hat’s with those old river gravels you see as you drive down to the boat ramp at Lees or float through Furnace Flats? Have you ever hiked up Nankoweap or Kwagunt or done the Carbon-Lava Chuar loop and wondered what all those flat terraces and gravels are doing up there? You may be familiar with the work of Ivo Lucchitta on these old gravels, but research done over the past few years using new tools for geologic dating is giving us more information on the age and meaning of these old gravels. Here, I try to relate what these recent studies tell us about the history of canyon cutting and the surprising influence of climate change on the Colorado River.

Pleistocene gravels and terraces in the big picture
To understand these gravels, we have to take a step back. Remember that overall incision of Grand Canyon began about six million years ago when the river finally found its way off the high Colorado Plateau and flowed out through the low country beyond Lake Mead. As river incision has continued since that time, it has occasionally stopped, and instead of cutting, has deposited sediment for several intervals of the Pleistocene (from 1.8 million years ago to ten thousand years ago). What is left behind from these cyclic changes are the gravels we see preserved here and there along the canyon, left high and dry by more recent incision. Why would the river have a split personality—incising sometimes and depositing other times? The answer is climate change.
We know from fossil plants and other things that lived in Grand Canyon during the peak of the last ice age (about 20 thousand years ago) that the average temperature back then was much lower (6-7 degrees c, or about 12 degrees f) and that annual precipitation was a bit higher. This would have significantly changed the rates of erosion and the amount of sediment getting to the river through side canyons and larger tributaries. The climate changes up in the Rocky Mountain headwaters were even more extreme, changing the discharge and flooding patterns of the river drastically. Looking over the whole Pleistocene, there have been many glacial-interglacial climate cycles, and each of them fundamentally changed the balance between the river’s flow and the sediment it was carrying. This sometimes caused the river to deposit sediment that it didn’t have the energy to carry, building up its bed before the next climate shift changed the balance back to downcutting again.
Previous researchers like Machette and Rosholt, as well as Ivo Lucchitta, recognized these gravels were probably formed by such climate cycles. But exactly when and how rivers respond to climate changes is one of the big questions in Geomorphology—especially considering our efforts to understand what is going to happen in the future as global warming continues. Ken Hamblin, with his research on the lava flows in western Grand Canyon, suggested instead that these gravels were all deposited at times when the river was backed up behind lava dams. Is this even possible? Below are three familiar examples of gravels my collaborators and I have worked on in order to answer these questions:

Example 1: outcrop across from Kwagunt on river left
Seeps and spring deposits, or travertine, are abundant along river left from Kwagunt (rm 56) to the confluence of the lcr. Straight across from Kwagunt camp one can see some of this travertine interlayered with old river gravel and hillslope deposits (Figure 1). This outcrop shows that, in the past, springs were seeping out along the edge of the Colorado River when it was depositing sediment rather than incising. A great thing about travertine is that its age often can be determined using a method called uranium-series dating, which we have used to figure out the age of the gravel at this outcrop.
A sample from travertine lying on the bedrock stratigraphically below and older than the river gravel is 151 thousand years old. Another sample of interfingering travertine up near the top of the river gravel is 118 thousand years old. Together, this means the river stopped incising and started depositing gravel sometime after 151 thousand years ago and deposition continued until some time just after 118 thousand years ago.

Example 2: downstream end of Tanner Bar
This is an example of the most prominent paleo-Colorado River gravel in eastern Grand Canyon (Figure 2). It is the lowest old gravel next to the modern river and the base of it is still below present river level. Sand lenses within the deposit can be dated by a method called optically-stimulated luminescence dating to tell us when the river was depositing and burying those sand lenses. Pebbles from the top surface of this landform also can be analyzed to tell us how long they have been exposed at the surface—that is, how long it has been since the river started downcutting again and left this flat terrace behind. For this, we use a different method called cosmogenic-exposure dating.
The summary here is that the paleo-river was depositing gravel during an episode that began prior to 71 thousand years ago and continued until after 69 thousand years ago. The exposure age from this terrace surface tells us that by 55 thousand years ago the river had changed processes and started downcutting again.

Example 3: terraces and deposits up the
Carbon-Lava Chuar hike
Perhaps when hiking up Carbon Canyon, across the Butte fault, and into Lava Chuar, you have hung a right and walked a ways up Lava Chuar creek. If you get up on one of the Pleistocene terraces up there you get an overview like that in Figure 3. Although there are cool old gravels along the mainstem Colorado, most of our recent work has concentrated on tributary catchments like Carbon and Lava Chuar because they hold a spectacular record of stream gravels. Five or more distinct gravel deposits that can be correlated (are the same age) have been recognized in each of these tributary canyons.
A disconnect seems to exist between the gravels in these side canyons and the gravels along the mainstem Colorado. It seems the tributaries, though in-synch with each other, are depositing gravels at distinctly different times than Colorado River. We don’t yet know why—more on that some other time.

Things to think about
The weird tributaries aside, the timing of when the mainstem Colorado River is dumping gravels rather than incising matches the timing of glacial ice advances up in the headwaters of the San Juans, the Uintas, etc. Specifically, this happens starting at about the peak of mountain glaciation and then continues during the time those glaciers were melting back.
Another weird thing: Pretty much everywhere else in the western u.s., and even across the world, geomorph-geeks find Pleistocene river gravels that date to the last ice age, about 20 thousand years ago. We cannot find any gravels this young in Grand Canyon. They should be there! A fun hypothesis is that they do, in fact, exist, but are hidden under the present-day channel.
What about those damned lava dams? Even though lavas poured into the western canyon and disrupted the river, all evidence argues against the idea that these volcanic eruptions somehow caused these gravels in eastern Grand Canyon to accumulate. Even if some of the proposed monster lava-dam lakes did exist, the timing is all wrong, and the gravel deposits themselves are all wrong sedimentologically. They were clearly put there by the same streams and rivers that cut down through them today.

Joel Pederson
Department of Geology
Utah State University

P.S. my collaborators in this work have been Matt Anders (grad student), Warren Sharp, Tammy Rittenour, and John Gosse who all produced the ages on the gravels, and Karl Karlstrom who took a break from bedrock geology to check out these gravels.

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