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
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
Bob Webb and Chris Magirl
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.