The Changing Rapids of Grand Canyon—
Crystal Rapid

In the beginning, Crystal Rapid wasn’t even noteworthy among Inner Gorge rapids. In the 1980s, it was the most-feared rapid on the river. Now, Crystal is kinder and gentler, owing in part to the 1996 controlled flood and the recent tendency for lower releases from Glen Canyon Dam. Crystal Rapid also represents the largest geomorphic channel change in the recorded history of the Colorado River, and its geomorphic history has spawned a mythology that transcends its difficulty as a whitewater run (Webb, 1996).
Because few people noticed the rapid amid the raging whitewater upstream and downstream, we know only a little about what the rapid used to be like. Robert Brewster Stanton photographed the rapid from several angles in 1890 (Figure 1). The only river trip that had a problem here was the 1915 Tadje-Russell trip; they eventually sank one of their steel-encased boats among rocks that probably came from Slate Creek in a prehistoric debris flow. In 1923, the U.S. Geological Survey surveyed the water-surface profile through Grand Canyon and worked on the reach from Hermit to Tuna Creek Rapids on August 30–31, 1923. They found that the rapid dropped sixteen feet (4.9 meters), and Claude Birdseye, the expedition leader, remarked in his diary that “the waves are high but the fall is distributed over about one-third mile, so it is easy to run.”
That all changed in December 1966. Much has been written about the 1966 storm in the southwestern United States, and some have greatly exaggerated its magnitude and significance. Between December 4 and 6, rainfall ranged from fourteen inches at the North Rim Entrance Station (8,700 feet elevation) to 2.08 inches at Phantom Ranch (2,570 feet elevation) and probably averaged five inches over the Crystal-Dragon Creek watershed (Webb, 1996). Debris flows occurred in Prospect Canyon (Lava Falls), Bright Angel Creek, Lava-Chuar Canyon, and Nankoweap Creek as well as in Crystal Creek. Webb and others (1989) estimated the discharge of the debris flow to be about 10,000 cfs, most of which was sediment. The change to Crystal Rapid was awesome (Figure 2); the debris flow constricted the river by about 80 percent and increased its drop, some of which was removed by the 1983 flood from Glen Canyon Dam.
Some hydrologists and geomorphologists have been fooled by Crystal Rapid, responding in part to its awesome whitewater and overlooking some readily available information. Cooley and others (1977) provided ample documentation of the Crystal Creek debris flow, despite the fact that computationally they treated it as if it were a clearwater flood. They found that the debris flow covered some archaeological sites, and some have chosen to interpret this as meaning the debris flow was the largest in Crystal Creek during the last thousand years (as discussed in Webb, 1996). Not so; another debris flow of similar size occurred sometime within the last 300 years, and a reasonable recurrence interval for the 1966 debris flow would be about 200 years (Cooley and others, 1977; Webb and others, 1989; Webb, 1996). The amount of deposition and size of boulders led Kieffer (1985) to conclude that a flood of 400,000 cfs would be required to remove the debris fan. However, the debris fan was reworked during both the 1983 and 1996 floods, with boulders more than six feet across being swept downstream. In fact, the Rock Garden that separates the rapid into two parts was mostly formed during the 1983 flood.
Between the 1983 and 1996 floods, and particularly at discharges below 30,000 cfs, Crystal posed severe challenges to motorboats and oar boats alike. No one in their right mind would purposefully go through the top center hole, and at most water levels the left side wasn’t an option. The typical run was either to slam the bow of a motorboat into the right bank just below the head of the rapid and pivot around, running the rest of the rapid backwards (the “turn-around run”), or with precise timing crash through the strong right lateral, simultaneously hoping not to be surfed into that center hole or the softer and less-hazardous hole just downstream. The hole that Kieffer characterized as a “hydraulic jump” forms downstream of a large block of schist in the left center of the channel just below the mouth of Slate Creek. This hole grows in the mid-20,000 cfs range, peaking in that stupendous breaking wave observed at about 70,000 cfs in 1983. The final challenge of Crystal Rapid is to avoid the center run over the Rock Garden, which can be real difficult if the motor isn’t running or an oar or two is missing.

