The wearing away of the sides and bottom of a river's channel is called

The wearing away of the sides and bottom of a river's channel is called corrosion. In the context of river erosion as used in many West African geography curricula, corrosion refers to the grinding and scraping action of the river's sediment load (sand, pebbles, boulders) against the bed and banks of the channel. This is also known as abrasion or corrasion in some textbooks.

The other types of erosion have different meanings:

• Attrition is the process by which transported rock fragments collide with each other and are gradually worn down into smaller, more rounded particles. It acts on the load itself, not on the channel.
• Hydraulic action is the force of the water itself against the banks and bed, including the pressure of water being forced into cracks. It does not involve the grinding action of sediment.
• Corrosion action is not a standard geographical term and serves as a distractor in this question.

When distinguishing between river erosion processes, remember: corrosion (abrasion) wears the channel using the river's load as a tool, attrition wears the load itself, and hydraulic action uses the sheer force of water alone.