A new geological study sheds light on a previously hidden phase of the Colorado River’s history.
Geologists have uncovered what happened to the Colorado River during a mysterious missing stretch of its history millions of years ago. Research published in Science shows that the river once flowed into a large inland lake for several million years before eventually carving a path into the Grand Canyon. This shift marked its evolution into a continent-spanning river that ultimately reached the Gulf of California.
“In some ways, you could really think of it as the birth of the Colorado River that we know today,” said first author and UCLA geologist John He. “There are rivers everywhere, but a river that carries water and sediment across the continent connects life throughout the region, and the entire ecosystem probably changed as a result of the arrival of the Colorado River into the basin.”
The study relies on sandstone analysis and aligns with fossil evidence, including fish remains, that points to increasing ecological connectivity across the Colorado River basin during this previously unknown phase.
Tracing a Lost Chapter of the Colorado River
The Colorado River was present in western Colorado about 11 million years ago and began exiting the Grand Canyon around 5.6 million years ago. However, its route during the intervening 5 million years has long been uncertain.
New findings indicate that the river collected in a basin east of the Grand Canyon, in what is now part of the Navajo Nation. From there, it eventually cut a downstream path that carried water toward the Gulf of California around 5 million years ago.
The Grand Canyon formed over multiple stages across a long span of time, and scientists still debate how much of that erosion can be directly attributed to the Colorado River.
“Geologists have proposed over a dozen hypotheses for the canyon’s formation and the Colorado River’s path,” said co-author John Douglass, a geologist at Paradise Valley Community College.
A major challenge along the river’s ancient route was the Kaibab Arch, a high region spanning northern Arizona and southern Utah. Scientists have suggested several ways the river may have crossed this barrier.
The new data supports a lake spillover model, in which water filled a basin until it overflowed and carved a channel toward the Grand Canyon.
“Other processes, such as karst piping, which involves water transport through rock, and headward erosion, may have also contributed to the establishment of the river’s course,” explained corresponding author Ryan Crow, from the U.S. Geological Survey. “Some reaches were likely newly carved, and others would have been significantly deepened by the integrated Colorado River over millions of years.”
Investigating the Ancient Bidahochi Lake
The investigation began when He, Douglass, and Emma Heitmann of the University of Washington met while studying deposits from Bidahochi Lake, an ancient lake located on Navajo Nation land. Much of the lake’s sediment has eroded, leaving its size and water sources unknown.
To trace the origin of these sediments, He examined zircon crystals found in sandstone samples.
Zircons form in cooling magma and remain chemically stable over long periods, preserving details about when and where they originated. Because they are common in granite and other volcanic rocks, they are widely distributed in sediments after those rocks break down.
Scientists use a method called detrital zircon geochronology to study these grains. By measuring uranium and lead isotopes in many zircons from a sample, researchers can identify sediment sources and estimate when deposition occurred. The resulting distribution of ages is known as a detrital signature.
“Zircons are some of the oldest fragments of our Earth,” said He. “They’re like little time vaults, and by looking at the age and geochemical signature of zircons, we can tell where a sediment that has been moved by a river originated.”
A Surprising Connection Emerges
While analyzing samples, He identified a zircon signature consistent with sediments previously linked to the Colorado River. When he shared the finding, it matched ongoing work by Douglass, Crow, and colleagues at the U.S. Geological Survey.
The team expanded to include researchers from several institutions, including the Arizona Geologic Survey, the University of Oklahoma, and the University of Washington. They compared thousands of zircon signatures from Bidahochi sediments with those from known Colorado River deposits and other potential sources.
The results showed that sediments deposited about 6.6 million years ago in Bidahochi Lake closely matched those from Colorado River deposits both upstream and downstream, including the Browns Park Formation in northern Utah and Colorado. Rock layers from this period show ripple patterns that indicate a strong river entering standing water, along with fossils of large fish adapted to fast-moving currents.
Together, this evidence suggests that the Colorado River once fed into the Bidahochi basin before overflowing and establishing its course through the Grand Canyon. This transition helped create the powerful river system that shaped the canyon and remains a key water source across the western United States.
“I think there is something unique and disquieting when the planet’s history is laid out before our eyes, but we cannot fully read it. We’ve always known the Grand Canyon is there, this solid towering wall of rock, but we’re learning more each day how it formed,” said He.
Reference: “Late Miocene Colorado River arrival in the Bidahochi basin supports spillover origin of Grand Canyon” by John J. Y. He, Ryan S. Crow, John Douglass, Christopher S. Holm-Denoma, Jorge A. Vazquez, Brian F. Gootee, Marsha I. Lidzbarski, Laura S. Pianowski, Harrison Gray, Emma Heitmann, Phil A. Pearthree, P. Kyle House and Shannon Dulin, 16 April 2026, Science.
DOI: 10.1126/science.adz6826
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