How Rivers Transport Load: The Four Methods

🚛 How Rivers Transport Load: The Four Methods

Transportation is the movement of eroded material (the load) downstream. The type of transport depends on the size of the particle and the velocity (energy) of the river. A river's ability to transport load depends on two properties: its capacity (the total amount of load it can carry) and its competence (the maximum size of particle it can carry). Both increase when velocity increases.

1. Traction

Traction moves the largest, heaviest particles — boulders and cobbles — by rolling them along the riverbed. The particles never leave the bed; they are simply dragged forward by the force of the water. You can sometimes see this happening in a fast-flowing upland stream after heavy rain: large stones visibly rolling and grinding along the bottom.

2. Saltation

Saltation moves medium-sized particles — pebbles and coarse sand — in a hopping or bouncing motion. The current lifts a particle briefly off the bed, carries it a short distance, then drops it. When it lands, it may dislodge another particle, sending it into saltation in turn. This creates a characteristic "bouncing" movement along the bed, visible in shallow upland streams.

3. Suspension

Suspension carries the finest particles — silt and clay — within the body of the flowing water itself. These particles are so light that the turbulence of the flow keeps them permanently lifted off the bed. Suspended load is what makes rivers look brown or cloudy during and after rainfall. It represents the largest proportion of total load carried by most rivers — the River Thames carries millions of tonnes of suspended sediment to the sea each year.

4. Solution

Solution carries dissolved minerals — the products of corrosion — invisibly within the water. There is no visible evidence of this form of transport; the river simply carries dissolved calcium carbonate, sodium chloride and other minerals as ions within the water. Solution transport is particularly important in limestone and chalk catchments.

The Hjulström Curve: Competence and Deposition (Higher Tier)

The Hjulström curve shows the relationship between flow velocity and whether a particle is eroded, transported, or deposited. The key insight is that deposition occurs when velocity drops below a threshold — the river no longer has the competence to carry a particle and drops it. This explains why deposition occurs in three key situations:

• On the inside of meander bends, where flow is slower (deposition builds point bars)
• Where a river enters the sea or a lake, and velocity falls suddenly (deposition builds deltas)
• When floodwater recedes after a flood event (velocity drops, load is deposited across the floodplain)

Interestingly, the Hjulström curve also shows that clay and silt are harder to erode than fine sand, even though they are smaller — this is because clay particles clump together (cohesion) and require a higher velocity to detach them from the bed.