AZGS Senior Geologist Jon Spencer's written a concise summary on the origin of the modern Colorado River and incision of the Grand Canyon starting about 4.8 million years ago (Mya), for our online Arizona Geology magazine.
Jon concludes that the integration of the Colorado River drainage basin with the Pacific Ocean occurred at 4.8 Mya. "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" He also cautions that "much remains to be learned about the cause of this transition." [Right, maximum inferred extent of the Miocene-Pliocene Hualapai-Bouse lake system. Southward spillover of Lake Blythe marks the final event in integration of the Colorado River drainage basin with the Pacific Ocean]