The Grand Canyon stands as one of North America’s most remarkable natural landmarks, a vast geological wonder in the southwestern U.S. state of Arizona. Its steep walls, dramatic contours, and layered colors reflect millions of years of Earth’s history, shaped primarily by the persistent force of the Colorado River.

New research is shedding light on how and when the Colorado River began carving its path through the region. By analyzing tiny grains of zircon found in sandstone—formed from ancient river sediments and volcanic ash deposits—scientists were able to reconstruct the river’s prehistoric routes and better understand its evolution over time.

The study suggests that about 6.6 million years ago, the Colorado River began flowing into a large surface depression in northeastern Arizona, forming a broad, shallow lake in what is now the region east of the Grand Canyon. Over time, this body of water grew until it eventually overflowed a low point along its edge around 5.6 million years ago. That overflow is believed to have initiated the river’s course through what would become the Grand Canyon.

As the river continued to evolve, it carved through a series of downstream basins before ultimately reaching the Gulf of California approximately 4.8 million years ago, where it emptied into the sea in what is now northwestern Mexico.

The ancient lake, which may have stretched more than 90 miles (150 kilometers) across, has long since disappeared. Researchers have informally referred to it as Bidahochi Lake, named after a local geological formation. It is believed to have been located largely within what is now the Navajo Nation.

“Scientists have long debated when the Grand Canyon was carved, and our study contributes to that conversation,” said UCLA geologist John He, co-lead author of the study published in the journal Science.

He explained the significance of zircon analysis in understanding Earth’s history. “In a handful of sand, there are hundreds of thousands of grains that look identical. But among them are microscopic zircon crystals that preserve information about where they originated,” he said.

The dating of volcanic ash layers helped researchers determine when sediment deposits containing the zircon grains were laid down, providing a timeline for the river’s development.

The Colorado River begins at La Poudre Pass in Rocky Mountain National Park in Colorado and stretches roughly 1,450 miles (2,330 kilometers) across the southwestern United States.

A key question has long been where the river flowed before it reached the Grand Canyon region. “We have long known the river existed in western Colorado 11 million years ago, and that it did not flow through the Grand Canyon until after 5.6 million years ago. But until now we knew almost nothing about where it was during the intervening time,” said study co-lead author Ryan Crow, a research geologist with the U.S. Geological Survey in Flagstaff, Arizona.

Researchers also noted that additional geological processes may have influenced how the river changed course over time.

Today, the Grand Canyon stretches about 280 miles (450 kilometers) in length, up to 18 miles (29 kilometers) wide, and more than a mile deep (about 6,100 feet or 1,860 meters). Its exposed rock layers include formations dating back as far as 1.8 billion years.

“The canyon continues to deepen as the Colorado River cuts through rock at an average rate of about 100 to 160 meters per million years,” Crow said. “The landscape we see today reflects roughly five million years of ongoing erosion and river incision.”

Despite the scientific complexity behind its formation, the canyon continues to inspire awe. “Grand Canyon, a natural wonder of the world, captures the attention and curiosity of almost everyone who sees it,” Crow added. “Even people who rarely think about geology often ask the same questions: How did it form, and when?”

He emphasized the scale of geological time visible in the canyon’s walls. “The architecture of the planet is exposed here in a way that is both remarkable and humbling,” he said. -Reporting by Will Dunham in Washington, Editing by Rosalba O'Brien/Reuters