Deposition occurs when transport can no longer sustain part of the load. Grain size, cohesion and turbulence make thresholds differ.
Discharge is water volume passing a point per unit time: cross-sectional area multiplied by mean velocity. Gradient supplies the downhill energy driving flow. Channel shape and roughness determine how much energy is lost through friction, so a steeper reach does not automatically have the greatest discharge.
Competence is the largest particle size a flow can move. Capacity is the total load it can transport under stated conditions. Both often rise as flow becomes more energetic, but sediment availability limits the actual load. A fast clear river may carry little material because little is supplied.
Deposition occurs when flow loses the ability to keep part of its load moving, or when sediment supply exceeds local transport capacity. A wider or shallower spreading flow, declining flood discharge, an obstruction or entry into still water can encourage it. Coarser grains commonly settle before finer ones.
Very fine clay particles need little turbulence to remain suspended once entrained, yet cohesive clay beds can resist initial erosion. Do not use a simple rule that the smallest particles are always easiest to erode. Dissolved material requires a chemical change to precipitate; slowing alone does not make it drop out as sand.
Vegetation increases resistance and traps sediment; dams slow water in reservoirs. Downstream of a sediment-trapping dam, reduced supply can encourage erosion if sufficient transporting power remains. Channel straightening, dredging and sand extraction alter local conditions, with effects that may extend upstream and downstream.
Step by step
Compare flow and supply
Consider both available energy and the sediment arriving.
Identify the change
Slower flow or excess supply can produce deposition.
Separate grain behaviour
Large grains, suspended clay and dissolved ions respond differently.
Worked example: An illustrative sediment pulse
A tributary delivers a large sand pulse after a slope failure. In the main channel, some sand settles where the river widens and flow spreads. The river can still transport fine silt farther downstream. Deposition is selective, not an instantaneous halt to all transport.
Watch out for this
A river deposits all of its load whenever velocity falls.
Some fractions settle while others remain suspended or dissolved. The size, cohesion and chemical state of the load matter.
Check your understanding
Which statement correctly distinguishes competence and capacity?
- Competence is river width; capacity is river depth.
- Competence concerns the largest particle moved; capacity concerns total load transported.
- Both mean annual rainfall over the basin.