Rocks, Rapids and the Hydraulic Jump
Water flows downhill. Side canyons make rapids.
Rocks make holes. These are the essential facts that face each of us as we travel down the
Canyon. But these are just the essentials; there is much more to the story. Why are the
rapids where they are? Where do the rocks come from and why does the water act like it
does when it meets a rock? Do rapids change? Shedding light on these and other questions
is the story told here. The geomorphology and hydraulics of the Grand Canyon are complex
subjects. As anyone knows who spends much time on a river, the dynamics of turbulent water
are anything but simple. But I will leave the equations to more technical papers and ask
those more knowledgeable in these subjects to excuse my simplifications.
One of the side benefits of the study of the operations of Glen Canyon Dam is
a more thorough look at the river and its rapids. Much of the information presented here
is extracted from the work done for the Bureau of Reclamation’s Glen Canyon
Environmental Studies (GCES). In particular I would like to thank Sue Kieffer for work on
the rapids and waves of the Canyon, to Bob Webb for insight into debris flows and an
analysis of the photos taken by Robert Stanton in 1890, and to Jack Schmidt for an
understanding of the Canyon’s dynamic geomorphology.
Rapids: a primer
In Grand Canyon virtually all rapids are formed by the rock debris carried into the river from side canyons. Tumultuous summer thunderstorms and severe winter storms wash large amounts of sediment into the river, narrowing the river as a fan of debris is built. Because side canyons tend to form along structural weaknesses (or faults) in the rock, canyons on both sides of the river are common and can provide twice the material. As the river is narrowed a sort of dam (technically called a weir) is formed which backs up the river and forms a quiet pool above the rapid. This pooled water then rushes over the weir in an effort to drop back to its original level, gravity speeds it up, and a rapid is formed.
Debris Flows: rocks that float
The fact that our side canyons are very steep allows water flowing down
them to pick up a great deal of energy. Flash floods normally come to mind when we think
of mechanisms that move rock down these canyons and flash floods do carry considerable
sediment. But a much more efficient mechanism exists to move really big rocks down our
normally dry washes, one that can actually float house sized boulders. It’s called a
debris flow.
When the Colorado is thick and muddy it contains less than 15% solids. A
hyperconcentrated flow such as a flash flood may be made up of as much as 40% solids. But
a debris flow can be 85% solids, so thick that large boulders are floated on top of the
flow. Because it acts only as a lubricant, a little water can move a lot of rocks. The
next time you pass the rock at President Harding consider that it probably floated down
the steep slope on river left and half way across the river. Debris flows may be the topic
of a later story in the news, so enough said here.
River reshapes the rapid
No sooner does a debris fan form at a canyon mouth then the river begins to remove it. The river’s success in this task is dependent on how much energy it can muster for the job. Higher energy comes from more velocity; the tighter the constriction the higher the velocity through it. In fact, the river’s ability to move material increases with the square of the velocity. If the river current’s speed doubles, the force is multiplied by four. A tripling of water speed increases force by nine. In short, high water removes the largest rocks in the shortest time. And the pre-dam Colorado often saw high water. Natural spring floods regularly brought 80,000 - 125,000 cfs through the Canyon. Floods of 125,000 cfs in 1957, 220,000 cfs in 1921, and an estimated 300,000 cfs in 1884 have been identified. Of course the closing of Glen Canyon Dam in 1963 put a stop to the high spring floods. Until 1983 that is....
A Common Width Ratio
In her investigations of the Colorado’s rapids, Sue Kieffer came upon an interesting phenomenon. It seems there is a “normal” ratio between the width of the river at constrictions formed by debris fans and the width of the river immediately upstream. At the majority of Canyon rapids the river narrows to about one half the it’s width upstream, a ratio of 0.5. We understand that as each new fan is subjected to the forces of high spring floods, rocks and debris will be washed downstream and the channel widened. It is less obvious why there should be such a “standard” width ratio. What force acts so uniformly on all debris fans to bring them to this standard? The answer may lay in a physical phenomenon we see on the river everyday, a hydraulic jump.
Hydraulic Jumps
Hydraulic jumps are common in our rapids. Most of the waves and holes we
try so hard to avoid are some class of jump. When you gaze at the ledge at Lava, or the
hole at 209 Mile, or the waves in the tongue of Crystal you are witnessing a hydraulic
jump in action. What causes them and why do they occur? There are at least a dozen
equations to describe the basic nature of water flow but the only one you need to know
here is Q = VA. The flow of water (Q) equals river speed (V) times the cross-sectional
area (A) of the channel. We see it all the time. As the dam releases more water the river
moves faster (greater velocity) and the river level rises (greater area). In narrow
stretches of the river the current is faster, in wider sections it is slower. Because
water does not compress, this is always true.
