River’s origin remains one of the biggest geological mysteries of the American West
Pete McBride/National Geographic Creative
Standing on a mesa high above the town of Rifle, Colo., Andres Aslan is having a hard time staying quiet. The lanky geologist from nearby Colorado Mesa University normally speaks in a low-key professorial drone. But here, looking down at a sprawling river valley blazing with autumnal cottonwoods, his enthusiasm cranks up his volume. “This could be it,” says Aslan, gesticulating wildly. “This may end up being the most important site anywhere.”
What’s important about this mesa, called Taughenbaugh, is the gravel under Aslan’s feet. It was laid down 1.75 million years ago by the Colorado River. The modern Colorado wends through the valley beneath. Over those millions of years, the river eroded away all the rock layers that once existed between the high mesa and the valley below.
Aslan has been striding up and down Taughenbaugh and neighboring mesas for years, gathering clues about how the famous river shaped the landscape. He hopes to help crack one of the biggest geological mysteries of the American West: how and when the mighty Colorado River came to be.
From its headwaters in western Colorado, the river makes its way west and south. It passes through the red rocks of Utah, carving dramatic landscapes such as Arches and Canyonlands National Parks, and surges onward through the Grand Canyon. Bottlenecked by dams, tapped along its course for drinking and irrigation, the Colorado eventually crosses into Mexico and trickles to the Gulf of California, 2,300 kilometers from where it began.
It is one of the world’s most storied rivers. Its deep canyons provided passage for the first geological explorations of the American West. Its waters, fought over from Phoenix to Los Angeles to Mexicali, make desert life possible for millions. Yet scientists know surprisingly little about the ancient history of the Colorado River.
They do know that by about 11 million years ago, rainfall was running off the western Rockies in a sort of proto–Colorado River. By about 5 million years ago, those waters had breached the Gulf of California and completed the entire river system drainage. But many mysteries remain.
One huge puzzle is when and how the Grand Canyon — the river’s most glorious stretch — came to be. In the last few years, a handful of geologists have put forward a startling alternative explanation of the canyon’s history. Rather than being carved in the last 6 million years or so, they contend, the Grand Canyon may date back some
70 million years. If they are right, then water has been running through essentially the same deep gorge since the time of the dinosaurs. Other researchers are far from convinced, and the two camps continue to argue over what the canyon’s rocks have to say about its history.
New research may illuminate the story of the Colorado River and the Grand Canyon. Crystals trapped in ancient sandstone are providing new time stamps for key stages in the river’s evolutionary past. Chunks of the mineral apatite offer clues to how long particular rocks have been exposed at the surface, revealing when canyons were carved. And good old-fashioned geological mapping, like Aslan’s, is tying lots of little river drainages together into a much fuller picture of how the Colorado arose over time.
“It’s among the world’s most fascinating rivers,” says Karl Karlstrom, a geologist at the University of New Mexico in Albuquerque. “It carved one of the most iconic landscape features on Earth. It’s a terrific laboratory.”
Drilling down to the early days
In the years after the Civil War, the explorer John Wesley Powell tried to make sense of the Colorado as he surveyed the length of the Grand Canyon. Powell knew that water was the key element that had shaped the otherworldly landscape. “The carving of the Grand Canyon is the work of rains and rivers,” he wrote. “Though storms are far apart and the heavens above are cloudless for most of the days of the year … an intermittent
rill called to life by a shower can do much work in centuries of centuries.”
After all, even the mightiest river begins as a tiny stream. Rainwater running off the land carves the underlying surface to create a gully. Over time, more and more stream-filled gullies connect in a branching pattern to form a large river.
Powell believed the Colorado was an ancient drainage, one that had followed essentially the same course from the Rockies to the Pacific for tens of millions of years. Later geologists discovered problems with this simplistic picture. Among other things, there’s no evidence that the river made it off the western edge of the Colorado Plateau — the raised area encompassing the Four Corners region where Utah, Colorado, New Mexico and Arizona come together — before about 6 million years ago. A big river spilling off the plateau should have left massive deposits of river gravel. But no one can find any.
In fact, the more geologists have looked at Colorado River history, the more complicated the story has become. Part of the problem is the sheer difficulty of measuring a landscape torn apart by water. “There just isn’t much data to go on,” says Kelin Whipple, a geomorphologist at Arizona State University in Tempe. “It’s a completely erosional system, so it hasn’t left much trace of how fast things happened.”
In the case of the Colorado, Aslan got lucky. Not far from Rifle, atop a mountain known as Grand Mesa, he and his colleagues discovered thick layers of gravel left behind by the ancient river. At one point a nearby volcano had erupted, burying the gravel in lava that hardened to solid rock. The researchers looked at the steady decay of radioactive elements in the hardened lava to deduce that it flowed atop Grand Mesa 10.7 million years ago. That means the river gravel it buried must be even older — and that the Colorado must have been flowing in this region by about 11 million years ago, the team reported in a 2010 Geological Society of America field guide.
Now Aslan wants to look even further back in time. He’s pinning his hopes on the mesas around Rifle, and getting a helping hand from the recent boom in natural gas exploration. The countryside is studded with drill rigs, and as Aslan gathers visiting geologists around him, drill truck after drill truck rattles by on the high country road. Soon the group is joined by Barbara Allen, a student in a bright pink Colorado Mesa hoodie. She’s working toward a geology degree while holding down a job at WPX Energy, a company that is drilling atop a neighboring mesa called Flatiron.
Aslan begins leading the group uphill, loping toward an outcrop of Colorado River gravel he promises is just over the next rise. Allen points out elk droppings for visitors to avoid as she describes the geology she and Aslan are hunting.
As at Grand Mesa, volcanic flows have covered and preserved Colorado River gravel in places. WPX has been drilling repeatedly through the mesas, plunging thousands of feet below ground in search of natural gas. Allen’s job is to race out to the mesa when engineers are spudding, or starting, the wells — and gather up precious samples of river gravel that the drillers would otherwise discard as useless. “They see me coming and they know what I’m looking for,” she says.
Aslan hopes to find drill cuttings that contain Colorado River gravel deposits going back millions of years. Though he eventually learns that the river’s gravel wasn’t preserved in the layers the WPX drill punched through, he continues to explore the mesas near Rifle, hunting for more signs of the Colorado flowing across this part of the country millions of years ago.
Signs of the ancient river
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