It could become a great Grand
Canyon trivia question: historically, which rapid had the greatest rise
in elevation at its head? Many informed people would reply that it must
be Crystal Rapid because of the infamous 1966 debris flow that changed
a riffle into a monster. More recent evidence suggests it could be Lava
Falls, which had six debris flows between 1939 and 1995. The answer, which
may surprise you, comes as a result of a major advance in our ability
to monitor changes in rapids in Grand Canyon. We’ve learned how
to compare airborne survey data, known as Lidar and flown in 2000 over
the entire river corridor, with the data surveyed in 1923 by the U.S.
Geological Survey. This is the second, and by no means the last, article
in this series to use this new comparison to evaluate how Grand Canyon
Rapids have changed.
Sheer Wall Rapid (mile 14.3 L)
Sheer Wall Rapid is an interesting piece of whitewater that very few people
pay attention to these days. It sure caught John Wesley Powell’s
attention in 1869: the scariest rapids he found were the ones he couldn’t
easily portage boats around. In his book (Powell, 1895), Powell describes
in great detail how they scouted the rapid by climbing through crevices
and ledges to get a view of how best to portage it. On higher water in
1872, Powell and crew ran it after another careful scout (Dellenbaugh,
1908). No one else has paid much attention in print to this rapid. Fortunately,
Franklin Nims, Robert Brewster Stanton’s photographer, scrambled
up on a ledge to capture Sheer Wall as it appeared in 1889.
Although Sheer Wall Rapid is benign compared to many others, it does have
the characteristics of its name. The rapid is in the middle of a long
reach of narrow canyon where the river cuts through Supai Sandstone. Tanner
Wash, which drains seventy square miles, cuts through a slot in the Supai
on river left, dumping all of its material into the river via a low waterfall.
Constrained laterally by the sheer sandstone walls, the river is forced
to pool up, then pour over the top of the debris fan in a unique and ordered
fashion. At low water, the debris fan is visible on river left, but it
is submerged at discharges higher than about 20,000 cubic feet per second
(cfs). Sometime between 1889 and 1992, at least one debris flow entered
from Tanner Wash, adding material and raising the head of the rapid by
a foot (Fig. 2). The lack of precision in determining the year of the
Tanner Wash debris flow is due to the scarcity of photos taken at river
level. Interestingly, due to pooling from the debris flow at House Rock,
two and a half miles downstream, the overall drop at Sheer Wall actually
decreased by roughly a foot between 1923 and 2000. Debris flows can enhance
the drop through rapids, and downstream debris flows can take it away.
Oh by the way, a new debris flow appears to have aggraded that debris
fan in September 2002. It didn’t seem to change the rapid, but it
sure felt friskier.
House Rock Rapid (mile 16.8 R)
In 1923, Colonel Claude Birdseye led a U.S. Geological Survey team through
Grand Canyon, evaluating the feasibility of dam sites and surveying their
potential capacity. The maps produced by the Birdseye team were a remarkable
achievement and represented the first comprehensive water-surface profile
of the river. Birdseye’s boatmen ran House Rock Rapid on August
4, 1923, while his survey team, walking around, measured a drop of nine
feet. They had portaged Soap Creek Rapid earlier in the trip but felt
House Rock posed no great danger. Birdseye boatman Leigh Lint described
House Rock in his diary as a “straight chute of large waves which
about half filled the cockpits of three of the boats.” The rapid
was considered by old-timers to be a wet, but simple, drop. In response
to Otis “Dock” Marston’s survey of difficult rapids,
Emery Kolb included Soap Creek, Hance, Horn Creek, Granite, Hermit, and
Lava Falls; House Rock was not on his list.
Experienced river guides know
all too well that things are different now: House Rock Rapid is the biggest
thing at lower water levels in Marble Canyon. At around 10,000 cfs, you
enter left and pull hard to the right to miss the big hole on the left
at the bottom of the rapid. If you pull too early, the right side is a
rumble run over shallow boulders. If you pull too late and go over the
guardian lateral, that hole will flip your 18-foot oar boat as if it were
a piece of balsa wood. Those boats doing their best flotsam impression
through House Rock tend to become flotsam (and jetsam) below the hole.
House Rock Rapid gained prominence sometime between 1966 and 1971 (Melis
and others, 1994). During that time, one or more debris
flows pushed into the river, constricting the current against the left
wall and forcing the current toward the large hole at the bottom of the
rapid (Webb and others, 2002). Martin Litton thought the big change occurred
during the 1966 storm that created Crystal Rapid; Brian Dierker didn’t
disagree but thought it kept changing through the late 1960s as more sediment
was added from Ryder Canyon. The rapid Birdseye experienced was formed
by a mature, modest, and unobtrusive debris fan on river right (Fig. 3a).
Though the waves were large, the run through the rapid was straight and
free of rocks. Today, however, a large collection of rocks cover the old
run and boats are forced down what used to be the left side. The hole,
present but easily avoided in 1923, is now more of a factor as they say.
Of all Grand Canyon rapids that we could compare for change between 1923
and 2000, House Rock had the greatest rise at its head of all—just
shy of six feet. Interestingly, not only did House Rock Rapid constrict
and aggrade, but Redneck Rapid (mile 17.4 l) and 18-Mile Wash (mile 18.0
l) downstream also added new material between 1923 and 2000. The rise
at 18-Mile Wash resulted from a well-documented debris flow in 1987 while
the rise at Redneck resulted from a rockfall in 1973 or 1974 (Melis and
others, 1994). Figure 2 shows how the drop through House Rock was decreased
by downstream pooling at Redneck Rapid. In 1923, Elwyn Blake noted that
below House Rock, “the river was smooth for…five miles,”
an observation confirmed by the Birdseye survey data. That’s certainly
not the case now.
Marble Canyon is filling in with boulders. In the past century, Sheer
Wall and House Rock Rapids had large, rapid-enhancing debris flows, while
Redneck and 18-Mile Wash saw events that created new rapids out of calm
water. Eventually, North Canyon, which has seen no debris flows historically,
will experience a large event, possibly drowning out the drops at Redneck
Rapid and 18-Mile Wash, thus returning the reach of river between House
Rock and North Canyon to a state Elwyn Blake would recognize. And part
of Boulder Narrows could be submerged in the process.
Bob Webb, Chris Magirl, & Diane BoyerReferences:
Dellenbaugh, F.S., 1908, A canyon voyage, the narrative of the second
Powell Expedition: Tucson, University of Arizona Press (1991 reprint),
Melis, T.S., Webb, R.H., Griffiths, P.G, and Wise, T.J., 1994, Magnitude
and frequency data for historic debris flows in Grand Canyon National
Park and vicinity, Arizona: U.S. Geological Survey Water Resources Investigations
Report 94–4214, 285 p.
Powell, J.W., 1895, The exploration of the Colorado River and its Canyons:
New York, Dover Publications (1961 reprint), 400 p.
Webb, R.H., 1996, Grand Canyon: A century of change: Tucson, University
of Arizona Press, 290 p.
Webb, R.H., Melis, T.S., and Valdez, R.A, 2002, Observations of environmental
change in Grand Canyon, Arizona: U.S. Geological Survey Water Resources
Investigations Report 02-4080, 33 p.