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!

Time Beyond Imagining - Unscrambling History and the Origin of the Grand Canyon

And a river runs through it...*

This is a continuation of my long-running exploration of the geological history of the Colorado Plateau which has been going on now for more than a year. If you've been a glutton for punishment, take heart, for it is almost over. We are closing in on the final 20 million years of a story that has lasted for more than two billion.

One river system totally dominates the Colorado Plateau: the Colorado River. The present incarnation of the river drains portions of seven states and part of Mexico, an area of just short of 250,000 square miles. In an average year (which we haven't had many of recently), it might provide something like 13 million acre-feet of water, which is used many times over for irrigation, power generation and domestic needs (most of the water agreements assumed a far higher average). Little, if any, water ever reaches the Gulf of California anymore.

The river flows 1,450 miles from the Rocky Mountains of Colorado to the Gulf of California. For about 217 of those miles the river flows across the Kaibab Plateau, a place which most people know as the Grand Canyon. The river flows through other beautiful canyons, notably at Glen Canyon and Cataract Canyon, but the Grand Canyon is unique. In places the canyon is more than a mile deep..

So, what made the Grand Canyon?

Well, the Colorado River carved it. Probably. That's the easy answer. Now, ask the hard question.

What made the Colorado River?

The entire river has mysterious twists and turns, and many can be readily interpreted, but the Grand Canyon is a marvelous geological mystery. How did this raging river cross an 8,000 foot high plateau? Some explanations immediately suggest themselves: the river was in place as the plateau rose 50 million years ago, and simply eroded downward as the crust rose. But this doesn't work because the sediments on the west end of the canyon reveal that the river has existed at that site for less than 17 million years. And in any case the Gulf of California is no more than 5 million years old, so the river had to be going someplace else. And that's what makes the mystery so interesting. By virtue of removing the landscape as it cuts downward, the evidence that can tell us the story of the river has been destroyed, except for tantalizing little bits here and there.

If anything has been learned from our explorations of the Colorado Plateau over the last year, it is that the earth's crust has been changing and will continue to change throughout time. Rivers don't just "start", and then begin removing a landscape. Drainages evolve along with the landscapes that they traverse, and what happens along one stretch of the river is not going to be the same as what happens someplace else. Sometimes rivers are disrupted by major events, like huge landslides, earthquakes and lava flows, and stream piracy.

The short answer as to the genesis of the Colorado river is that we don't know all the answers, but we know a lot about it. The river has existed in its present course for no more than about 5 million years, and it carved much of the Grand Canyon in that relatively short period of time. It was aided and abetted by the Pleistocene Ice Ages, which produced glacial melt runoff many times the natural flow of today. Imagine spring flood runoff with sections of the river hundreds of feet under water.

We know that from the end of the Cretaceous to about 20 million years ago, the plateau region was surrounded by highlands, and that most rivers flowed northeast, but may have ended in dry basins or freshwater lakes. The height and timing of the uplift of the plateau during the Cenozoic is one of the controversial aspects of the story (early uplift related to Cretaceous convergence, or relatively recent uplift?). But what is known for sure is that the surrounding regions subsided to form the Basin and Range province and the Rio Grande Rift 15-20 million years ago, leaving the plateau as an elevated land mass.

Rivers and streams began eating away at the edges of the plateau by way of headward erosion, and over the years different stretches of isolated rivers were integrated into the single river system we see today. The precise method by which they were integrated is the focus of current research, and a number of interesting ideas have emerged over the years.

I've written a couple of times about an interesting version of a stream piracy event that was a prominent hypothesis in the 1970's when I was first learning geology in the Grand Canyon region. It suggested that headward erosion of a "Hualapai Drainage" cut entirely across the Kaibab Plateau until it captured the ancestral Colorado, which at the time was flowing southeast towards the Gulf of Mexico. The idea of seeing the moment of stream capture has always appealed to me, so I love the hypothesis, but it has some possibly insurmountable problems.

Another very creative idea made use of the fact that there are numerous cavern systems throughout the older Paleozoic limestones on the plateau. Perhaps the first route of the waters of the Colorado River across the Kaibab Plateau was under the surface, flowing through caves. Eroded and enlarging the cave openings led to the eventual collapse of the ground above, and the formation of a new canyon. Again, there are problems with the scale of this hypothesis, but it had the virtue of explaining some of the inconsistencies of other models.

In the modern era, one hypothesis suggests that the Colorado River may have been forced to the present path across the Kaibab Plateau early on, but that the river drained to the northwest into what is now the Basin and Range Province. By five or six million years ago, stream piracy robbed the old channel of the Colorado, but west of the Kaibab uplift. The avenue to the sea resulted in a rapid cutting of the Grand Canyon soon after.

An alternative modern hypothesis suggests that the integration of the Colorado River system involved the development of huge lake systems which one by one overflowed into adjacent basins, carving deep canyons as they went.

I couldn't begin to argue for one hypothesis or another, but luckily others are doing the discussion in my place, and the Grand Canyon Natural History Association has been kind enough to publish the latest book discussing the origin of the canyon on the internet at
http://www.grandcanyon.org/booksmore/booksmore_epublications.asp. The whole entire book! If you want to learn more about the research that is taking place these days, check it out.

*borrowed from Norman MacLean, referring to a different river, the Blackfoot in Montana. Great book and subsequent movie, including some unexpected geology references concerning J Harlan Bretz and the Spokane Floods.

Accretionary Wedge #17: Caught in a time warp!

Lockwood has graciously consented to host the new Accretionary Wedge, and asks where we would want to go in the geologic past with a time machine. I could imagine many moments, including some from childhood (I really wanted to see living dinosaurs), but I come back to one thing: I think I most would like to see the moment the Grand Canyon began!

One could argue that it didn't just begin, but some imaginative hypotheses have been suggested over the years that include some form of stream piracy, where a Colorado River once flowed elsewhere, possibly into the Gulf of Mexico, or points northwest in the Basin and Range. A separate drainage system related to extensional stresses to the southwest evolved which eventually captured the river, and from there the giant canyon was carved in just a few million years.

I have many times imagined seeing that first trickle of water that spilled over the low divide, and how it would have become a torrent, and within hours the work of building a new canyon would have begun. Whether the hypothesis is correct or not, it is a wonderful piece of work for the imagination!