The long and winding Colorado

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

To find the other bookend to the story of the Colorado, you have to travel way down near the Arizona-California-Mexico confluence. Here, geologists have pieced together the story of a flood appearing a little more than 5 million years ago. That water, they say, must be the full-blown arrival of the Colorado.

In far southern California and northern Mexico, the movement of the San Andreas fault has caused a huge chunk of land, known as the Salton Trough, to drop below sea level. Rebecca Dorsey, a geologist at the University of Oregon in Eugene, has been scouting this region for signs of the ancient river. Because of its low elevation, the land serves as a sort of sink where sediments accumulate. Dorsey has found massive layers of river deposits dating to 5.3 million years ago; studies of the mineral grains show that they came from all across the watershed of the modern Colorado River.

Over the last 10 million years, some 340,000 cubic kilometers of rock washed across the western United States downstream to this part of the Southwest — enough to fill NASA’s enormous Vehicle Assembly Building in Cape Canaveral almost 100 million times. “This huge volume of rock eroded out of the Colorado Plateau, and it’s the river that did the job,” says Dorsey. She and Greg Lazear, of the Grand Junction Geological Society of Cedaredge, Colo., described the calculations in the August Geosphere.

So the river started in western Colorado at least 11 million years ago, and its waters reached California after 6 million years ago. In between, rivers may have drained off the Colorado Plateau to the north, through Idaho and beyond. Or they may have ponded in huge lakes like today’s 4,400-square-kilometer Great Salt Lake. In this scenario, the lakes existed until a river captured their waters and spilled off the plateau to form the Colorado.

This may also have been the time when, according to classical theories, the river began carving the Grand Canyon. Most geologists hew to the idea that the Colorado River started to cut through the rocks that form the canyon beginning about 6 million years ago. (The rocks exposed in the canyon walls are far more ancient, dating back as much as 1.8 billion years.)

Dueling dates

But some geologists say that the carving of the canyon took place way before that. The evidence comes in the form of crystals of apatite, a mineral commonly found in rocks along the canyon’s 433 kilometers. Starting in 2008, Rebecca Flowers of the University of Colorado Boulder and her colleagues studied helium inside the apatite, which is created when uranium decays. This helium can be used to record the cooling history of the rocks.

The key is that helium diffuses out of apatite crystals, and is permanently lost, when temperatures are hotter than about 50° Celsius. If you find helium in an apatite crystal, you know the rock has been cooler than 50°. The more helium, the longer it has been cool. Rocks get cool when they move from being buried deep within the Earth to the surface — like when a river erodes away the overlying rock layers. Flowers and her colleagues used helium in rocks exposed in the Grand Canyon to calculate how long they had been cool, and thus how long ago the canyon was carved.

“These rocks have been cold for a very, very long time,” says Flowers, about 70 million years. She and Kenneth Farley of Caltech published their findings in 2012 in Science (SN: 1/12/13, p. 15).

Flowers runs a highly regarded geochronology laboratory. But Karlstrom, who grew up hiking and rafting the Grand Canyon, didn’t let her calculations go unchallenged. “It just goes against decades of geological understanding,” he says. Karlstrom fired off a technical response to Science, cross-questioned Flowers at a conference weeks after her paper was out and organized four follow-up sessions for a Geological Society of America meeting last October in Denver.

Those sessions at the Denver conference were as close as geologists get to friendly fire. Flowers, Karlstrom and other players in the canyon dating game lined up against a meeting-room wall and passed a microphone back and forth, arguing over what apatite grains were telling them.

Flowers stood by her dates, even as Karlstrom whipped out another set of data. Last April, he and colleagues led by John Lee of the U.S. Geological Survey in Denver published a paper in Geosphere with additional helium dates from dozens of rock samples from the canyon. The researchers used the same helium dating method that Flowers employs, as well as a second technique: They looked at marks left behind by radioactive atoms spontaneously fissioning and plowing through the apatite crystal.

If the crystal is hotter than about 110° C, it naturally heals itself and very few of these “fission tracks” are preserved. As temperatures start to cool below 110°, though, more and more of the tracks remain in the crystal. That means the existence of the tracks can be used like a stopwatch to calculate when and how fast the apatite cooled. Lee’s team reported that, for the eastern Grand Canyon at least, there could have been no canyon as deep as today’s until after about 25 million years ago — and probably much more recently than that.

There’s no clear answer for why the “young canyon” and “old canyon” proponents are so far apart. In part, the disagreement involves how to interpret the record in the apatite crystals. In places, the helium data from Flowers’ team and Lee’s team are essentially identical; the difference is in how scientists infer a temperature history for the rock. Use a slightly different temperature for how cool the surface of the Earth is, for instance, and suddenly an apatite crystal might look like it’s been buried for much longer.

Karlstrom thinks there may be ways to “honor all the datasets,” as he puts it. One possibility is that different small canyons formed at different times. “Variable cooling histories along the river corridor reflect a ragged escarpment where places are cooling faster or slower,” he says. And those smaller bits could be confusing the much grander story of the river as a whole.

In the end, the answer may come by combining the best geologic mapping with additional rock-cooling dates. Some researchers are beginning to use a third dating method, which measures the ratio of helium isotopes within apatite. It can be used to track the final part of a rock’s cooling history, between about 60° and 30° C. Only by “triple dating” lots of rocks with all three methods, Karlstrom says, can geologists begin to agree on the canyon’s history.

Whatever happens with the fight over the Grand Canyon’s age, the Colorado River will continue to flow, as much as it can, given all the demands on its water. It will trickle down from the western slope of the Rockies, past the high mesas of the Colorado Plateau and through the Grand Canyon, until it empties into the sea.