Riverwash consists of very deep alluvial materials in stream channels that are frequently flooded. These areas are highly dynamic and change with each flooding event. Little or no vegetation grows on Riverwash because of the flooding. No attempt is made to classify these materials because of the instability of the unit. The unit is subject to constant erosion and deposition during flooding events.

Riverwash materials are erratically deposited and stratified layers of sand, gravel and cobbles. Small layers of finer soil materials may be stratified with the coarser sediments which dominate riverwash materials.