The Colorado River disappeared from the geological record for 5 million years. Scientists now know where it went

Geologists have solved the mystery of the disappearance from the geological record, millions of years ago, of one of North America's most important waterways: the Colorado River. A new paper published in Science shows that the river flowed into an upstream lake over the course of a few million years, then likely flowed for the first time into the Grand Canyon. The moment marked the Colorado River's transition to a continental-scale river as it made its way down to 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 finding, based on the analysis of sandstone samples, complements paleontological evidence, such as fish fossils, that suggests life began to become part of an integrated ecosystem throughout the Colorado River basin during this hidden chapter of its history.

How and when did the Colorado River reach the Grand Canyon?

The Colorado River existed in western Colorado 11 million years ago and first exited the Grand Canyon around 5.6 million years ago. But how it navigated the terrain between the two points for around 5 million years had been a mystery. Now, new evidence suggests it pooled just east of the Grand Canyon, in what is now part of the Navajo Nation, before charting a downstream path that ultimately led to the Gulf of California around 5 million years ago.

The Grand Canyon was carved in multiple phases over a long period of time, but precisely when and how much the Colorado River incised it remains debated 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.

One obstacle in the ancient river's path is the Kaibab Arch, a topographic high point located in northern Arizona and southern Utah. Geologists have proposed different scenarios for how the river crossed it, but one theory that the new evidence makes more plausible is lake spillover. In this scenario, the Colorado River would have filled a lake and eventually exited it along a course to 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."

The collaborative work began when He, Douglass and Emma Heitmann at the University of Washington, met in the field while studying the remnant deposits of Bidahochi Lake, an ancient lake on Navajo Nation land. Most of the deposits of this enigmatic lake have eroded away, so no one knows how large the lake was. Geologists also didn't know what rivers fed the lake, or why Bidahochi Lake eventually disappeared.

To understand where the sediments in Bidahochi Lake came from, He searched for zircons in the sandstone they collected.

Zircons are microscopic crystals that form in cooling magma. They do not degrade or change much over time and therefore contain an accurate geochemical signature of the moment they were created. Zircon is found in granite and other volcanic rocks, so it occurs abundantly in many sediments after the source rocks erode.

Geologists have developed a technique called detrital zircon geochronology that uses lasers or ion beams to measure the ratios of uranium and lead isotopes in hundreds of zircons in a sample. The unique age and history of each zircon can thus be traced to learn the sources of a sediment and estimate when it was deposited. The age spectrum derived from hundreds of zircons in a sample is called its 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."

He was studying the detrital zircon signatures of the samples he collected when, to his surprise, he detected what he thought was the signature of sediments known to have been deposited by the Colorado River. When he brought this up to Douglass, his colleague said that was exactly what he, Crow and some of his colleagues at the U.S. Geological Survey were looking for at the same time.

The researchers teamed up with USGS geologists and colleagues at the Arizona Geologic Survey, the University of Oklahoma, and the University of Washington. Together, they compared the detrital signatures of thousands of zircons in the sand that He and coauthors collected with those from other known deposits of the ancestral Colorado River and a few other possible sources.

The results showed that signatures of the sediments deposited about 6.6 million years ago in Lake Bidahochi closely matched those of other Colorado River deposits downstream and upstream, including the Browns Park Formation in northern Utah and Colorado. Study of rock layers in the field from this time period showed signs of rippling that indicated a strong river flowed into standing water, and fossils of large fish species characteristic of fast-flowing waters.

These lines of evidence strongly indicated that the Colorado River was supplying water and sediment to the Bidahochi basin before it spilled over and the river began to flow through the Grand Canyon. This set the stage for the mighty Colorado River that carved much of the Grand Canyon and upon which much of the West depends for water.

"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.

*Any person(s) wishing to conduct unmanned aerial vehicle flights on the Navajo Nation must first apply for and receive a permit from the Navajo Department of Transportation.