What Does a River Deposit When It Slows Down?

When a river's velocity decreases—at a bend, a confluence, a reservoir, or its mouth—it loses the competence to transport sediment. Deposition follows a predictable sequence:

Sorting by Grain Size (Hjulström Curve Principle)

1. Coarsest first: Boulders, cobbles, gravel (bedload) — deposited at steep gradient breaks, channel bars.
2. Sand: Saltation load — forms point bars (inside meanders), mid-channel bars, delta foresets.
3. Silt: Suspended load — settles on floodplains (overbank deposits), delta topsets, prodelta.
4. Clay: Colloidal suspension — travels farthest; deposits in quiet water (lakes, estuaries, prodelta, abyssal plain).

Resulting Landforms

• Alluvial fans: At mountain fronts (sudden gradient drop).
• Meander point bars: Inside bends (helicoidal flow).
• Floodplains & natural levees: Overbank fines + coarse ridge at channel edge.
• Deltas: Topset (braided), foreset (steep), bottomset (fine) — at river mouths.
• Braided channel bars: High sediment, variable flow.

Controls on Deposition

• Sediment supply: Erosion rate upstream (tectonics, climate, land use).
• Flow regime: Flood magnitude/frequency.
• Base level: Sea level, lake level, dam pool.
• Channel geometry: Width, depth, sinuosity.

Deposition builds the river's own landscape—creating habitat, fertile soil, and the stratigraphic record.