Cutback or “cutting back” as it relates to rivers, streams, and other bodies of water is the process where soil and rock are eroded away creating a sharp drop off and subsequent waterfall. The Niagara Falls are one of the more spectacular examples of “cutback”.
Cutting Back of the New River
Notice the person standing on the bank of the river to the far left of the image.
Click on the link for a larger version of the image.