For more than 150 years, scientists have debated one of North America’s biggest geological mysteries: When did the Colorado River carve the Grand Canyon, and how did it happen?
A new study suggests the answer is much slower and more gradual than many researchers once believed.
The study, led by scientists at the University of New Mexico and published in Nature Communications, combines two very different types of evidence—tiny volcanic crystals and the DNA of living fish—to piece together the ancient history of the Colorado River.
The researchers found that the Colorado River did not form in one dramatic event.
Instead, it developed slowly over about three million years, between 8 million and 4.8 million years ago.
During this time, several smaller rivers gradually connected to form one large river that eventually flowed from the Rocky Mountains, through the Grand Canyon, and all the way to the Gulf of California.
To uncover this history, the team studied around 10,000 tiny volcanic crystals called sanidine. These crystals were originally created by volcanic eruptions between 60 million and 40 million years ago.
Using a dating method that measures the age of individual crystals, the scientists tracked where the grains traveled over millions of years.
They found that the crystals first moved from volcanic regions in present-day Idaho into Utah.
Later, they reached Arizona, passed through what is now the Grand Canyon between about 6 million and 5 million years ago, and finally arrived at the Gulf of California by about 4.8 million years ago.
The researchers then compared this geological evidence with something completely different—fish DNA.
They examined genetic studies of native Colorado River fish, including pikeminnow and chub species. By studying how the fish evolved over time, scientists estimated when different populations became separated from one another.
Remarkably, the fish told the same story as the volcanic crystals. Their DNA suggests the fish gradually spread south through the growing river system, reaching Arizona between 7 million and 6 million years ago before moving through the Grand Canyon between 6 million and 5 million years ago.
Because these two completely independent sources of evidence produced nearly identical timelines, the researchers believe they have found the strongest explanation yet for how the Colorado River developed.
The team also believes that powerful geological forces deep inside the Earth played the biggest role. Over millions of years, movements in the Earth’s crust slowly tilted the Colorado Plateau toward the southwest. Volcanic activity and shifting land created large basins that temporarily trapped water, sediment and fish before the river continued flowing south.
The findings challenge several older ideas that suggested the Grand Canyon formed quickly because of a giant lake overflowing, one river capturing another, or rapid erosion.
Instead, the new evidence points to a much longer and more gradual process shaped mainly by tectonic forces acting over millions of years.
The researchers say there are still unanswered questions, especially about exactly what happened between 6 million and 5 million years ago. They hope future discoveries will uncover more physical evidence from this important period.
By combining geology with genetics, this study offers the clearest picture yet of how one of the world’s most famous natural wonders slowly took shape over millions of years.
