Known for carving the Grand Canyon, the Colorado River is one of the most famous waterways in North America. However, this river hasn’t always flowed through the Grand Canyon and it’s unclear exactly when, and how, the relationship between the two began.
This mystery has long been debated by geologists, but a new study published last week in the journal Science may have finally provided an answer. The study, led by the University of California’s John He, examined crystals from samples of sandstones and found that the river flowed into an upstream lake before it made its way into the Grand Canyon.
“In some ways, you could really think of it as the birth of the Colorado River that we know today,” said He, describing the moment the river reached the famous canyon and transitioned to a continental-scale river.
“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 changes as a result of the arrival of the Colorado River into the basin.”
Today, the Colorado River supplies much of the water for the American West, supporting over 150 threatened species and more than 40 million people in one of the most arid areas on Earth.
The river serves as a critical habitat for many birds, such as the endangered Ridgway's rail, and several large mammals, including moose, elk, bighorn sheep and black bears.
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Based on the geological record, geologists know the Colorado River existed in western Colorado roughly 11 million years ago. They also know the river first exited the Grand Canyon roughly 5.6 million years ago, but where it flowed in the intervening 5 million years remained a mystery - until now.
New evidence suggests the river pooled just east of the Grand Canyon in what is now part of the Navajo Nation, creating the now-empty lake known as Bidahochi Lake. Most of the deposits of this ancient lake have eroded away, so it’s unclear exactly how large it was at its peak. It’s also unknown what rivers fed it or why the lake ultimately disappeared.
To understand where the sediments in Bidahochi Lake came from, He and colleagues collected samples of sandstones and later searched them for zircons - microscopic crystals that form in cooling magma. These crystals are incredibly robust and therefore record an accurate geochemical signature of the moment they were created.
Using a technique known as 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, geologists can work out their age and, crucially, where they originated. From this, they can trace the sources of a sediment and estimate when it was deposited.
“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.”
The results of He’s analyses showed that signatures of the sediments deposited roughly 6.6 million years ago in Lake Bidahochi closely matched those of other Colorado River deposits downstream and upstream. This, combined with signs of rippling in rock layers studied in the field and fossils of large fish species characteristic of fast-flowing waters, indicated a strong river flowing into standing water.
These lines of evidence led He and colleagues to conclude that the Colorado River was supplying water and sediment to Bidahochi Lake before it spilled over and the river began to flow through the Grand Canyon. The ‘lake spillover’ hypothesis has long been debated, but this study adds significant new evidence in support of it.
“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.
Top image credit: RJ Sangosti/MediaNews Group/The Denver Post via Getty Images
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