gentle gradients to work laterally
- 2) Unconsolidated alluvium irregularities
- 3) Coriolis force deflecting water (NCERT)
Concave vs. Convex Bank
- Concave bank: Active undercutting (Cut-off scarp)
- Convex bank: Deposition (Slip-off slope)
- Makes meander grow into deep loops
Oxbow Lakes (Cut-Offs)
- Deep meander loop gets cut off at inflection point
- Abandoned crescent loop = Oxbow Lake
- Gradually silts up into marshes & dies
Braided Channels (NCERT)
- Less discharge + heavy sediment load
- Central sand/gravel bars divert flow
- Interlacing network of shifting threads
III. Deltas: Stratification, Gilbert Beds & Morphological Types (NCERT)
Deltas (Coastal Cones)
- Like alluvial fans but at the sea coast
- River dumps load into ocean/sea
- Cone spreads outward as a Delta
Delta vs. Fan Stratification
- Unlike fans, delta deposits are CLEARLY
- WELL-SORTED and STRATIFIED (NCERT)
- Coarsest settles first; finest carried to sea
Topset, Foreset & Bottomset
- G.K. Gilbert's 3-layer delta structure
- Topset (flat), Foreset (inclined prograding),
- Bottomset (horizontal deep marine clay)
Electrolytic Flocculation
- Fine clay stays suspended in fresh water
- Saltwater ions ($\text{Na}^+, \text{Mg}^{2+}$) neutralize charge
- Clays clump (flocculate) & sink at mouth
Arcuate (Fan-Shaped) Delta
- Convex rounded seaward margin
- Many shifting distributaries + wave smoothing
- Ganga-Brahmaputra, Nile, Mahanadi, Godavari
Bird's-Foot (Digitate) Delta
- River discharge dominates weak waves/tides
- Levee-lined channels jut far into sea
- Mississippi River & Subarnarekha
Estuarine & Cuspate Deltas
- Estuarine: Submerged funnel mouth (Narmada, Tapi, Seine)
- Cuspate: Tooth-shaped wave delta (Tiber, Ebro)
Delta Favourable Conditions
- Abundant river load & no big lakes upstream
- Shallow continental shelf at mouth
- Weak tidal currents & stable coast
IV. Indo-Gangetic Depositional Zonation & Indian Exemplars
1. Bhabar Pebble Belt
- 8–16 km porous boulder-pebble alluvial fans
- Parallel to Shiwalik foothills
- Streams disappear underground here!
2. Terai Marshy Belt
- 10–20 km south of Bhabar
- Underground streams resurface
- Swampy fine silt, tall grass & Dudhwa tigers
3. Bangar vs. 4. Khadar
(OLD)
(NEW)
- Bangar: Old upland terrace + Kankar nodules
- Khadar: Young active floodplain renewed annually
- Khadar is super-fertile fine silt
Kosi Mega-Fan & Kanwar Lake
& KANWAR OXBOW
- Kosi ("Sorrow of Bihar"): World's largest alluvial mega-fan
- Kanwar (Kabartal) Lake, Bihar: Asia's largest freshwater Oxbow!
Majuli Braided River Island
- Brahmaputra braided channel in Assam
- World's largest inhabited river island
- Formed by channel avulsion & mid-bars
East Deltas vs. West Estuaries
DELTAS
ESTUARIES
- East (Ganga, Mahanadi, Godavari, Krishna, Kaveri): Gentle slope $\to$ Deltas
- West (Narmada, Tapi): Hard rift + steep drop $\to$ Estuaries
Core Distinctions
• Location: Alluvial fans form at mountain foot-slope plains (inland), whereas Deltas form where rivers enter the sea or ocean.
• Stratification & Sorting: Unlike alluvial fans (which are coarse and poorly sorted), deposits making up deltas are very well sorted with clear stratification—coarsest materials settle out first and finest clays are carried out to sea.
• Climatic Fan Shape: Alluvial fans in humid areas show normally low cones with gentle slopes, whereas in arid and semi-arid climates they appear as steep high cones.
• "Meander is not a landform but is only a type of channel pattern."
• Point Bars (Meander Bars) are found on the convex (inner) side of meanders of large rivers, whereas active undercutting occurs on the concave (outer) bank.
• Natural Levees are low, linear, parallel ridges of coarse deposits along the banks of large rivers.
From the Shiwalik foothills southward: Bhabar (porous pebble fans where streams vanish) $\to$ Terai (marshy belt where streams re-emerge) $\to$ Bangar (older upland alluvium with calcareous Kankar) $\to$ Khadar (young, fertile active floodplain renewed every monsoon).
Fluvial Depositional Matrix
NCERT Chapter 7 & Indian Exemplar Key