THALWEG & RIFFLES
TRELLIS · INLAND
ARAB 77%
B.o.B
SUPERIMPOSED
River Channels & Drainage
Channel Planforms & Sinuosity · Basin Morphometry · Antecedent vs. Superimposed · NCERT Drainage Patterns & Regimes (28 Micro-Realms)
From Channel Planform Kinematics to Continental Drainage Architecture
A river system operates simultaneously at two scales: the local Channel Planform (governed by the Thalweg line, riffle-pool spacing, and Sinuosity Index across straight, meandering, braided, and anastomosing reaches) and the regional Drainage Network (governed by rock structure, tectonic history, and slope—yielding Antecedent and Superimposed rivers alongside the four classic NCERT drainage patterns: Dendritic, Radial, Trellis, and Centripetal).
I. Channel Planform Morphology, Sinuosity & Hydraulic Geometry
Channel Planform Spectrum
- Leopold & Wolman classification
- Controlled by slope, discharge & load
- Single-thread vs multi-thread channels
Sinuosity Index ($SI \ge 1.5$)
SI ≥ 1.5 → MEANDERING
- Ratio of channel length to valley length
- $SI < 1.05$: Straight; $1.05\text{–}1.5$: Sinuous
- $SI \ge 1.5$: True Meandering channel
The Thalweg Line
- Line connecting lowest bed points
- Carries maximum velocity filament
- Swings from bank to bank in bends
Riffle & Pool Sequences
- Riffle: Shallow, fast coarse gravel bar
- Pool: Deep, slow concave scour hollow
- Spaced every $5\text{ to }7\times$ channel width
Meander Scaling Laws
- Meander wavelength $\lambda \approx 10\text{–}14 \times W$
- Radius of curvature $r_m \approx 2\text{–}3 \times W$
- Scales directly with bankfull discharge
Braided vs. Anastomosing
SHIFTING BARS
STABLE ISLANDS
- Braided: Unstable shifting sand/gravel bars
- Anastomosing: Cohesive vegetated islands
- Separated by stable permanent channels
Avulsion & Palaeochannels
- Abrupt abandonment of river channel
- Forms new course on lower floodplain
- Kosi avulsions & Ghaggar palaeochannel
II. Basin Morphometry & Genetic Stream Classification (NCERT)
Catchment vs. Watershed
- Drainage Basin: Large river catchment area
- Watershed: Small rivulet/stream unit
- NCERT Chapter 3 scale distinction!
Bifurcation Ratio ($R_b$)
NORMAL RANGE: 3.0 TO 5.0
- Horton's Law of Stream Numbers
- Ratio of streams of order $u$ to $u+1$
- High $R_b$ = elongated structural control
Drainage Density ($D_d = \Sigma L / A$)
- Total stream length per unit basin area
- Coarse/Low $D_d$: Permeable sandstone/forest
- Fine/High $D_d$: Impermeable shale/badlands
Concordant Stream Quartet
- Consequent: Follows initial dip slope
- Subsequent: Erodes strike weak beds ($90^\circ$)
- Obsequent (opposite dip) & Resequent
Insequent Streams
- Develops without structural control
- Homogeneous granite or flat alluvium
- Builds classic Dendritic networks
Antecedent Rivers (Discordant)
- Existed BEFORE Himalayan folding began
- Downcut matches rate of mountain rise
- Indus, Sutlej, Ganga, Kali, Kosi, Subansiri
Superimposed Rivers
- Formed on younger horizontal cover strata
- Maintains path into buried older structure
- Damodar, Subarnarekha, Chambal, Banas, Son
III. Classic Drainage Patterns (NCERT Core — Prelims High-Yield)
1. Dendritic Pattern
- Resembles branches of a tree (NCERT)
- Uniform rock resistance & flat plains
- Northern Plains (Indus, Ganga, Brahmaputra)
2. Radial Pattern
↙ ↓ ↘
- Rivers originate from a hill/dome
- And flow outward in all directions
- Amarkantak range (Narmada, Son, Mahanadi)
3. Trellis Pattern
- Primary tributaries flow parallel to each other
- Secondary tributaries join them at right angles
- Folded Himalayan ranges & Singhbhum
4. Centripetal Pattern
- Rivers discharge waters from all directions
- Into a central lake or depression (NCERT)
- Sambhar Lake (RJ) & Loktak Lake (Manipur)
Rectangular Pattern
- Both trunk & tributaries make $90^\circ$ bends
- Follows intersecting fault & joint grids
- Vindhyan Sandstone & Palamau region
Annular & Parallel Patterns
- Annular: Ring-like concentric dome (Pithoragarh)
- Parallel: Steep coastal escarpment streams
- Short swift West-flowing Sahyadri rivers
Barbed & Deranged Patterns
- Barbed: Boat-hook junction from river capture
- Deranged: Unintegrated glaciated swamps
- High Karakoram & Ladakh glacial valleys
IV. The Great Indian Water Divide, River Regimes & Stream Piracy (NCERT)
Indian Water Divide (77/23)
- Delhi Ridge $\to$ Aravallis $\to$ Vindhyas $\to$ Sahyadris
- 77% area drains to Bay of Bengal
- 23% area drains to Arabian Sea (NCERT)
Indo-Brahm (Shiwalik) River
- Miocene mega-river from Assam to Punjab
- Pleistocene Potwar Plateau uplift +
- Malda Gap down-thrusting split it into 3!
River Capture Anatomy
- Steeper stream cuts headward across divide
- Sharp $90^\circ$ turn = Elbow of Capture
- Dry notch = Wind Gap; shrunk river = Misfit
Himalayan Stream Piracy
SARASWATI
TSANGPO
- Yamuna captured Tons/Saraswati headwaters
- Bhagirathi/Alaknanda trans-Himalayan cuts
- Arun (Kosi) captured Tibetan tributaries
River Regimes (NCERT)
- Pattern of seasonal flow in a channel
- Himalayan: Glacial melt (summer) + Monsoon
- Peninsular: Purely rainfall-controlled
Ephemeral Inland Drainage
- Endorheic streams never reach the ocean
- Luni, Machhu, Rupen, Saraswati, Banas
- Vanish in Rann of Kutch salt flats
Master Drainage Matrix
- Completes Fluvial Geomorphology triad
- Links Realms 44, 45, 46 & 47
- Transitions to Realm 48 (Karst/GW)
Core Distinctions
• Dendritic: Resembles the branches of a tree (Rivers of the Northern Plains).
• Radial: Rivers originate from a central hill/dome and flow outward in all directions (Amarkantak range — Narmada, Son, Mahanadi).
• Trellis: Primary tributaries flow parallel to each other and secondary tributaries join them at right angles.
• Centripetal: Rivers discharge their waters from all directions into a central lake or depression (Sambhar Lake, Loktak Lake).
Drainage Basin and Watershed are both areas drained by a river and its tributaries, but they differ in scale: "Catchment areas of large rivers are called river basins while those of small rivulets and rills are often referred to as watersheds. Watersheds are small in area while the basins cover larger areas."
3. Trellis vs. Rectangular PatternBoth feature $90^\circ$ right-angle junctions, but Trellis forms in folded sedimentary topography (parallel anticlines and synclines), whereas Rectangular forms in faulted and jointed hard rock where both trunk streams and tributaries make sharp right-angle bends.
Channels & Drainage Matrix
High-Yield UPSC Prelims Reference