But what happens to water that is flowing through constrictions? Depending on
the velocity and depth of the stream, the flow is described as being subcritical,
critical, or supercritical. The slow, placid river above President Harding is certainly
subcritical. The swift, turbulent water down the right side of Lava is just as certainly
supercritical. Critical flow exists as a transition between these two. When water flows
very swiftly it builds up a lot of energy. This internal energy can be thought of as a
combination of the turbulence and velocity of the water. If the velocity increases very
swiftly or the depth decreases quickly, as in our rapids, the flow can become
supercritical and very unstable. The internal energy of the water, a function of its
velocity and depth, can become greater than the force of gravity which is holding it down.
Suddenly the river is trying to cram too much water through too small a tube. Some of the
internal energy must be released and, being constrained on three sides by the channel, it
does so by rising upward and slowing back to its subcritical state. A hydraulic jump is
formed. Most of these we see as waves, the most severe we call holes.
The wall of water formed can be steep and dramatic. Our Q = VA equation must
be satisfied, so the river behind the jump (or hole, remember) has a greater depth and
slower velocity, and reestablishes the balance of energies. Because the main ingredients
for a hydraulic jump are swift water and quickly changing depths, rapids are perfect
incubators. Three things common in a rapid can bring on a jump: 1) the river channel can
suddenly shallow, 2) a large obstruction (such as a rock) can cause the water to shallow
as it moves over it, and/or 3) the channel can quickly narrow. In the Canyon it’s
usually some combination of these. But how does that effect the width ratio of rapids?
The Normal Wave
When the high water of a spring flood hits the severe constriction of a recent debris flow a hydraulic jump or hole of enormous size and power can form. Such a hole is not associated with individual rocks as we are used to experiencing. It is a broad, backbreaking wave created by supercritical flow formed by the sudden constriction of the river and the high velocity of the water. This wave, called a normal wave, is perpendicular to the river flow, often spanning the width of the river, and can be tens of feet in height. The wave has tremendous turbulence and can quickly erode and widen the channel until the flow again becomes subcritical and the jump subsides. Sue Kieffer attributed the uniformity in width ratios to this process. As long as the channel is erodible the process is essentially self regulating. If the constriction is too narrow, high spring floods create a normal wave which in turn erodes the channel sufficiently to remove the wave. But the closure of Glen Canyon Dam in 1963 ended the natural spring floods and up until 1983 river levels rarely exceeded 30,000 cfs. Any rapid that formed during those 20 years had not yet fully matured. As we shall see, this is exactly the case at Crystal.
Crystal: A rapid’s rapid.
Crystal Rapid, the rock garden, Slate Creek eddy, Crystal Hole,..... Crystal. With the
possible exception of Lava Falls few rapids in the world evoke such universal respect and
awe. How many sweaty palms have climbed to the top of the bluff and gazed down on the
tumultuous waters? Has anyone climbed up there without sweaty palms and a dry mouth?
But Crystal is more than a large rapid. It is long; one of the longest we
have in the Canyon. It is situated in the ominous depths of the upper Granite Gorge. And
it’s not a pretty place to get in trouble. An upside down boat or swimmer has a long
swift float ahead all the way to Tuna Creek or beyond. That is if the rock garden is
safely negotiated. There are as many different runs as there are boatmen to run them.
While the runs in Lava have stayed pretty much the same over the past 25 years, Crystal
has changed dramatically. Its recent history gives us a glimpse into the evolution of the
rest of the rapids in Grand Canyon. So if you’re interested in what makes them tick,
whether you’ve climbed to the bluff with sweaty palms yet or not, here is the story
of Crystal of Grand Canyon.
Pre-1966
Little changed in Crystal Rapid between Robert Stanton’s first photos in 1890 and 1966 . But over the past 25 years it has been the one of the most dynamic in the Canyon. At the turn of the century Crystal was a long but relatively minor rapid. The 1923 U.S.G.S survey party measured a drop of 17 feet. (See pictures) The run was wide and the river pushed to the right or Crystal side. There was no rock garden. The main pre-1966 obstacles were rocks on the left, the result of a large debris flow out of Slate Creek. The force of this flow, which occurred sometime before 1890, was such that it pushed material tens of feet upstream. The large rock we still see on river left just above the mouth of Slate Creek came from that debris flow and hasn’t budged in more than 100 years.
