Last reviewed 16 Sept 2026 · 9 min read
Classification of rivers
Rivers change character from source to mouth:
| Stage / type | Features |
|---|---|
| Mountain (incised / rocky) stage | Steep slopes, high velocities, rocky or boulder beds, deep gorges; little sediment deposition; hydropower sites |
| Boulder (sub-mountain) stage | Beds of boulders, gravel and shingle; wide shallow channels in the foothills (bhabar/terai); unstable in floods |
| Alluvial (trough) stage | Flat slopes, sandy/silty beds and banks; channels constantly adjust — meandering, braided or straight; subdivided into aggrading, degrading and stable (regime) rivers |
| Delta stage | Near the sea — very flat; river splits into distributaries; heavy deposition |
| Tidal stage | Levels and flows influenced by tides |
Behaviour of alluvial rivers
Meandering
A meandering river flows in a series of alternate bends. On a bend, secondary (helicoidal) currents carry surface water to the outer (concave) bank and bottom water with sediment towards the inner (convex) bank — the outer bank erodes and deepens, and a point bar is deposited on the inner bank. The meanders therefore migrate progressively downstream and sideways.
- Sinuosity (meander ratio) = channel length ÷ straight valley length; a river with sinuosity above about 1.5 is generally called meandering.
- Meander length, width and amplitude increase with the dominant discharge (roughly in proportion to ).
- Cut-off — when a loop becomes very pronounced, the river breaks through its narrow neck during a flood; the abandoned loop becomes an ox-bow lake. After a cut-off, the slope and velocity locally increase and new meanders begin to form.
Braiding
A braided river flows in several interlacing channels separated by sand bars and islands. Causes: excess sediment load that the river cannot carry, steep slopes, highly erodible banks, and widely fluctuating discharges. Example: Kosi and Brahmaputra.
Aggradation and degradation
- Aggrading river — bed rises by deposition when sediment supply exceeds transport capacity (e.g. upstream of a dam or weir — backwater zone; below a confluence with a heavily loaded tributary; rivers confined by embankments).
- Degrading river — bed lowers by erosion when the river carries less sediment than its capacity (e.g. downstream of a dam, which traps sediment and releases clear water; after a cut-off).
Sediment transport
| Mode | Description |
|---|---|
| Bed load | Particles roll, slide or jump (saltation) along the bed |
| Suspended load | Finer particles kept in suspension by turbulence |
| Wash load | Very fine particles (not present in the bed) carried from the catchment |
DuBoys' tractive force on the bed of a wide channel:
Sediment motion begins when exceeds the critical tractive stress .
Shields' criterion: motion starts when the dimensionless shear stress
exceeds a critical value (about 0.03–0.06 for fully rough turbulent flow, depending on particle Reynolds number).
Bed-load formulas: DuBoys, Shields, Meyer-Peter–Müller, Einstein; suspended load from the Rouse distribution.
Bed forms (as velocity increases): plane bed → ripples → dunes (lower regime, high resistance) → transition (washed-out dunes) → plane bed with sediment movement → antidunes and standing waves (upper regime). Bed forms change the roughness and hence the stage–discharge relationship.
Waterway and scour depth
- Regime waterway (wetted perimeter):
- Normal scour depth (below HFL) when the waterway is not contracted:
- When the waterway is contracted (discharge intensity per metre):
The maximum scour depth at a particular point is a multiple of the normal scour depth — about 1.25R in straight reaches, increasing at bends, and about 2R or more at noses of guide banks, piers and severe bends (values are prescribed in codes such as IRC for bridges). Foundations, cut-offs and launching aprons are designed for these depths.