Rocks carried by the river smash together and break into smaller pieces. this process is called
1)abrasion 2)attrition 3)suspension 4)mass wasting

(1) Abrasion means wearing away of something because of depression
(2) Attrition is the process of breaking  down of something due to pressure or some kind of attack
(3) Suspension happens when something made of fine particles is discharged and cause blurness in the water.
(4) Mass wasting is the movement of of rocks down a slope because of its mass of an outside force such as water.

When rocks smash together, they break down into smaller pieces due to the attack. This is attrition.