Transportation (Traction, Saltation, Suspension, Solution)

Transportation Processes

Traction

Traction is the process where large, heavy particles like boulders and pebbles are rolled along the riverbed by the force of the water. These particles are too heavy to be lifted, so they move by rolling or sliding.

Saltation

Saltation involves smaller particles such as sand grains that are too heavy to be suspended but light enough to be briefly lifted off the riverbed. These particles bounce or hop along the riverbed in a series of short jumps.

Suspension

Suspension occurs when very fine particles like silt and clay are carried within the water itself. These particles remain suspended in the flow, giving the river a cloudy or muddy appearance, especially after heavy rain.

Solution

Solution is the process where minerals dissolve in the water and are transported in solution. This includes substances like calcium carbonate from limestone, which cannot be seen but are carried invisibly by the river. This process involves chemical weathering dissolving minerals into the water.

For instance, in the River Thames, large pebbles are moved by traction near the upper course, while fine silts are carried in suspension downstream.

Factors Affecting Transportation

River velocity and discharge are crucial. Faster water has more energy to transport larger and more sediment. After heavy rainfall, discharge increases, so the river can carry more material.

Particle size and shape affect how easily sediment is transported. Smaller, smoother particles are easier to carry, especially in suspension or solution. Larger, angular particles tend to move by traction or saltation.

Water turbulence helps lift and keep particles suspended. Turbulent flow creates swirling eddies that can pick up sediment from the riverbed.

River gradient (steepness) influences velocity. Steeper gradients in the upper course mean faster flow and more energy for transportation, while gentler slopes in the lower course reduce velocity and transportation capacity.

For example, after a storm, a steep upland river like the River Severn will carry more sediment by saltation and traction due to increased velocity and turbulence.

Role in River Landscape Formation

Transportation moves sediment downstream, shaping river channels and influencing the formation of features. Sediment carried by traction and saltation can abrade the riverbed and banks, linking closely with erosion processes.

Suspended and dissolved materials affect water quality and sediment load, which influence how and where deposition occurs (deposition is a separate process covered in other topics).

The amount and type of sediment transported affect the river channels shape, width, and depth. For example, a river carrying a high sediment load may have a wider, shallower channel.

Learning Example: Calculating River Velocity's Effect on Transportation

If a rivers velocity increases from $1\,\mathrm{m\,s^{-1}}$ to $3\,\mathrm{m\,s^{-1}}$, the energy available to transport sediment increases significantly. This means larger particles can be moved by saltation or even traction, and more sediment can be carried in suspension.

For example, a pebble that was previously too heavy to move might now be rolled along the bed (traction), and fine sand might be lifted into suspension.