Load

The word load is used to describe the rock particles that are carried by a river. There are three categories:

Bedload – large particles carried only at times when the river has much energy.

Suspended load – small particles that can be carried by the water, making it look muddy.

Dissolved load – minerals carried by solution that are not visible.

A river obtains its load from two main sources:

About 90% comes from the movement of material down the slopes of the river valley by weathering and mass movement.

10% comes from the erosion of the bed and banks of the river channel.

The ability of a river to transport depends on:

Location and supply of material

Amount of material available

Character of the material

Volume of water

Amount of energy available to do the work.

River capacity – the total amount of load that a river can carry at any one time. Capacity increases as discharge increases.

River competence – the maximum size or weight of material a river can transport. In times of flood, a river’s competence will increase – it will be able to carry bigger particles.

Capacity and competence increase downstream, as discharge increases, and during times of flood.

Processes of River Transportation

Traction – large boulders are rolled along the river bed.

Saltation – pebbles, sand and gravel bounce along the river bed.

Suspension – fine particles are carried by the water, making the river look muddy.

Solution – minerals that are dissolved in

the water, and therefore not visible.

Animation –click here!

The Hjulstrom curve is a graph that shows the relationship between the velocity of a river and the particle sizes that may be transported. It shows the velocity needed to pick up material of a certain size (velocity needed for erosion to take place), and the settling velocity when deposition will occur.

Clay Silt Sand Pebbles Cobbles Boulders

1000

100

10

Vel

ocit

y (c

um

ecs)

Particle Diameter (mm)

0.04 0.6 2 64 256

Particles eroded

Particles deposited

Particles transported