River Processes: Transportation

This is for all levels:

Rivers can either deposit, erode or deposit material.

Transportation

Load is either transported through suspension, solution or bed load (traction & saltation). For sediment to move resisting forces have to overcome, competent velocity has to be achieved (this is the lowest velocity at which particles of a particular size are set in motion), and critical tractive force must be achieved (This is when drag and embedded particle inertia is overcome and the particle begins to move).

Traction: Traction occurs when the largest cobbles (100-1000mm) and boulders (bed load) roll or slide along the bed. The largest of these may only be moved during times of extreme flood (high discharge).

Saltation: Bed load is either moved through saltation or traction. Saltation occurs when pebbles (1-100mm), sand (0.1-1mm) and gravel are temporarily lifted by the current and bounced along the bed in a hopping motion.

Solution: If the bedrock of the river is readily soluble, it is constantly dissolved in flowing water and removed in solution. Except in limestone areas, the material in solution forms only a relatively small proportion of total load.

Suspension: Very fine particles of clay and silt (0.001-0.1mm) are dislodged and carried by turbulence in a fast-flowing river. The greater the turbulence and velocity, the larger the quantity and size of particles which can be picked up. The material held in suspension usually forms the greatest part of the total load; it increases in amount towards the river’s mouth, giving the water its brown or black colour.