Over 5 million years ago, the Colorado River disappeared from the geological record, but now geologists have finally solved the mystery behind this ancient vanishing act.
The Colorado River’s path can be traced back 11 million years to western Colorado, exiting the Grand Canyon 5.6 million years ago, and finally ending at the Gulf of California. According to a new paper from UCLA researchers published in Science, previously unknown evidence for the Colorado River’s trajectory has been revealed by studying zircon deposits in Navajo land near the Grand Canyon.
The Disappearing Colorado River
Finally solving a long-term geological mystery, the geologists from UCLA traced the Colorado River’s path through an upstream lake and, likely, into the Grand Canyon, on its journey to becoming one of North America’s most important waterways.
“In some ways, you could really think of it as the birth of the Colorado River that we know today,” said lead author John He, Ph.D., of UCLA. “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 team’s analysis of sandstone samples corroborates fossil evidence indicating that an integrated Colorado River basin ecosystem developed during this period.
The Colorado River’s Journey
How the Colorado River made its way from western Colorado through the Grand Canyon in about five and a half million years has long been a mystery to geologists. The recent investigation finally solved this geological cold case by examining a region now part of the Navajo Nation, suggesting that east of the Grand Canyon, the river pooled for a period before heading downstream about five million years ago to the Gulf of California.
Although it’s agreed that the Grand Canyon was not carved all at once, the extent to which the Colorado River is responsible for the iconic landmark’s present shape has long been a matter of debate among geologists.
“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.
Crossing High Ground
The Kaibab Arch, a portion of high ground along the river’s path, has long mystified geologists, who have offered various theories over the years about how water could have overcome gravity to continue its journey. Among the various theories, lake spillover finally has some evidence thanks to the new research.
“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,” said co-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.”
But discovering the evidence for this theory was largely accidental. The ancient Bidahochi Lake, which existed between 6 and 16 million years ago and may have been even larger than Lake Michigan, captured Dr. He’s interest. Why the lake eventually evaporated or how it was fed remained mysterious, leading him to investigate sandstone samples from the dry bed, which eventually provided evidence supporting the spillover theory.
Investigating the Colorado River
He was looking for microscopic zircon crystals formed by cooling magma, which are remarkably stable over immense periods of time and preserve a geochemical signature of their formation. Measuring the uranium and lead isotopes in these tiny crystals using laser or ion beams allows geologists to precisely date the deposits they are sourced from.
“Zircons are some of the oldest fragments of our Earth,” Dr. He said in a statement. “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.”
Dr. He was surprised to discover a signature in samples from Bidahochi Lake suggesting they originated in the Colorado River, supporting the lake spillover theory. Following the discovery, a team of researchers from the US Geological Survey, UCLA, the Arizona Geological Survey, the University of Oklahoma, and the University of Washington assembled to conduct further investigations. Through their work, the team identified the Lake Bidahochi samples as close matches for others collected from the Colorado River, both upstream and downstream. Additionally, rippling in the rock layers indicated a strong river flowing into a lake, along with certain fish fossils that corroborated the theory.
“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,” Dr. He concluded. “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.”
The paper, “Late Miocene Colorado River Arrival in the Bidahochi Basin Supports Spillover Origin of Grand Canyon,” appeared in Science on April 16, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.