A river transports its sediment load in a variety of ways. The methods of
transport are also used to describe the various loads of a river, i.e. the bed
or traction load, suspension load, dissolved load, and suspended load).
The sediment load varies from river to river, along the course of one river or
in the same place at different times.
This is because the velocity of the water is crucial in determining the way
that sediment is transported. The relationship between erosion, transport and
deposition of sediment is complex and can be shown by the Hjulstrom diagram.
This is based on experimental work rather than natural channels, but it shows
the principles involved.
Entrainment is the process of starting the particles of sediment moving - the
opposite of settling.
High velocities result in sediments being transported in the river flow, while
low velocities result in sediment being deposited.
Medium sand (0.25-0.5 mm in diameter) is moved at the lowest velocities.
Larger, heavier sediments need higher velocities to start movement.
Silt and clay need higher velocities than their size would suggest
are cohesive (they stick together) and so, in fact, are bigger than they should
Once set in motion, fine particles can be transported even if the velocity
Larger coarse particles are deposited rapidly as velocity falls. In
with mainly boulders and gravel, transport only occurs at high flows.
In natural channels the situation will be more complex. For example, small
particles may be sheltered by larger particles and therefore they are not moved.
Velocity of flow is variable across and vertically in a natural channel,
this will affect the processes.
Sediment transported under lower-flow velocities, as bed load, may become
suspended load under high velocities.
A river must have energy available to perform the work of transporting
sediment. A river's power is the energy available to overcome friction and to
move sediment. This will be greatest in rivers with high discharge, high
downstream gradient, and an efficient channel. If the river has just enough
energy in a particular section (reach) to transport the sediment available,
then it is in equilibrium. If it has more power available than needed to
transport its sediment load, the river will have excess energy and will erode
its channel. If there is less energy available than is needed to transport its
load, then deposition of the sediment will result.
Two further terms are useful in describing sediment transport. The competence is the maximum sediment particle size that can be carried at a particular
velocity. The capacity is the total load of sediment that the river can carry.
These will vary with both discharge and velocity.
The velocity at which a sediment particle drops to the channel bed is called
the settling velocity . This depends upon the size and shape and density of the
sediment particle. Deposition may be temporary on the channel bed and the
sediment may be moved again at a time of higher flow. In other situations there
is a net deposition of sediment, and a deposition landform results, e.g.
floodplains and point bars on the inside of meander bends.