We estimated the timing of paleodrainage connections in the Colorado River Basin using mitochondrial deoxyribonucleic acid (DNA) sequence divergences among populations of the speckled dace, Rhinichthys osculus . Cytochrome b and ND4L sequences were analyzed by maximum likelihood methods to estimate phylogenetic branch lengths, which were calibrated to geological time with a fossil age estimate. We assume that heterogeneity in rate of evolution of mitochondrial DNA is caused in part by differences in body size, temperature, and correlated life-history traits; therefore, branch lengths are used directly to calculate rates of nucleotide substitution and ages of nodes on the phylogenetic tree. Rhinichthys osculus is estimated (by the corrected age of the oldest fossil) to have diverged from its sister species at 6.3 Ma. We estimate that speckled dace have been in the Colorado drainage for 3.6 m.y., and they have dispersed through the drainage and to former connectives, such as the Los Angeles Basin, in the past 1.9 m.y. Divergence among lineages of the upper and lower Colorado River drainages (above and below Grand Canyon) is estimated to have occurred ca. 1.9–1.3 Ma. Genetic divergence of allopatric lineages in the lower Colorado River drainage was accompanied by morphological adaptations to different stream gradients, but small genetic distances among these forms indicate recent gene flow and lack of reproductive isolation.

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.

Abstract A rockslide took place at 10.33 a.m on November 29, 1987, starting from a location 4350 m above sea level on the western slope of Cerro Rabicano in the headwaters of Estero Parraguirre, a tributary of the Río Colorado, Regíon Metropolitana, Chile. It consisted mostly of limestone with gypsum and andesites, and was ~6.0 × 10 6 m 3 in volume. The slide quickly became a rock avalanche, and later a hyperconcentrated debris flow with a volume of at least 15 × 10 6 m 3 due to the incorporation of snow, ice, and sediments from the floor of Estero Parraguirre. Frontal waves were 20–30 m high; the high-energy flow moved with enormous destructive power, first through the channel of Estero Parraguirre, and later through the Río Colorado valley, and finally emptied into the Río Maipo after traveling ~57 km and descending a vertical distance of ~3400 m. During its rapid movement down the Río Colorado, the debris flow killed at least 37 people, and caused serious damage to the Maitenes hydroelectric plant (24 MW) and another hydroelectric plant (Alfalfal, 160 MW) that was under construction. The identification of several diamicton units deposited by prehistoric mud and debris flows in the valley of the Río Colorado, together with the enormous energy and great magnitude of destruction of the November 29, 1987, landslide event, confirm that this type of process is one of the major geological hazards in the Andean valleys of central Chile.