Fluvial erosion, the process by which a river shapes the landscape, depends largely on the energy of its flowing water. This energy transports a considerable load of sediment, ranging from fine silt to large boulders. These transported materials are the tools that enable the river to cut into the surrounding terrain. Utilizing this load, a river gradually wears away its channel, deepening and widening the path it follows.
Identifying the Core Erosional Term
The specific term for the erosive action caused by stones and other sediment carried by a river is fluvial abrasion, also known as corrasion. Abrasion is a mechanical action where the load of sand, gravel, and larger stones scrapes, grinds, and wears away the solid rock of the riverbed and banks. The sediment acts like sandpaper, continually smoothing and breaking down the channel material through friction and impact. This continuous scouring is an effective way a river deepens and widens its channel.
The Mechanics of Fluvial Abrasion
The effectiveness of fluvial abrasion is directly tied to the hydraulic conditions and the characteristics of the sediment load. Water velocity must be high enough to lift and transport the sediment particles, turning them into abrasive instruments. When these particles are carried along the bed, they possess kinetic energy that is transferred to the stationary rock surface upon impact. The rate of erosion is directly proportional to the hardness and concentration of the abrasive material. Harder rock fragments, such as quartz, are more efficient at wearing away the river channel than softer ones.
Sharp, angular stones are significantly more abrasive than those that have been tumbled and rounded over a long distance. A higher concentration of sediment also increases the frequency of impacts, accelerating the rate at which the bedrock is chipped and scraped away. This grinding action removes small rock fragments, allowing the river to incise into its bed, which is especially pronounced during periods of high discharge.
Landforms Created by Abrasion
The persistent, localized grinding of abrasion is responsible for creating several characteristic river landforms. The most distinct feature is the formation of potholes, which are cylindrical depressions drilled into the solid bedrock of the riverbed. These are created when a pebble or small stone becomes trapped in a slight natural depression and is swirled by the turbulent water currents. This rotational grinding action deepens the hollow, and new stones become trapped to continue the process as the original abrasive stone wears down.
Abrasion also contributes to the general smoothing and polishing of the entire channel surface. The constant scouring action wears away any irregularities, leaving the bedrock of the bed and the banks with a scoured appearance. This process is crucial in shaping the overall form of the river channel, especially in its upper and middle courses where the energy carries a substantial abrasive load.
How Abrasion Differs from Other River Erosion
To understand the specific action of abrasion, it is helpful to distinguish it from the river’s other primary erosive mechanisms. Abrasion involves the contact between the river’s transported load and the channel itself, acting like a scouring tool. This mechanism is distinct from attrition, which is the process of the transported sediment particles colliding with each other. Attrition does not directly erode the bed or banks but instead reduces the size of the load, making the stones smaller, smoother, and more rounded as they move downstream.
Abrasion is also different from hydraulic action, which relies only on the physical force of the water itself. Hydraulic action occurs when the fast-flowing water forces air and water into cracks and crevices in the rock. The subsequent compression and release of this air, or the sheer pressure of the water, weakens the rock structure until fragments break off. Unlike abrasion, this process does not require the use of sediment as a tool for erosion.