The Changing Rapids of the Colorado River—
Suturing of Boulders

At certain places in Grand Canyon, where debris flows have not recently occurred, one can catch a glimpse of the net effect of an important process that stabilizes rapids. Suturing results when rocks grind against one another under water, resulting in an array of particles that appear to fit together, much like the pieces of a jigsaw puzzle (Webb, 1996). Sutured rocks resemble flagstone paving, as if constructed by a master mason. As such, sutured boulders form large, seemingly immobile masses of rock impervious to the power of the river. This process mostly occurs in rapids and is especially noticeable in Cataract Canyon, where debris flows are less frequent than in Grand Canyon and the rocks are softer. Suturing also is apparent along the margins of the more stable rapids in Grand Canyon (e.g., 217-Mile Rapid, 232-Mile Rapid, Ruby Rapid) as well as at other nondescript places (Fig. 1).
The crew members of the second Powell Expedition were the first to notice suturing. Frederick Dellenbaugh described it at the Big Drops, possibly at Big Drop 2:
An interesting feature of this canyon was the manner in which huge masses of rock lying in the river had been ground into each other by the force of the current. One block of sandstone, weighing not less than six hundred tons, being thirty or forty feet long by twenty feet square, had been oscillated till the limestone boulders on which it rested had ground into it at slowly and regular rocking as the furious current beat upon it, and one could feel the movement distinctly (Dellenbaugh, 1908).
As Dellenbaugh notes, suturing results from vibration induced by strong currents impinging on rocks. Flow in a rapid pulsates, as manifested by breaking waves and swirling eddy fences, and the pulses are of sufficient amplitude and frequency to cause boulders to vibrate in place. What is remarkable is how quickly this process occurs (Webb et al., 1999); recently, we observed a lower unit and a whiskey bottle sutured into rocks in Cataract Canyon (Figs. 2 and 3).
Once a debris flow pushes boulders into a rapid, the river works constantly to move them downstream or erode them in place. Entrainment occurs when boulders are plucked from the bed and swept downstream; this mostly happens shortly after a debris flow occurs, when the river reaches a high enough stage and has sufficient power to start a particle in motion. It may be difficult to imagine, but large boulders are bounced downstream, moved along by the force of river water at flood stage. Boulders accumulate in an orderly fashion downstream, creating secondary rapids and debris bars that usually alternate from one side of the river to the other. The classic example of this is at Fossil Rapid, where the primary debris fan is on river left, the first debris bar is on river right, and the river moves through an S-turn at low water. Granite Rapid provides another example.
Most new debris fans have extremely loose rocks, making walking on them hazardous, and in some cases large air voids can be seen among the particles.

When a new debris flow is inundated by the Colorado River, some particles are moved short distances and rearranged. Particles are rotated by the current, and the air voids may be packed with smaller particles. This simple rearrangement makes entrainment of individual particles much more difficult. Once dropped in place, these boulders then sit and are subject to the other forces in the river’s bag of tricks.
The sediment load of the unregulated Colorado River is (was) high, both in Cataract Canyon and in Grand Canyon before Glen Canyon Dam was built. The load consists of considerable amounts of sand and gravel, particularly at flood stage. These particles collide with boulders in a process termed corrasion, pitting them much like sand hitting a windshield. Because most of the rocks along the river contain soluble calcium carbonate or other salts, the rocks also dissolve, albeit slowly, as water circulates among them. Dissolution is most effective in removing the cement between the grains in sandstone, but dissolution can also occur in massive limestones or granite. It is difficult to determine how effective these processes are or how quickly they operate, but they may be the ultimate means by which the river eliminates obstructions in its path.
In the process of entraining boulders, the force of the river flow vibrates boulders in place, much in the same way as a telephone wire vibrates in a wind. Vibration affects the rock matrix in two ways. At contact points, each boulder rubs its neighbors, removing material on each particle. If a soft particle, such as a sandstone, rubs against a harder particle, such as a solid limestone, the softer rock loses more mass in the process. Because of their weight, particles apply forces to their neighbors, but in the act of vibrating, these forces increase to include not only the weight of the boulder but also dynamic lateral forces created by pulsating water flowing past the rock. Dissolution of some minerals, particularly those containing calcium carbonate, increases with pressure, leading to the second mechanism of suturing. Dissolution is expected to occur more quickly at the contact points, where forces are high, than at other places exposed only to water.
Because suturing occurs relatively quickly, it is an important concern related to management of Glen Canyon Dam. If removal of aggraded debris fans is a management priority, floods like or larger than the one released in 1996 must be scheduled relatively frequently. In the absence of frequent floods, low flows in the Colorado River, which have insufficient power to entrain boulders, can vibrate them in place, resulting in suturing and a debris fan more resistant to particle entrainment. The next time you walk along the right bank at Crystal Rapid, notice how some of the Supai boulders have been shaped at their contact points with other boulders. Relatively young rapids such as Crystal can quickly become immobile masses of sutured boulders, much like Hance Rapid already has.
Bob Webb, Chris Magirl, & Diane Boyer
References:
Dellenbaugh, F. S., 1908, A canyon voyage: Tucson, University of Arizona Press, 1984 reprint, 277 p.
Webb, R.H., 1996, Grand Canyon: A century of change: Tucson, University of Arizona Press, 290 p.
Webb, R.H., Griffiths, P.G., Melis, T.S., Elliott, J.G., and Pizzuto, J.E., 1999, Reworking of debris fans by the 1996 controlled flood in Grand Canyon, in Webb, R.H., Schmidt, J.C., Valdez, R.A., and Marzolf, G.R., The 1996 flood in Grand Canyon: Scientific experiment and management demonstration: Washington, D.C. American Geophysical Union, Geophysical Monograph, p. 37–51.