Now, if you want to go one further, after the sun has contributed the energy to make clouds which turn to rain, the falling rain contains carbonic acid (and others due to man made pollution), which also has a chemical action on such strata as limestone, aiding the above mentioned erosion by particulate friction with the channel surface, again, mainly comprised of clay and silt. Of course, the size of particles is dependent on the characteristics of fluid flow, and is not limited to clay and silt.

Gioks's post also reminds me of another erosion process. I think it's time for a brew. (Yeah, I know what to do with it.)