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.
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. Its called a
River reshapes the rapid
No sooner does a debris fan form at a canyon mouth then the river begins to remove it. The rivers 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 rivers ability to move material increases with the square of the velocity. If the river currents 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 Colorados 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 its 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 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.
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 rapids 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?
Little changed in Crystal Rapid between Robert Stantons 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 hasnt budged in more than 100 years.
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
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.
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 Kieffers 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.......