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The Rapids and the Pools- Grand Canyon

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The Rapids and the Pools- Grand Canyon The Rapids and the Pools-Grand Canyon By LUNA B. LEOPOLD THE COLORADO RIVER REGION AND JOHN WESLEY POWELL GEOLOGICAL SURVEY PROFESSIONAL PAPER 669-D The rapids and the profile of the Colorado Rzuer Contents Illustrations Page FIGURE86. Photograph showing flow-measurement gear being readied for observations of velocity and depth- -.~ - .- - - .- -- 134 87. Cumulative curves showing distribution of river widths and depths in Grand Canyon- - ~ ___._--_. .~ ----___.-- 135 88. Photograph showing Hance Rapids ______.__.____.-----.-----....-----..-.-----.--~.--.-..--~---------136 89. Map and profile of Colorado River near Unkar Rapid ____._____.._.__________________________.----------137 90. Photograph looking downstream at Unkar Rapid .____._._______ .______________ ._..__~ ___________ ._.___- 138 91. Diagrammatic sketches showing four types of water waves occurring in rapids _._____.._______._.___________139 92. Photograph showing a rapid due primarily to convergence where rockbound channel narrows in a part of Granite Gorge____________.._..--~-..-----.--.----------.------.-..----------------.--------..--.-..--- 139 93. Photograph showing rapids unrelated to any tributary entrance and presumably caused by large gravel bar deposited on streambed____.______--.--.-..-------..-------..-..----.------.-.--.----.-.-.----~-. 140 94. Graph showing relation of fall in elevation of Colorado River to river distance- ____________________--------140 95. Diagrams showing channel cross sections, bed and surface velocities, and water-surface appearance near Unkar Rapid_____________..____________________------.------------.-----------.~.-------------------142 96. Graphs showing changes in flow characteristics during a 10-month period at Lees Ferry- - - - __--._. .-. -. - .-- 143 97. Profile of the Colorado River through the Grand Canyon.. ___________. ___-________._.___.-----.-. .-. .-- - 144 THE COLORADO RIVER REGION AND JOHN WESLEY POWELL THE RAPIDS AND THE POOLS- GRAND CANYON By LUNAB. LEOPOLD Abstract They had more reason than we for a deep and troubled fear, On that first trip, no one knew whether high and Through the Grand Canyon the Colorado drops in elevation vertical waterfalls might block completely any passage abont 2,200 feet in 280 miles ; moht of this drop occurs in rapids that account for only 10 percent of the distance. Despite the by boat. Clearly, there was no return upstream. xmportance of rapids, there are no waterfalls. Depth measure Powell (1875, p. 62), halfway through his trip, ex- ments made at Xo-mile intervals show that the bed profile pressed his feelings this way: "* * * there are great is highly irregular, but the apparent randomness masks an descents yet to be made, but, if they are distributed in organized alternation of deeps and shallows. Measurement 'of the age of a lava flow that once blocked the canyon near Toro- rapids and short falls, as they have been heretofore, we weap shows that no appreciable deepening of the canyon has will be able to overcome them. Hut, may be, we shall tnhen place dnririg the layt million years. It is reasoned that the come to a fall in these canyons which we cannot pass, river has had both the time and the ability to eliminate the where the walls rise from the water's edge, so that we rnpids. The long-continlied existence and the relative straight- cannot l~nd,arid where the water is so swift that we ness of the longitndinal profile indicate that the river main- tains a state of rluasi-ecluilibriiun whith provides the hydraulic cannot return. Such places have been found, except that requirements for carrying the debris load brought in from up- the falls were not so great but that we could run them stream without continued erosion of the canyon bed. The main- wit11 s:!fety. How will it be in the future!" trnance of the alternating pools and rapids Seems to be a neces- In the hundreds of miles through which the river sary part of this poised or equilibrinm condition. flows in a canyon section, the channel consists of :in al- ternation of flat pools and steep rapids. Yet there are no GENERAL STATEMENT waterfalls in the usual sense of the word. What ,John In the dry glare of a sun-drenched afternoon, in the Wesley Powell feared the most does not exist. Why not ? bitter chill of a thunderstorm wind, or in the purple This seems a simple enough question, yet the answer is evening, there is no respite from the incessant boom of neither simple nor obvious. the great river. One finds at times he has forgotten the This chapter is an attempt to explain, albeit incom- ever-present roar of the rapids and, as if suddenly pletely, why rivers characteristically develop a uniform awakened, he hears it apniii. So persistent is the sound profile downstream, gradually decreasing in steepness. that 1 often wonder how the mind can put away the