A new USGS professional paper (co-authored by AZGS geologist Dr. Phil Pearthree) proposes the Chemehuevi Fm as a formal lithostratigraphic unit and offers an explanation for its origins. I've excerpted the following from the abstract:
The Chemehuevi Formation forms a conspicuous, widespread, and correlative set of nonmarine sediments lining the valleys of the Colorado River and several of its larger tributaries in the Basin and Range geologic province. This is one of the most prominent stratigraphic units along the river below the Grand Canyon, and the formation records an important event or set of events in the history of the Colorado River. Our preferred interpretation of the Chemehuevi Formation is that it contains the remnants of deposits formed during a single major episode of fluvial aggradation, during which the Colorado River filled its valley with a great volume of dominantly sand-size sediment. The most likely cause for the aggradation is an extraordinary increase in sand supply, likely due to widespread climatic change. However, other explanations have not been ruled out. However, the Chemehuevi Formation contains the remnants of the most recent large magnitude (>100 m) aggradation of the Colorado River.
Ref: Malmon, D.V., Howard, K.A., House, P.K., Lundstrom, S.C., Pearthree, P.A., Sarna-Wojcicki, A.M., Wan, Elmira, and Wahl, D.B., 2011, Stratigraphy and depositional environments of the upper Pleistocene Chemehuevi Formation along the lower Colorado River: U.S. Geological Survey Professional Paper 1786, 95 p., available at http://pubs.usgs.gov/pp/1786/.