Q. Discuss the concept of graded profile of a river and the factors that control it.
Grade is the fluvial expression of equilibrium. A graded river is one whose longitudinal profile — the thalweg from source to mouth — has been adjusted so that the slope available yields exactly the velocity needed to move the load delivered to it, with neither net cutting nor net filling over a span of years. The resulting long profile is a smooth concave-upward curve. Grade is therefore a self-regulating tendency, not a finished landform.
The concept and its authorship
- G. K. Gilbert (1877), in the Report on the Geology of the Henry Mountains, first gave the term technical content: by his law of declivities a stream adjusts its declivity until capacity matches the load supplied, and a stream so adjusted, neither cutting nor filling, is at grade.
- W. M. Davis absorbed grade into the geographical cycle as a stage, employing the term for “the balanced condition of a mature or old river”, a balance between “the capacity of a river to do work, and … the quantity of work that a river has to do” (Davis, 1902). Grade was attained at maturity and thereafter degraded slowly towards the peneplain.
- J. H. Mackin (1948) supplied the definition still quoted: a graded stream is one in which, over a period of years, slope is delicately adjusted to provide, with the available discharge and the prevailing channel characteristics, just the velocity required for transportation of the load supplied from the drainage basin.
- Mackin’s second sentence carries the theory. The graded stream is a system in equilibrium, and any change in any of the controlling factors displaces that equilibrium in a direction that tends to absorb the effect of the change. Grade is defined by negative feedback, not by any particular gradient value.
- The difference is decisive: Davis’s grade is a stage in a sequence; Mackin’s is a condition of self-regulation that a reach may hold, lose and recover repeatedly within one cycle.
Why the long profile is concave upward
- Discharge increases downstream as tributaries and groundwater are added, so the same load can be moved on a progressively gentler slope.
- Calibre falls downstream through attrition and hydraulic sorting; silt and fine sand demand far less shear stress than headwater boulders, releasing the river from steep gradients.
- Hydraulic efficiency improves with channel size — a larger cross-section has a greater hydraulic radius and proportionately less wetted perimeter, so frictional loss falls and velocity is sustained on a lower slope.
- Erosion is accordingly minimal at the source (little water, few tools) and minimal again near the mouth (gentle gradient, low velocity), but maximal in the middle course — the distribution that bends the profile into a concave curve.
The factors that control grade
Leopold and Maddock (1953) sorted the variables: independent controls the river can only adjust to — discharge, load and ultimate base level; semi-dependent channel variables it can alter — width, depth, roughness, grain size, velocity and pattern; and the dependent outcome, the water-surface slope that is the graded profile.
Discharge and its regime
- Mean discharge sets transporting power; a rise in discharge unmatched by load produces surplus energy, incision and a flattening of slope.
- Variability matters as much as volume: monsoon rivers do most of their annual work in a few weeks, so the channel-forming flow is the bankfull discharge of about 1.5-year recurrence, not the mean.
Sediment load — quantity and calibre
- Load supplied from the basin, not load already in the channel, is the control: a river fed coarse debris holds a steeper slope, fed silt a far gentler one.
- Load exceeding capacity forces aggradation, raising the bed until transport is restored — the Kosi, delivering a huge Himalayan sediment yield onto a low-gradient plain, has built its megafan and repeatedly avulsed across it, most destructively at Kusaha in 2008.
Channel geometry, roughness and pattern
- Width-depth ratio, bed roughness and planform absorb change before slope does — a river narrows, deepens or switches between meandering and braided form long before it regrades a whole profile.
Lithology and structure
- Resistant bands interrupt grade and generate knickpoints, breaks of slope that migrate headward and mark reaches not yet graded.
- Peninsular rivers are largely graded across hard rock, and their falls are structurally controlled rather than products of rejuvenation: Jog on the Sharavati, Chitrakote on the Indravati, and Dhuandhar where the Narmada crosses the marble gorge at Bhedaghat.
Base level, tectonics and climate
- Lowering of base level rejuvenates the profile, producing incision, terraces and a new graded segment extending upstream; a rise drowns the lower course and forces deposition.
- Active uplift renews slope faster than the river consumes it, which is why the Himalayan headwaters of the Ganga, Sutlej and Brahmaputra remain ungraded bedrock reaches while their plains courses approach grade.
- Climate and vegetation govern runoff and sediment delivery together: deforestation raises load and drives aggradation, while stable cover with higher rainfall raises discharge relative to load and drives incision.
Human intervention
- Dams trap sediment and release hungry water — the Sutlej below Bhakra and the Mahanadi below Hirakud show downstream degradation and bed coarsening, as the Colorado does below Glen Canyon.
- Barrages, embankments and sand mining reset the controls: the Kosi embankments confine sediment and raise the bed between them, while mechanised sand extraction lowers local base level and triggers headward incision.
Grade as a tendency, not an end state
- Graded and ungraded reaches coexist in one river. The Ganga is effectively graded across the plains yet ungraded in its Himalayan gorges, and the profile is disturbed afresh by every tributary delivering surplus load.
- Hack (1960) replaced the Davisian sequence with dynamic equilibrium, in which form is adjusted to process at every moment and relief differences express rock resistance rather than stage.
- Schumm’s work on complex response and geomorphic thresholds showed adjustment to be episodic: one base-level fall yields alternating incision and deposition before a new grade settles, and a river may stay at grade while the character of that graded condition itself changes.
Conclusion
The graded profile is a statement about behaviour rather than shape: a river holds the slope that just moves its load, and returns towards it whenever disturbed. Perfect grade is rare, because tectonics, climate and now dams, embankments and sand mining keep resetting the controls. The concept earns its place by being predictive — any interference with discharge, load or base level is repaid by an adjustment elsewhere along the profile.