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4.8 Ma Age for Inception of the Modern Colorado River

Article Author(s): 

Jon Spencer

Age of the Colorado River

Initial arrival of Colorado River water to the Lake Mead area is recorded by evaporite and lacustrine sedimentation east of Las Vegas, Nevada (Castor and Faulds, 2001; Duebendorfer, 2003). Later, at 4.83 Ma, the eastern Mojave Desert area in the Blythe basin area was flooded. How much time elapsed between these two events? The water that accumulated in Blythe basin, the southernmost of the Bouse basins, was sufficiently salty that it was hospitable to three marine species (Smith, 1970; Todd, 1976) that were probably introduced inadvertently by birds (Spencer and Patchett, 1997; Spencer et al., 2008a). Calculations show that such salinities would develop in a similar-size lake fed by the modern Colorado River water over about 30,000 years due to evaporation and salt concentration in lake waters (Spencer et al., 2008a). This suggests that a geologically insignificant amount of time separated initiation of Colorado River flow through the Grand Canyon region and filling of Lake Blythe. Complete filling of Lake Blythe, overflow and incision of an outflow channel, and partial basin filling with sediments (necessary for sand to be transported by the Colorado River to the Salton Trough), probably also occurred quickly because of the large water volume and erosive power of the Colorado River and its high sediment load.


Southward spillover of Lake Blythe marks the final event in integration of the Colorado River drainage basin with the Pacific Ocean. This event resulted in a major eastward shift in the continental divide, which was perhaps the single largest transition in the Cenozoic hydrologic evolution of southwestern North America (Spencer et al., 2008b). While much remains to be learned about the cause of this transition, it now appears to have occurred at 4.8 Ma. Rapid incision of the Grand Canyon began at this time.

Senior Geologist
Arizona Geological Survey


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