The correct option for the question regarding the wearing away of the sides and bottom of a river's channel is D. corrasion. Detailed Explanation 1. Understanding Corrasion: - Corrasion, also known as abrasion, refers to the process where the river's load (which includes rocks, pebbles, and sediment) scrapes against the riverbed and banks. This mechanical action gradually wears away the material, deepening and widening the river channel over time. - This process is particularly effective in rivers with a high sediment load, where the particles carried by the water act like tools that erode the channel. 2. Why Option D is Correct: - Corrasion specifically describes the physical wearing away of the river's channel due to the friction and impact of sediment and rocks. It is a key process in river erosion and is essential for shaping river landscapes. - The term "corrasion" emphasizes the action of materials being carried by the water that actively erode the channel, making it the most accurate term for this process. Analysis of Other Options A. Hydraulic Action: - Hydraulic action refers to the force of moving water itself. It occurs when water exerts pressure on the riverbanks and bed, causing cracks and weaknesses to form. While hydraulic action contributes to erosion, it does not specifically describe the wearing away caused by the load of the river. - Therefore, while hydraulic action is a significant erosive force, it does not directly refer to the wearing away of the channel by sediment. B. Corrosion: - Corrosion in a geological context typically refers to the chemical weathering of rocks and minerals, often involving the dissolution of soluble materials. This process is more about chemical reactions than physical wear and does not accurately describe the mechanical wearing away of the river channel. - Thus, corrosion is not the correct term for the physical erosion of the riverbed and banks. C. Attrition: - Attrition is the process where rocks and pebbles collide with each other in the river, causing them to break down into smaller, smoother pieces. While attrition contributes to the overall sediment load and can enhance corrasion, it does not directly refer to the wearing away of the channel itself. - Therefore, attrition is related but does not specifically address the erosion of the river channel. Summary of Key Points

• Corrasion is the correct term for the wearing away of the sides and bottom of a river's channel due to the mechanical action of sediment.
• Hydraulic action involves the force of water but does not specifically describe the wearing away caused by sediment.
• Corrosion refers to chemical weathering, not physical erosion.
• Attrition describes the breakdown of particles through collision, not the erosion of the channel.

Revision Summary

• Corrasion (abrasion) is the wearing away of river channels by sediment.
• Hydraulic action is the pressure exerted by moving water, not sediment.
• Corrosion involves chemical processes, not physical erosion.
• Attrition refers to the breakdown of particles, not the channel erosion itself.