How Did the Grand Canyon Happen? It's a Mystery, a Delightful Mystery!

And that's something that I really love about science and geology: the acknowledgement that there are things we don't know...yet. No matter how ingenious our technology becomes, no matter how much data we collect, there will always be new things to discover, and each week research reveals something new about our Universe that no one has ever known before. And that's the fun of exploring a wonder like the Grand Canyon. We don't completely understand why it's there!

There are many things we can say about how the Grand Canyon formed. For instance, the Colorado River carved the Grand Canyon. That's an answer that satisfies many, because the river is big, drains a large region, and lies at the bottom of the present-day canyon. It carries millions of tons of silt and sand downstream for temporary storage in Lake Mead before it ends up in the Gulf of California. The canyon is getting a tiny bit deeper and wider each and every day. That much is clear.

Source: http://en.wikipedia.org/wiki/Kaibab_Plateau

The first problem is a big one: how did the Colorado River get over the Kaibab Plateau (above)? The Kaibab tops out at over 8,000 feet, but the lands east from which the Colorado flows are 3,000 feet lower. The easy answer is that the river was there and the land rose around it while the river cut down, keeping pace with the uplift. That is an appropriate hypothesis, but unfortunately can't be true because the plateau is 70 million years old while the Colorado River cannot have exited the plateau at the present Grand Wash Cliffs locale prior to 16 million years ago. The main lesson of several different possible explanations is that there has not always been a single through-going Colorado River. There have been several river systems that were later integrated into a single river.

So which of those Colorado Rivers was it that carved the Grand Canyon? Was it the Colorado River we see today that flows (or once flowed) into the Gulf of California? If that's the case, the Grand Canyon can be no more than 4-6 million years old, because that's when the Gulf of California opened up. Was it the "Colorado River" that carved these entrenched meanders near Peach Springs (photo below)? If it was, then we are talking about a river that carved a 4,000 foot deep canyon more than 50 million years ago. A river that flowed northeast, almost opposite of what the Colorado River does today.

Or was it this river (below)? Looking at the shape of the tributary streams, it looks like this river is flowing northeast...but it's not. That's Marble Canyon, occupied by the Colorado River just downstream of Lees Ferry, and just upstream of the Grand Canyon itself. The river flows southwest, but the topography is strongly suggestive of a river that once flowed in exactly the opposite direction.

How in the world can a river completely change direction? It seems to fly in the face of physics and the laws of gravity. But there are ways that it can happen and we don't have to break the law (of nature). One of them is called stream piracy, and it is happening even today on the Colorado Plateau, at another familiar national park, Bryce Canyon.

The gradient of a river is the relative slope of the river, the altitude it drops over a given distance along the riverbed. Higher gradient streams flow faster, and therefore tend to have greater erosive capacity. Since the highest gradients are often at the headwaters of a stream, erosion often acts to eat away at the edge of the drainage or plateau where the river originates. This upstream lengthening of the river is called headward erosion.

Check out the Google Earth image below of Bryce Canyon National Park. The famous hoodoos, or spires, form at the edge of the plateau and are light orange in color. The erosional amphitheaters represent the headwaters of the Paria River and other tributaries to the Colorado River. The amphitheaters completely surround a second north-flowing drainage, the East Fork of the Sevier River. The headwaters are carving away at the plateau edge, getting closer and closer to the channel of the Sevier River. In the near geologic future, the water that flows north towards Salt Lake City will instead flow southwest towards Las Vegas. Early settlers in the region took advantage of the impending piracy; they dug a canal that diverts some of the Sevier River into the Paria River drainage to support agricultural fields downstream.

The picture below highlights the edge of the plateau and the two drainages. Ultimately it is tectonic activity (raising or sinking of the crust) that determines the response of river channels. And there have been many tectonic changes in and around the Colorado Plateau where the Grand Canyon was eroded.

How are we to make sense of these discrepant pieces of data? In essence, it is the discrepancies that lead us to better understand how the Grand Canyon might have happened. While we don't have a full story to explain the development of the canyon, there are certain things we do know (this information is summarized nicely in Wayne Ranney's excellent book Carving Grand Canyon: Evidence, Theories and Mystery, published by the Grand Canyon Association).

