The lower Colorado River corridor below Grand Canyon developed sometime between ∼5.3 Ma and ∼4.8 Ma, as evidenced by the first appearance of Colorado River-derived sediments of the Bouse Formation in basins along its modern course and the abrupt appearance of distinctive hematite-stained “C-suite” Colorado River sand grains near the base of the Wind Caves member of the Latrania Formation in the Salton Trough. The oldest recognized evidence for a through-flowing Colorado River is the ∼4.5 Ma to ∼3.5 Ma Bullhead Alluvium. Interest and debate focuses on whether a “pre-Bullhead” Colorado River might have first reached the Salton Trough at 5.3 Ma, prior to deposition of some of the Bouse Formation. Fine-grained but undated Colorado River sediments have been recognized on the Yuma Proving Grounds, Arizona, since the 1970s. These sediments have been correlated by some to the Bouse Formation, and thus, would pre-date the Bullhead Alluvium and might provide evidence for the existence of a “pre-Bullhead” Colorado River. In this study, I use the extent of amino acid racemization in ostracode valves and microfossil assemblages to test correlations between the fine-grained sediments on the Yuma Proving Grounds with other independently dated lower Colorado River corridor alluvial packages that were deposited at ∼5 Ma (Bouse Formation) and at ∼75 ka to ∼60 ka (Chemehuevi Formation). The results of this study confidently demonstrate that the fine-grained sediments on the Yuma Proving Grounds are not late Miocene or early Pliocene in age but instead are correlative with the late Pleistocene Chemehuevi Formation. Evidence for a “pre-Bullhead” Colorado River remains elusive.