Q. Discuss the development of the cross-profile of a River.
The cross-profile, or transverse profile, is the section taken at right angles to a river’s course — the valley from bluff to bluff, and within it the channel from bank to bank. It is the counterpart of the longitudinal profile, which records gradient. Its development records a shifting balance between vertical incision, which cuts the notch, and the subaerial and lateral processes that open the notch out.
What the Cross-Profile Measures
- Two nested scales. The valley cross-profile records valley width, depth and side-slope angle; the channel cross-section within it records bank-to-bank width, mean depth and the width–depth ratio.
- The river does not cut the whole profile. It erodes only at bed and banks; the sides above are shaped by weathering, rainwash and mass movement. The form is a ratio between these two rates, not the work of the channel alone.
The Mechanisms Involved
- Valley deepening — vertical erosion of the floor by hydraulic action, corrasion, pot-hole drilling and solution, greatest where gradient is steep and coarse tools abundant.
- Valley widening — undercutting of the lower valley sides, provoking slumping and retreat of the walls above, assisted by sheet-wash and by gullying of the banks. Meander migration is the most effective widening agent of all.
The Developmental Sequence
Incision and the V
- Where stream power greatly exceeds resistance — steep gradient, active uplift, coarse bedload — downcutting outruns retreat of the sides and the section closes into a narrow, deep V, its floor potholed and its walls projecting as interlocking spurs.
- Where the rock is massive or weathering slow the V tightens into a gorge: the Bhagirathi and Alaknanda gorges above Devprayag, the Indus gorge at Nanga Parbat, and the Narmada’s marble slot at Bheraghat below the Dhuandhar fall.
Opening Out of the V
- As the bed nears base level, downcutting slackens while weathering of the sides continues; the sides retreat faster than the floor falls and the V opens to a wider angle.
- The side-slope angle is set by rock strength and weathering regime, not by the river — bare steep walls in arid settings, gentle convexo-concave slopes under the deep weathering of the Chota Nagpur margins.
Spur-Trimming and the Flat Floor
- Erosion shifts from the bed to the outer, concave banks; the channel swings and meanders impinge on the valley sides.
- Interlocking spurs are truncated into bluffs, and a strip of floor wider than the channel is planed across — the first floodplain.
- The meander-belt width now governs valley-floor width, which grows one meander sweep at a time.
The Open Valley
- The floor becomes many times the channel width, thickly alluviated and bounded by residual bluffs, with levees, backswamps and oxbows upon it.
- The Ganga plain below Prayagraj is the type case — a floor so wide the valley sides are invisible from the channel.
Controls on Cross-Profile Form
- Gradient and stream power set the share of energy taken by incision rather than lateral corrasion; a flashy monsoon regime widens more than a steady one.
- Lithology and structure are decisive. Resistant rock gives a deep narrow section, weak rock a broad open one; alternating hard and soft beds give structural benches and a stepped profile, as on Deccan trap flows and the Bhander caprock. Dipping strata cause uniclinal (homoclinal) shifting, which migrates the channel down-dip and leaves an asymmetrical section — steep scarp side, gentle dip side; faulting does the same abruptly.
- Climate and weathering regime fix the side-slope angle and the debris supply.
- Base level and rejuvenation superimpose a fresh narrow V inside an older open valley — the valley-in-valley form, with paired terraces and a headward-migrating knickpoint. This topographic discordance is the clearest evidence that the sequence restarts; the Chambal’s lower reach, incising into unconsolidated alluvium, has been dissected into ravines by exactly this response.
- Tectonics keeps incision ahead of widening: Himalayan valleys stay V-shaped along their whole mountain course, while peninsular rivers of like discharge opened out long ago.
The Channel Section Within the Valley
- The width–depth ratio is the working index of channel shape and turns on bank cohesion — silty-clayey banks hold a narrow deep section, non-cohesive sand a wide shallow one (Schumm) — and vegetation narrows it further.
- At a meander the section is strongly asymmetric: a pool scoured at the outer, concave bank beneath an undercut cliff slope, a point bar built against the inner slip-off slope, and helical secondary circulation carrying bed load inward to maintain both.
- Braided reaches carry a very wide, shallow, multi-thread section over shifting bars — the Brahmaputra at Majuli, whose belt far exceeds the width any single thread needs.
- The section is not fixed even at one point: at-a-station hydraulic geometry shows width, depth and velocity all changing as a flood passes.
Fluvial V and Glacial U
- A glacier fills its valley and abrades floor and walls together, giving a parabolic U-trough with truncated spurs and hanging tributaries; a river erodes a narrow bed only, so its section is a V.
- The Himalaya preserves the composite — a glacial trough above, with a sharp post-glacial V notched into its floor: a valley-in-valley of climatic, not tectonic, origin, as below Gangotri.
- A wide, flat-floored mature fluvial valley is loosely called U-shaped. It is not: its floor is alluvial fill between bluffs, not an abraded rock trough.
Critical Assessment
- The sequence is not a timetable. Its ancestry is Davisian and assumes uninterrupted development toward base level, whereas real basins are polycyclic and their profiles polygenetic.
- Hack’s dynamic equilibrium (1960) reads a cross-profile as the present ratio of resistance to erosive process, so a gorge and an open valley may coexist in one basin at one time, as the Narmada does within a few tens of kilometres of Bheraghat.
- Dating has replaced inference. Cosmogenic-nuclide exposure ages and luminescence dating of terrace fills measure incision rates directly, turning a descriptive sequence into a testable chronology.
Conclusion
The cross-profile is best read not as a stage in an ageing sequence but as an instantaneous balance between the rate at which the bed is lowered and the rate at which the valley sides are consumed. Where uplift or resistant rock keeps incision ahead, the V persists indefinitely; where base level is approached, lateral swinging widens the floor faster than the bed falls. Structure, climate and tectonics decide which term dominates, and the profile is that arithmetic made visible.