From 80 to 30 million years, river in the Colorado Plateau region flowed northeast into large inland lakes. The Claron Formation, which forms the hoodoos of Bryce Canyon, was one of these lake systems. The highlands from which the rivers flowed originated in the compressional tectonics of the Laramide Orogeny.

From 30 to 16 million years there was a dark period for which there is little evidence revealing the type of river activity. According to Ranney, the drainages became ponded, dry, rerouted, reversed, or "confused".

From 16 to 6 million years, basin and range style faulting severely disrupted the landscape, creating deep sedimentary basins that were isolated from one another. The sinking of these basins may have formed the high gradients that initiated deep canyon cutting, but until 6 million years ago, no Colorado River flowed of the Colorado Plateau at any particular point with any relationship to today's course.

Between 6 and 4 million years ago, the opening of the Gulf of California caused the integration of different river drainages into the unified Colorado River we know and love today. Much of the cutting of the Grand Canyon probably took place during this time period, aided in large part by extremely high river discharges fed by melting glaciers in the Rocky Mountains during the last ice age. 

What we can say is that the Grand Canyon and the Colorado River exist today, and the canyon continues to be carved and excavated by the river. We are lucky to have evolved just in time to see the canyon, because in a geologically short period of time, the entire plateau can be expected to erode away, removing nearly two billion years of geological evidence. It's true that other canyons will form, but it's hard to imagine any place as grand as this one. Get out and see it while you can!

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...

Accretionary Wedge #8 One Heck of a Beheading!

This month's Accretionary Wedge host is Julian over at Harmonic Tremors, and the theme of the month is what particular geologic event, no matter how small or large in scale, no matter how recent or ancient, is most significant to you as a scientist and an individual. As usual, I had to put a great deal of thought into the topic; and many things come to mind.

One of the final exam problems I present to my historical geology students is to devise a David Letterman style "Top Ten" list of the most significant events in the Mesozoic and Cenozoic eras, and many good answers come from that exercise, including asteroid impacts, dinosaurs and dinosaur extinctions, rise of mammals, continental break-ups, and so on (my favorite is the arrival of grass, and the effect it had on mammal evolution). These are big effects, and I enjoy discussing them, but as I approach the new Wedge topic of the month, I am thinking of a smaller event that has always intrigued me.

On the last day of 1996 and first few days of 1997, Central California was ground zero for an extraordinary meteorological event, a warm tropical storm that dumped feet of rain onto a record Sierra Nevada snowpack. The resulting floods were catastrophic, and quite simply are not likely to be experienced again (some drainages were described as having 250 - 500 year floods). My in-laws lived on the Walker River, an eastern Sierra river whose greatest historical flood had topped out at 6,000 cubic feet per second. In the 1997 flood, the river reached 12,000 cfs. Needless to say, many homes were washed away, and others (including my folk's) were damaged. The river channel was completely reshaped, and in many places a new channel had formed.

The government decided that a river should stay put, and so the Army Corps of Engineers was called in to "put the river back" where it was supposed to be, and so I got to watch over a period of weeks as the giant bulldozers prepared the old channel to have a river in it again. The big day arrived, and right there on my father-in-law's porch I was able to watch as the dozer made the cut the brought the river back into the old channel for the first time in months. It was a marvelous sight to watch the whole river change course. Of course I had no digital camera at the time, although an old-fashioned video of the event exists somewhere out there in storage.

It was a great geological event (well, geological engineering event), but it is not what I am going to talk about!

I became a geology major in the Grand Canyon. Literally IN the canyon, as I was on my first overnight geology field trip, and we spent five days below the rim, hiking the New Hance Trail down and the Grandview Trail on the way out. One of the details I remember the best is a discussion of the origin of the canyon itself. In those olden days of 1976, it was an unsolved mystery (I am glad it is all solved now...) but the most intriguing hypothesis, to me at least, was the stream piracy model. It posited that the ancestral Colorado River was flowing southeast towards the Gulf of Mexico, but that it was pirated by a "Hualapai Drainage" that worked its way east through the Kaibab Plateau by way of headward erosion. The part that intrigued me the most was to imagine the moment, the minute, that the Colorado River started to flow over that low divide that separated the two giant drainages, and the Colorado started a new channel that eventually gave us one of our greatest national treasures, the Grand Canyon. Imagining that moment, along with many others, made me curious enough to become a geologist and teacher.

So....the most important geologic event in my life happens to be one that may never have happened. But...it could have!