The greatest stream erosion occurs at the point where the greatest force of the river hits against the side of the bedrock before eroding. Streams on the earth's surface are eroded in three ways: Hydraulic action, abrasion, and solution. Dependent upon the makeup of the soil or surface upon which the force of the river hits, determines the movement of the river's path.
I would say site because they need to know what place they are talking about.Hope this helps :)