Despite this progressive flattening of slope, they teiid noise into some recess, even momentarily. to maintain an alternation of low-gradient deep pools The river's boom is associated with a pervasive un- and higher gradient riffles or rapid reaches. The general easjness which never leaves a man while he is clamped explanation will then be applied to the Colorado River within the cliffs of the caiiyoii. This uneasiness is not in the Grand Canyon section to inquire in what ways, the reflection of a queasy stomach for, in fact, the dry if any, a canyon alters a river's characteristir bed profile. air, the sun-dappled water, and the intense color tend to give a sense of exhilaration. Rather, the uneasiness CONCEPT OF QUASI-EQUILIBRIUM is a subdued but undeniable cold fear which never departs. A river is both the route and the transporting agent> To anyone who has been down the big river, the words by which rock and soil eroded off the continent are car- in Powell's journaLls convey clearly the fact that even ried to the sea. The necessity for such movement~lies those courageous men had the same constant, unrest. merely in the energy possessed by any object as a result 131 132 THE COLORADO RIVER REGION AND JOHN WESLEY POWELL of its elevation. Water falling 011 mountains as precipi- gravel. This debris will accumulate anywhere along the tation will flow downhill because of the pull of gravity, river where the flow conditions make the capacity to and in the course of its movement it will carry along bits transport less than the load brought in from upstream. of rock and soil. The water moving downhill is con- The factors governing transport capacity, especially stantly replenished by more falling as precipitation, and width, depth, velocity, and slope, adjust among thein- therefore, through the action of tlie hydrologic cycle, selves to keep in balance the transport capncity and the the continents are gradually worn clair-n. Though the load to be carried. The ubiquitous forni.of the river water falls over a widespread area, it does not long re- profile-steep in the headwaters and gr:tdually decreas- main so dispersed and gathers in the well-defined rib- ing in gradient downstream-results from the internal bons of a channel network. adjustments among the hydraulic factors :is tribut:iries No aspect of the work of rivers can be cliscussed with- introduce additional water and their debris load. out some reference to the concept of quasi-equilibrium There is another constraint on the tendency for uni- and least, work. The pool-and-rapid sequence, which is form river gradient which is of controlling importance the major concern of this essay is integrally related to in the present discussion of pools and rapids. Coarse the concept. debris, especially gravel, will not move downstream in Power is expended-that is, energy is mechanically a uniform sheet but will tend to bunch up in mounds converted into heat-throughout the natural world. Wa- separated by troughs. This concentration of coarse par- ter converts its energy of elevation into lieat as it flows ticles at some places on the riverbed, separated by zones downhill. A rolling rock does the sanie us it moves down of relative scarcity of similar rocks, results from the ef- a slope. Wind dissipates its energy :LS it blows from a fect of one rock on another in close proximity. The high-pressure area to one of low pressure. The work closer rocks are spaced, the greater is the water flov done during such energy conversion tends to be uni- required to move them. Gravel bars in rivers, then, are formly distributed because any nonuniformity causes a the result of the tendency for rocks to accumulate in concentration of work on the dissident or anomalous groups. The phenomenon is strikingly similar to the feature. tendency for automobiles on a highway to accumulate For example, a carpenter sawing a board strikes a in groups separated by stretches of open road nearly nail. All the work of the saw is concentrated on the nail devoid of cars, even though the highway is free of ob- and little on the woocl until the nail is eliminated. So structions or causes for local slowdown. also in planing a board. Any slight prominence or The river channel, then, is a result of complicated bump on tlie surface is reduced by the plane faster than interactions among marly factors that teiid to reinforce tlie surrounding uniform surface. or oppose each other. The net result of their interaction These examples are analogous to the work done by is a more or less stable and self-adjusting system, hav- flowing water in a river channel. The channel bed- ing overall characteristics of uniformity, and, within considered over some miles of length-tends toward a restraints, of minimization of work. This stable but self- uniform down-channel slope. If some unusual feature adjusting condition is often described as quasi- exists, such as a ledge of especially hard rock, a very equilibrium.
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