Scientists Have a New Clue to How Grand Canyon Formed

The Grand Canyon may owe its shape to a giant ancient lake that, after collecting water for a million years, finally couldn't hold any more. A new study in Science backs the long-argued "lake spillover" idea, suggesting the ancestral Colorado River once pooled in northern Arizona about 6.6 million years ago, forming Lake Bidahochi (or Hopi Lake), per Smithsonian. The lake slowly filled over hundreds of thousands of years, researchers say. Then, around 5.6 million years ago, water likely spilled westward over a geologic formation dubbed the Kaibab Arch and began slicing out what would become the Grand Canyon, the study argues, per Live Science.

To reach that conclusion, scientists led by the US Geological Survey compared zircon crystals locked inside sandstone from the Bidahochi Basin, just east of the canyon, to those found in early Colorado River sediments, finding a close match. This and other evidence, including from chemical signatures and fossil fish, suggests the Colorado River once fed into the basin. The work doesn't close the book on the matter, however. While some outside geologists call the spillover scenario "reasonable," they say it doesn't eliminate rival explanations involving other rivers, groundwater, or erosion. Some even argue a pre-existing notch in the Kaibab Arch would've prevented water from pooling in that area.