By Andrew Stuart
It’s the lifeline of the Southwest. The Rio Grande flows for almost 2,000 miles, from the snowy San Juan Mountains to subtropical plains and the Gulf of Mexico. Yet most of its journey is through the desert, and its impact here is immense. Many communities wouldn’t exist without it. Wildlife relies upon it. And in the canyons of the Big Bend, it’s created some of the most spectacular landscapes in Texas.
This desert lifeline owes its existence to a powerful tectonic phenomenon known as the Rio Grande Rift. The river didn’t form overnight – the evolution of the Rio Grande is a multi-million-year story, one that geologists continue to decipher.
Beginning more than 30 million years ago, and continuing today, our continent is being slowly torn apart by tectonic forces at the Rio Grande Rift. From Colorado to West Texas, the river follows the course of that “tear,” which creates mountains and valleys. Yet the through-flowing river is, geologically speaking, a recent phenomenon.
The Rift is segmented into separate basins. And for millions of years, each basin contained a lake or playa – like the Dead Sea today, where the Jordan River ends in a rift valley.
Dan Koning, a senior field geologist with the New Mexico Bureau of Geology and Mineral Resources, studies the river’s evolution.
“That’s an important thing to keep in mind when talking about the river and the Rift,” Koning said. “It turns out when the Rift was really active – between 20 and 10 million years ago – the river couldn’t connect the different basins, because it kept being trapped in really highly subsiding basins.”
It was only when the Rift’s activity slowed that streams could fill the basins with sediment, and the lakes could connect.
By 13 million years ago, streams from the southern Rockies joined to flow past present-day Taos and Santa Fe. But that “paleo-Rio Grande” ended in a lake, first south of Albuquerque, then near Socorro, New Mexico.
William Blake famously wrote of “seeing the world in a grain of sand.” Koning and his colleagues use a technique that echoes that poetry.
From ancient river deposits, they collect, and date, grains of sand – specifically, a mineral called sanidine. Sanidine is produced in volcanoes. By analyzing it, geologists can determine where it formed – and, by extension, the headwaters that fed the river. They can also track the river’s downstream progress.
It arrived at Truth or Consequences about 5 million years ago. Koning’s current research focuses on this lake, and when the river breached it to continue south.
Beginning about 3.5 million years ago, the Rio Grande ended as a series of lakes around Las Cruces. Geologists can date the next stage of the river’s progress from a cataclysmic event: a massive eruption of the Yellowstone volcano – which blanketed western North America in ash.
“Geologists love it,” Koning said, “because it makes a nice time marker. We know that ash is found in the river sand that came from the 2.1-million Yellowstone eruption. So we know that the river was flowing past El Paso at 2.1 million.”
That ash is found in river sands as far south as Presidio.
Before the Rio Grande’s arrival, the Rio Conchos likely ended in a lake at present-day Presidio-Ojinaga. But the combined rivers “escaped” the basin – and began to carve the vast Big Bend canyons.
Compared to its earlier development, the river moved quickly through the present-day borderlands. In studying deposits at the Gulf, geologist William Galloway has found that the river arrived there between 1 million and 600,000 years ago.
It had reached the ocean, but the river had more work to do upstream.
Throughout its progress, the river’s primary headwaters had not been in the eastern San Juan Mountains, where they are today. Its main trunk was the Rio Chama. In northern New Mexico, massive volcanoes had built up the landscape – damming water from the Rockies into a lake: “Lake Alamosa.” But about 400,000 years ago, that lake spilled over, and flowed south. It cut through a thousand feet of volcanic rock to create the Taos Gorge.
“So, the present configuration just happened in the last 400,000 years,” Koning said, “even though the river itself was existing on a scale of 13 million years or so.”
Today, human activity has made the Rio Grande one of the world’s most endangered rivers. But it’s still life-giving. And its epic story is worthy of a Great River.