Eroded rocks picked up by the river scrape and rub against the channel wearing it away
River water dissolves some types of rock, e.g. chalk and limestone.
The force of the river water colliding with rocks breaks rock particles away from the river channel.
Eroded rock picked up by the river crash into each other and break into smaller fragments. Their edges also get rounded off as they rub together. The further material travels, the more it is eroded - causes particle size to decrease from the river's source to its mouth.
