A model is proposed whereby a Miocene Colorado River precursor canyon, deeper than 600 m, formed on the western Hualapai Plateau by headward erosion along a strike-valley drainage. Basin and Range faulting of the margin of the Colorado Plateau initiated canyon formation. This canyon was occupied by a long narrow lake, and the surface of the lake was at or above the level of the Hualapai Limestone. Such a hypothesized lake would have trapped any coarse sediment derived from the surrounding basin at the head of the lake, well upstream from the Grand Wash Trough. The drainage area feeding into the lake would have included the Hualapai Plateau and the combined ancestral drainages of Kanab and Cataract Creeks. This >13,000 km 2 basin has been dominated by surface exposures of Paleozoic carbonates since at least late Eocene time and generates no more than 1%–2% of the runoff associated with the modern (predam) Colorado River discharge. Such a carbonate-dominated, sediment-deficient basin would supply carbonate-rich runoff to the structural depocenter in the Grand Wash Trough, possibly explaining the upward transition to the Hualapai Limestone facies in late Miocene time. The upstream canyon delta produced in this proposed model could have been removed by the Pliocene-Pleistocene integration and younger incision of the more powerful, modern Colorado River.