1966 Flood
In December of 1966 a severe winter storm struck the western United
States. It was neither the largest nor most severe to hit this region but it set off a
sequence of events that was to dramatically effect all who subsequently floated the
Colorado. Instead of snow this warm storm brought rain to the high elevations of the
Colorado Plateau. It is estimated that only about 5 inches of rain fell in intense
cloudbursts along the upper drainages of the Crystal amphitheater but this rainfall
triggered 19 slope failures in the Hermit Shale, Supai Group, and Muav Limestone. These
failures provided the material for several debris flows that joined in Dragon Creek and
flowed 13 miles to the Colorado River at an estimated 10 to 12 miles per hour. At the
river 10,000 cfs of rock debris collided with 10,000 cfs of river water, severely
constricting the river to a width of less than 100 feet and increasing the fall of the
rapid by 16 feet. “Lake Crystal” was formed, drowning the tail waves of Boucher
Rapid upstream.
In the span of a few minutes Crystal became a completely new rapid. But it
was a young and immature rapid. The river cleared what it could from its channel and
formed the rock garden below. With a drop of 33 feet it was now certainly one of the
longest and swiftest rapids in the Canyon. But it differed from most other rapids in two
important ways. First, the main drop and narrowest constriction (barely 100 feet wide) did
not occur at the head of the rapid as is common. Most of the fall was spread relatively
evenly between the tongue and the constriction well below the Slate Creek eddy. Standing
within this constriction was a large rock. The famous Crystal Hole was formed when the
river, accelerating swiftly down the long slope of the upper debris fan, became
supercritical when forced over the large rock and through the narrow constriction
simultaneously.
The second and most important difference was in the width ratio. The
constriction in Crystal was barely one quarter the width of the river upstream instead of
the more common one half. This severe narrowing certainly contributed to the size and
power of the old Crystal Hole and played an important role in the events of 1983. Crystal
had yet to completely come of age.
1983 Flood
By 1980 a series of wet years had completed the filling of Lake Powell and
the dam stood at near capacity. The winter of 1982-83 produced twice the normal snowpack
in the southern Rocky Mountains. Just enough room remained to accommodate a normal spring
runoff. But this spring was anything but normal. A series of warm, wet storms spread rain
along the snowpacked watersheds and the rivers feeding the Colorado rose dramatically.
Glen Canyon Dam began releasing excess water in early June, 1983. By June 7th the Colorado
River was flowing at 50,000 cfs; by June 22nd, 70,000 cfs; and finally peaked on June 26
at 92,000 cfs. The inflow to Lake Powell peaked shortly before at 120,000 cfs.
During that period enormous changes took place in Crystal. As the river rose
the old “Crystal Hole” was replaced by a large hydraulic jump, (Normal Wave)
perpendicular to the river current and about 100 feet downstream of the old hole. As flows
reached 50,000 - 60,000 cfs much of the debris fan was covered with slow water and the run
of choice was through the tammies there. Loud cannon-like booms from the main channel
announced the movement of large boulders. The Normal Wave surged to heights of 30 feet. At
92,000, cfs water (and boats) entered the wave at almost 30 mph while velocities through
the wave were only a little over 10 mph. Going through the wave was like hitting a two
story wall at 20 mph. Even the largest rafts flipped.
Meanwhile the rising power of the river increased the width of the channel by
removing the shoreline along the crystal debris fan. At the same time it deepened its
channel by eroding upstream toward the head of the rapid. When the water finally dropped
in October, Crystal was very different. Gone was the old Crystal Hole, replaced by a
strong hole or two at the entrance of the rapid. The narrowest point and the steepest drop
now occurred at the head of the rapid where a strong hole was now the focus of river
running. The rock garden received the majority of the material torn out of the main rapid.
Most significantly the width ratio had increased from .25 to about .40. It was expected
that the increased width ratio would be sufficient to eliminate the supercritical
conditions that created Crystal’s Normal Wave. But a few springs later, as the river
rose again to more than 45,000 cfs, I personally watched a large breaking wave appear just
where the Normal Wave had been, perpendicular to the current and nearly shore to shore.
One of our rafts actually flipped twice exiting the rock garden right side up but worse
for the wear. A second raft flipped, and with the boatman perched on the floor, floated
all the way to Elves before finding an eddy.
Epilog
While Crystal can now be considered a full grown rapid, it is by no means completely mature. Its width ratio of 0.40 is still less than the average of 0.50. Higher flows would almost certainly result in supercritical flow and another Normal Wave. Sue Kieffer’s calculations show that the 0.50 width ratio is probably the result of river levels in the range of 400,000 cfs. Crystal will have to wait for the silting of Glen Canyon Dam before experiencing those flows. It may just do so. In spite of the 1966 debris flow, Crystal drainage is not considered particularly active. An event of that magnitude may not occur there again in the next 1000 years. Interestingly, according to Bob Webb, the side canyon which will most likely produce the next new rapid is none other than Prospect Canyon at Lava Falls. Now that could be interesting.......
Tom Moody
