For years, geologists in the Southwest have pondered a big question: Where did the Colorado River go before it carved the Grand Canyon?
Researchers knew the river flowed near Grand Junction, Colo., 11 million years ago, and that it exited the Grand Canyon about 5.6 million years ago. But where did it go in the interim?
A study published in the journal Science on April 16 proposes a possible answer.
By analyzing zircon mineral grains in the Bidahochi Basin on the Navajo Nation, the study’s authors say the Colorado River likely pooled into a lake, eventually spilling over an uplift called the Kaibab arch and cutting the Grand Canyon.
“Our new study has shown, I think pretty conclusively, that indeed these lake deposits east of Grand Canyon on the Navajo Nation were deposited by the Colorado River,” says Ryan Crow, a research geologist with the U.S. Geological Survey and a coauthor of the study. “So I think, by far, lake spillover is the most likely mechanism to transfer water from the lake into a new river course that would've eventually … over millions of years, carved Grand Canyon.”
Crow says the study’s conclusion should have a role in future scientific and academic work involving the proto-Colorado River.
“Our ideas about the Grand Canyon are constantly evolving,” he says. “I think our new work shows that this lake has to be incorporated into any kind of new models about how the river and the canyon formed.”
John Douglass, a professor at Paradise Valley Community College and a coauthor of the study, says there are lots of ideas for how the canyon formed, but that the lake spillover theory detailed in the paper has been contested for years.
A series of articles published starting around 2010 posited there was an older Grand Canyon where the Grand Canyon is today that started to form about 15 million years ago.
“[It was] this relic that kind of formed a notch in the Kaibab to allow the lake, when it started rising, to pour through that notch and cut the rest of the Grand Canyon,” says Douglass. “I completely disagree. I do not think there was a paleo canyon.”
Douglass argues his paper’s conclusion is simpler. To visualize what the paper proposes, he suggested picturing going into your backyard and digging a hole in the ground.
“And as long as it wasn't totally flat, and you got a hose and you filled up that hole with water, the water will fill up, it'll pour over the spot, and it will cut a little canyon in the rim of whatever your hole is,” he says. “That's fundamentally all that happened. Our big finding is … that we showed the Colorado River actually arrived just east of the Grand Canyon right before the Grand Canyon formed, and that had been contested and not known previously.”
The environment in the Bidahochi Basin back when the spillover first occurred probably would’ve been saturated with water, Crow explains.
“It was flowing into this basin, and there's kind of a rich fossil record in this particular part of the Bidahochi formation,” he says. “So it was probably a lush environment, lots of animals, fish, mammals, birds, kind of congregating in this wet area in the desert.”
The moment that ancient lake spilled over marks the Colorado River’s transition to the continental-scale waterway we know today, which 40 million Americans rely on for water amid a 26-year megadrought.
