River Processes

Rivers shape the landscape through the processes of erosion, transportation, and deposition. Erosion is the wearing away of the riverbed, banks, and material carried within it by the flowing water. Transportation occurs when the river carries these eroded materials downstream, moving them through suspension, saltation, and traction. Deposition happens when the river loses energy and drops the carried materials, creating features such as deltas, floodplains, and sandbanks. These processes continuously alter the landscape, carving valleys, forming meanders, and building new landforms.

How does a river erode?

A river erodes in four ways:

• Hydraulic Action: The force of the moving water itself breaks off pieces of rock and soil from the riverbank and bed, creating cracks and gradually wearing them away.
• Corrosion: Rocks and pebbles carried by the river collide with the riverbed and banks, grinding them down through friction and impact.
• Solution (Corrosion): Water, often slightly acidic due to dissolved carbon dioxide, chemically reacts with soluble minerals in the rocks, dissolving them and carrying them away in solution.
• Attrition: Rocks and pebbles transported by the river collide, breaking into smaller, smoother, and more rounded pieces as they are carried downstream.

How does a river transport material?

A river transports material in four ways:

• Traction: Large particles like boulders and pebbles are rolled along the riverbed by the force of the water, mainly in the upper course where the flow is strongest.
• Saltation: Smaller stones and pebbles are lifted and bounced along the riverbed in a series of short jumps, typically occurring in the middle course of the river.
• Suspension: Fine particles such as silt and clay are carried within the water column, giving the river a muddy appearance, and can be transported over long distances.
• Solution: Dissolved minerals and substances are carried within the water, resulting from the chemical dissolution of soluble rocks, and occur throughout the river’s course.

Processes of river transportation

Where and why does a river deposit material?

A river deposits material when its velocity decreases, and it can no longer carry the load it has been transporting. This typically happens where the river slows down, such as on the inside bends of meanders, in the lower course where the gradient flattens, or at the river’s mouth, where it enters a larger body of water like a sea or lake. Deposition occurs because the reduced energy of the river means it can no longer support the weight of the sediments, causing them to settle out of the water and accumulate, forming features like deltas, floodplains, and sandbanks.