A new study suggests the Colorado River may have pooled as a giant lake in what is now northern Arizona and carved out the Grand Canyon after spilling downstream.
Scientists have discovered that small sediment particles in the Bidahoti Basin, in the upper reaches of the canyon, were transported from the upper Colorado River basin as early as 6.6 million years ago.
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“Our new evidence shows that it accumulates just east of the Grand Canyon and feeds a vibrant ecosystem,” he told Live Science.
The discovery, in turn, suggests that about 5.6 million years ago, a huge ancient lake in the basin slowly filled and overflowed, allowing the Colorado River to flow into it and forming what is now the Grand Canyon.
But this study probably isn’t the last word on the origins of the Grand Canyon. “I don’t think their data supports that.” [lake spillover] Karl Karlstrom, a geologist at the University of New Mexico who was not involved in the new study, told Live Science that the researchers are focusing on some consensus about when the canyon formed.
The question of how the Grand Canyon formed is how the waters of the Colorado River gathered from its headwaters (now in Colorado’s Rocky Mountain National Park) and flowed into what is now northern Arizona, gouging out a 5,000-foot (1,500-meter) deep canyon. The Colorado River dates back 11 million years to western Colorado, but it only reached the ocean between 4.6 and 4.8 million years ago.
Scientists already knew that more than 5 million years ago, downstream from Lake Mead, the river flowed into a series of previously isolated lakes, filling each lake with sediment and water until the water level reached such a height that it flowed out of the lake basin and down to another lower area. Whether something similar happened in the upper reaches of the Grand Canyon, where rivers slowly carve their way from their sources to the ocean, is hotly debated.
There are other mysteries as well. The Colorado River cuts through Kaibab Arch, a high point visible from the South Rim today, raising questions about how and why the river passed over the high-elevation feature rather than around it.
To learn more, he and his colleagues examined zircons in the Bidahoti Basin. Zircons are small, weather-resistant mineral particles that contain chemical information about their age and where they formed. Layers of volcanic ash helped researchers determine the age of these zircon deposits.
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He said the zircon in the basin matched that of the ancestral Colorado River. This indicates that an ancient lake in the basin (also known as Hopi Lake) was fed by the Colorado River, making the lake-outflow hypothesis plausible, he said. This would not have been a catastrophic flood, but rather a steady flow from a flooded lake high enough to cross Kaibab Arch.
Other evidence, such as fossils of large fish species adapted to life in fast-flowing waters and increased sediment flowing into Bidahoti, also points to the development of fast-flowing river systems, the researchers wrote.
“I think it’s pretty convincing in terms of the argument that lake outflow was more important to the high-altitude valleys further north than previously thought,” Rose Peek, a postdoctoral fellow in Earth and Planetary Sciences at the University of Texas at Austin who was not involved in the study, told Live Science.
But not all researchers are convinced. Karlstrom and his colleague (and spouse) Laura Crossey, a geochemist at the University of New Mexico, dispute Karlstrom and his colleagues’ interpretation that there was a large lake in the Bidahoti Basin. They also point to data that suggests that 10 million years before the main Colorado River flowed into the area, Kaibab Arch, carved by the Little Colorado River (a tributary of what is now the Colorado River), had a notch in it that would have allowed water to flow rather than pool. (Crossey and Carlstrom are collaborating with some of the new paper’s co-authors on related research, but were not directly involved in the research.)
The disagreement highlights some of the differences and uncertainties in the interpretation of data from around the canyon, he said.
But both sides of the debate are beginning to agree on some basic facts, including the timing of the river’s movement, its route through Bidachochi, and its north-south development in several stages, Karlström and Crossy told Live Science.
“We are moving toward a consensus toward resolving these long-debated issues,” Karlstrom said.
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