What is the term for when boulders collide and break into smaller pieces in a river?

The term for when boulders collide and break into smaller pieces in a river is attrition. This process occurs as rocks and pebbles are transported by the river's current and come into contact with one another, causing them to gradually wear down and break apart. Over time, this leads to the formation of smaller, rounder sediment that can be more easily carried along by the water.

In contrast, hydraulic action refers to the force of water impacting the riverbanks and bed, which can erode material but does not specifically involve the breaking apart of boulders through collision. Corrosion, also known as abrasion, involves the wearing away of rocks by sediment that is held in suspension in the water, not by the direct collision between larger boulders. Solution refers to the process where minerals dissolve in water, which is a different type of weathering altogether. Each of these processes plays a role in shaping river landscapes, but attrition specifically describes the action of boulders colliding and breaking into smaller pieces.