Colorado River may have pooled and spilled over to form the Grand Canyon, solving a long-standing mystery ‪—‬ but not everyone agrees

The Colorado River may have carved out the Grand Canyon after pooling as a giant lake in what is now northern Arizona and spilling downstream, a new study suggests.

Scientists found that tiny sediment grains in the Bidahochi Basin, upstream of the canyon, were carried from the upper Colorado River watershed by 6.6 million years ago.

The findings fill in a 5 million-year gap about where the Colorado River was during this early period, said John He, a geologist at UCLA and the first author of the new study, published today (April 16) in the journal Science.

"Our new evidence shows that it pooled just east of the Grand Canyon, feeding a vibrant ecosystem," He told Live Science.

The findings, in turn, suggest that a giant ancient lake in the basin slowly filled and overflowed, causing the Colorado River to flow through and carve out what is now the Grand Canyon around 5.6 million years ago.

However, this study will likely not be the last word on the Grand Canyon's origins. "I don't think their data support that [lake spillover] conclusion," said Karl Karlstrom, a geologist at the University of New Mexico who was not involved in the new study. But researchers are zeroing in on some points of agreement about when the canyon formed, he told Live Science, even if they are still hashing out the "how."

The question of the Grand Canyon's formation comes down to how the waters of the Colorado River gathered from their headwaters (now in Rocky Mountain National Park in Colorado) and funneled through what is now northern Arizona, gouging out the 5,000-foot-deep (1,500 meters) canyon. The Colorado River dates back 11 million years in western Colorado but didn't reach the sea until 4.6 million to 4.8 million years ago.

Scientists already knew that, downstream of Lake Mead more than 5 million years ago, the river flowed into a series of previously isolated lakes, filling each with sediment and water until the water level reached high enough to flow out of the lake basin and head downhill to another low spot. It's hotly contested whether something similar happened upstream of the Grand Canyon as the river slowly forged a path from its origins to the ocean.

There are other mysteries, too. The Colorado River cuts through the Kaibab Arch, a high point visible from the South Rim today, raising questions about how and why it went over a high-elevation feature rather than around it.

To learn more, He and his colleagues examined Bidahochi Basin zircons — tiny, weather-resistant mineral grains that contain chemical information about their age and where they formed. Beds of volcanic ash helped the researchers pin down the ages of these zircon deposits.

The zircons in the basin matched those of the ancestral Colorado River, He said. This shows that an ancient lake in the basin (sometimes known as Hopi Lake) was fed by the Colorado River, which indicates the lake-spillover hypothesis is plausible, he said. This would not have been a catastrophic flood, but rather a steady flow from an overfull lake that would have been high enough elevation to cross the Kaibab Arch.

Other evidence, such as fossils of large fish species adapted for a living in fast currents and an increase in sediment flowing into the Bidahochi, also points to the development of a fast-moving river system, the researchers wrote.

"I think it's pretty convincing in terms of arguing that lake spillover was important for the canyon higher and farther north than it had previously been thought to be the case," Barra Peak, a postdoctoral researcher in Earth and planetary science at the University of Texas at Austin who was not involved in the study, told Live Science.

However, not all researchers are convinced. Karlstrom and his collaborator (and spouse) Laura Crossey, a University of New Mexico geochemist, contest He and his colleagues' interpretations that the Bidahochi Basin held a large lake. They also point to data suggesting there was a notch in the Kaibab Arch carved by the Little Colorado River (a tributary of the modern-day Colorado) 10 million years before the main Colorado River made its way to the area, which would have let water flow rather than pool. (Crossey and Karlstrom collaborate on related research with some of the co-authors of the new paper but were not directly involved in the study.)

That disagreement highlights some of the differences in interpretation and uncertainties in the data from around the canyon, He said.

But both sides of the debate are starting to agree on some basic facts, such as the timing of the river's travels, its path through the Bidachochi, and its development north to south in several steps, Karlstrom and Crossey told Live Science.

"It's heading in the direction of a consensus toward solving these long-debated issues," Karlstrom said.