The Colorado River – which did not always flow through the Grand Canyon region – may have begun to carve its path through it after an ancient lake overflowed roughly 5.6 million years ago, say researchers. Their results suggest lake spillover was more important than processes like groundwater flow or erosion in informing the river’s modern path. The timing and mechanisms leading to the Colorado River’s integration into the Grand Canyon and its role in the canyon’s formation remain poorly understood. Geologic evidence indicates that the Colorado River existed in what is now western Colorado by about 11 million years ago; however, it did not exit the Grand Canyon on its modern course until about 5.6 million years ago, leaving a significant gap in the river’s early history. Previous studies have proposed that the river’s integration into its modern path involved a complex, multi-stage history of canyon carving and capture. One leading idea is that a series of closed basins once held large lakes that gradually filled and spilled over, eventually linking isolated drainage systems into a continuous river system reaching the ocean. Geological formations such as the Bidahochi Formation have been interpreted as evidence of such a paleolake system, supporting the spillover hypothesis, yet these conclusions remain contested.

Here, John He and colleagues used precise uranium-lead dating of zircon crystals from volcanic ash layers and sandstones to determine the age and origin of sediments within the Bidahochi Formation and other areas along the Colorado River’s course. These zircon age patterns act as a “fingerprint” to trace sediment sources and river connections over time. He et al. discovered that the composition and age distribution of zircon grains in the upper Bidahochi Formation closely match those found in known early Colorado River deposits. This similarity suggests a shared sediment source and indicates that the upper Bidahochi Formation was likely connected to the ancestral Colorado River by 6.6 million years ago. Other geological evidence, including increased sediment accumulation, strontium isotope ratios, and fossil fish assemblages, further indicate that Colorado River water was flowing into and gradually filling the basin for hundreds of thousands to over a million years before extending downstream. What’s more, the elevation and structure of ancient lake deposits indicate that a Colorado River-fed lake once rose high enough to overtop the Kaibab arch to spill over into the canyon region. According to the authors, while other mechanisms like groundwater flow or erosion may have played supporting roles, their findings suggest that lake spillover was the primary process establishing the Colorado River’s course through the Grand Canyon.

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