The Abandoned Lands...A Journey Through the Colorado Plateau: Where the Rivers Changed Direction

...it's gone away in yesterday
Now I find myself on the mountainside
Where the rivers change direction
Across the Great Divide...

from Kate Wolf (a folksinger sorely missed...)

Abandonment as a theme for my current blog series about the Colorado Plateau doesn't always involve people and cultures. It can include rivers and other natural features as well.

Peach Springs Canyon and Diamond Creek provide the only vehicular access to the Colorado River in the entire length of Grand Canyon. As noted in a previous post, it is a spectacular introduction to the geology of the Paleozoic and Proterozoic rocks of the region. The route also reveals a neat little riddle.

Peach Springs Canyon is a very deep gorge, one of the larger tributaries of the Grand Canyon. Peach Springs Creek is a dry wash most of the time, but it would be understandable that it could have been carved by huge flash floods over the years. But what gets interesting is that Peach Springs Canyon also has tributaries, and they are also huge. But they also have practically no drainage area that could efficiently collect cloudburst runoff. How did these tributaries get carved?

It gets better...check out the GoogleEarth image below (click it for a larger image). Why do the winding canyons bend away from the linear fault-line creek bed, and then bend back? And notice how they don't drain in one direction, but instead each canyon has a drainage divide, with water flowing two directions from a high point within each gorge. Shouldn't a drainage in a single canyon segment be one way?

Before I point out the third strange item, understand that the Grand Canyon and the Colorado River as we see them today didn't exist prior to six million years ago. The present course of the Colorado River was not even possible until the Gulf of California (Sea of Cortez) formed around 5 millon years ago. And deposits along the Grand Wash Cliffs where the Grand Canyon ends at Pierce Ferry show that no large river existed there 17 million years ago. Why is this a problem?

As one starts down the road into Peach Springs Canyon, the rolling hills reveal sediments that are filling some of these odd tributary canyons. and they are old sediments; some are as old as Paleocene, the earliest part of the Cenozoic era which began 65 million years ago. These abandoned channels were carved in latest Cretaceous time. These tributaries are older than the present day Grand Canyon! Much older...they've been exhumed by the carving of the present day Colorado River. But these valleys may once have been inhabited by the dinosaurs!

Today, the Peach Springs area lies near the western edge of the Colorado Plateau. The Basin and Range province lies to the west and south, a much lower area on average, providing an avenue for the Colorado River to reach the sea. 65 million years ago, the region was much different. What are today the low areas of the Basin and Range were once a high mountainous region that we call the Mogollon Highlands. Rivers drained from these highlands through the Peach Springs region, carving the channels and canyons towards the northeast, pretty much the opposite direction that the Colorado Plateau flows today. At some point in the last few tens of millions of years, the rivers changed direction through a combination of headward erosion and stream piracy, and ultimately the "old" Colorado River of Colorado and Utah was diverted west and the Grand Canyon was the happy result.

What an incredible adventure of the mind! The history of the Colorado River and the carving of the Grand Canyon have been complex problems that have kept some of the very best field geologists busy for decades, and the full story of how the grandest canyon on the planet came to be is still only partially complete. For more information, check out

Colorado River Origin and Evolution (free ebook at http://www.grandcanyon.org/booksmore/booksmore_epublications.asp)

and Richard Youngs recent work in Peach Spring Canyon at:
http://repository.azgs.az.gov/sites/default/files/dlio/files/nid1362/cr-11-o_peach_springs_quad_0.pdf

Time Beyond Imagining: A Scrambled Landscape in Unaweep Canyon

In our last exploration of the Cenozoic story of the Colorado Plateau, we were looking at a sort of mystery canyon, where a deep gorge had been cut into very tough Proterozoic rocks, but where no river exists today. The name of this beautiful canyon is Unaweep, which in the Ute language means "Canyon with two mouths". That's another view of the canyon in the top picture. To help explain what went on here, I have included a picture of another incredible place in the same region, Black Canyon of the Gunnison National Park. I wrote about the rocks of Black Canyon last year in this post, but had little to say about the location of the canyon itself. Like Unaweep, it doesn't make a whole lot of sense at first.

Notice how similar the two canyons are in certain respects. The main gorge is cut through the hard crystalline metamorphic rocks which date back to around 1.7 billion years ago, and there are softer sedimentary rocks along the canyon rims which have eroded back into a more muted topography. And then observe the big difference: Black Canyon is 'V' shaped, and Unaweep has a flat floor giving it more of a 'U' shape. A 'U' shape should cause even a basic geology student to think "glaciers", and glacial action would make a certain amount of sense. The ice ages ended only a few thousand years ago, and glaciers could have accounted for the shape of the valley and the fact that no obvious erosional agent exists in the canyon today. The region is elevated enough, and glacial action has indeed been suggested by researchers in recent times. I am unconvinced, though, because of the general lack of glacial features beyond and above the rim of the canyon. But it is a reasonable hypothesis.

On the other hand, the similarity of the canyons cannot be dismissed. If you removed the Gunnison River, and let debris fall off the cliffs into the valley, it would start to fill in. And what if the river were dammed in some fashion? Again, debris would fill the resulting lake, forming a flat valley floor. But how can a river disappear?

When canyons erode, they don't just get deeper. They get wider as a result of mass wasting of the canyon walls, but they also get longer as a result of headward erosion. The steepest part of a canyon system is often at the headwaters of the stream, and erosion tends to eat into the slopes of the mountains in an upstream direction. If a canyon is vigorously eroding into a mountain, it may actually intercept and divert drainages that were flowing in other directions. In other words, a river can steal the water from another river, an act of stream piracy. With the addition of more water from the stolen stream, the original river may erode even more rapidly, causing extensive changes to a landscape in a short period of time.

You can probably see where this is going: what about the beheaded stream? Bereft of water, the canyon downstream of the diversion no longer has a river of any consequence and erosion ceases. Mass wasting of the canyon slopes will start to fill it in, and one is left with a mysterious gorge with no river inside.

This is almost surely what happened at Unaweep Canyon. There is still a great deal of debate about the origin of the gorge; geologists are not actually sure which river once flowed here. Was it the Gunnison, the Uncompahgre, or was it actually an ancient path of the Colorado River? It is a neat little problem, and is well worth a visit if you are ever traveling in the country between Black Canyon and Grand Junction, Colorado. Follow Highway 141 south from the village of Whitewater to Gateway.

The location of the Gunnison River as it flows through Black Canyon is another mystery altogether! More on this soon...