Several basins in S. Nevada and NW Arizona record a transition from lacustrine to fluvial deposition between ~5.6 and 4.2 Ma that represents local inception of the Colorado River (CR). In the Lake Mead (LM) region near the modern CR, a series of lakes dominated the late Miocene landscape. In the eastern LM region, the Hualapai Limestone (HL) accumulated in such lakes in the Grand Wash trough, just west of the mouth of the Grand Canyon, and in northern Detrital basin. Geochemical, petrographic, and paleontologic data demonstrate a lacustrine origin for the HL. 40Ar/39Ar and tephrochronologic data bracket the HL between 11 and 6 Ma. It is one of the youngest deposits formed prior to integration of the eastern LM region into a through-flowing CR. In the Grand Wash trough, a 4.4 Ma basalt flow is intercalated in CR gravels. In the western LM region, a lacustrine limestone just north of Frenchman Mt. rests on the ~5.6 Ma Wolverine Creek tephra and interfingers eastward with a gypsum deposit that extends to near the modern CR. These relations bracket CR inception in the LM region between ~5.6 and 4.4 Ma. To the south in the Lake Mohave and Laughlin-Bullhead City area, alluvial fans dominated the late Neogene landscape until ~5.6 Ma when small lakes formed. Near Bullhead City, the ~5.6 Ma Wolverine Creek tephra lies directly below a thin limestone. Nearby, younger CR gravels contain the 3.6-4.2 Ma “lower Nomlaki” tephra (A. Sarna-Wojcicki, pers. com. 2002). Thus, inception of the CR in the Lake Mohave area is bracketed between ~5.6 and 4.2 Ma, similar to that in the LM region.

These relations indicate relatively rapid, regional inception of the CR in the early Pliocene. The chemistry of late Neogene lakes may hold important clues to the mechanisms of inception. Sr isotopic data from the HL (87Sr/86Sr to 0.7196) imply that source waters for some lakes differed significantly from modern CR water and may have been largely derived from groundwater issuing from the W. Colorado Plateau and/or central Nevada carbonate aquifer. Available data suggest, however, that lacustrine deposits in the western LM and Lake Mohave regions are slightly younger than the HL in the eastern LM region. This may imply that the lakes partly served as temporary sinks for CR water and were progressively formed and breached in a cascading downstream sequence.