Surprisingly, Crystal changed during the 1996 controlled release. About 1,100 square feet of area was removed from the debris fan, mostly boulders. The top-center hole is now softer and no longer breaks perpendicular to the current. The right lateral, too, is softer, owing to the removal of a key boulder; this makes the pull to the right less strenuous even if the adrenaline is pumping hard. In recent years, few motor guides seem inclined to do the turnaround run, opting instead for a straight-forward right run. It even seems as if the left run is more available now, although that could just be because of the nature of recent flow releases from Glen Canyon Dam. The 1995 flood in Crystal Creek may have been partially responsible for some of these changes, because a few new boulders were thrown into the river and then rearranged by the 1996 flood. As the story of Soap Creek shows, a few new rocks here and there can tame a rapid’s waves.
There is no doubt that Crystal has had a major impact on the Colorado River through the Inner Gorge. The exact nature of these changes was unknown until 2000, when the Grand Canyon Monitoring and Research Center arranged a Light Detection And Ranging (lidar) overflight of the river corridor with one intent of developing a new longitudinal profile for the Colorado River. Lidar is a sophisticated laser-based imaging system used to develop very detailed topographic data. Despite all the years of research in Grand Canyon, the 1923 usgs was the only systematic data available on the water-surface profile of the river. We recently analyzed the 2000 lidar data, matching both the 1923 usgs data (normalized to 8,000 cfs discharge) and the lidar data (obtained at 8,000 cfs discharge) to a common river-mile distance. Because of inaccuracies and difference of interpretation of the center of the river, the two data sets do not precisely overlap and must be adjusted. Part of our adjustment was based on the fact that the elevation at the heads of both Hermit and Tuna Creek Rapids have not changed historically. In contrast, both Boucher and Crystal Rapids aggraded appreciably between 1923 and 2000.
Comparison of the lidar and 1923 data (Figure 3) shows several things about Crystal Rapid and its effect on its neighbors. The drop through the rapid is now 21.3 feet (6.5 meters), down from its post-1966 high but considerably higher than what the rapid had in 1923. Because of the 1951 debris flow in Boucher Creek and the backup from the Crystal Creek debris flow, Boucher Rapid is now raised, on both the upstream and downstream sides, above what it was in 1923. As previously discussed in the bqr, Boucher simultaneously is drowned out by Crystal Rapid and also drowns out the bottom of Hermit Rapid. Returning to Crystal’s direct effects, the lidar data clearly shows “Lake Crystal” and subtly shows that some of the debris from Crystal may have washed downstream towards Tuna Creek Rapid, raising the bed slightly between the two rapids.
The tributaries giveth, and the river taketh away. When the water drops from the floods that occasionally are released from Glen Canyon Dam, some rapids can be significantly changed. Although we think that much of the debris fan at Crystal Rapid would disappear if a flood the size of the 1884 event (300,000 cfs) occurred, or even if the 1921 event (220,000 cfs) were repeated, the fact is that it is unlikely a flood larger than the 1996 event (47,000 cfs) will occur anytime soon. Crystal is here to stay, and fortunately, for the time being at least, it isn’t the raging monster that some of us once beheld. And this rapid continues to change.
Bob Webb and Chris Magirl

References:
Cooley, M.E., Aldridge, B.N., and Euler, R.C., 1977, Effects of the catastrophic flood of December, 1966, North Rim area, eastern Grand Canyon, Arizona: U.S. Geological Survey Professional Paper 980, 43 p.
Kieffer, S.W., 1985, The 1983 hydraulic jump in Crystal Rapid: Implications for river-running and geomorphic evolution in the Grand Canyon: Journal of Geology, v. 93, p. 385–406.
Webb, R.H., 1996, Grand Canyon: A century of change: Tucson, University of Arizona Press, 290 p.
Webb, R.H., Pringle, P.T., and Rink, G.R., 1989, Debris flows in tributaries of the Colorado River in Grand Canyon National Park, Arizona: U.S. Geological Survey Professional Paper 1492, 39p.