Geological Survey Professional Paper 669 The Colorado River Region and John Wesley Powell

In the dry glare of a sun-drenched afternoon, in the bitter chill of a thunderstorm wind, or in the purple evening, there is no respite from the incessant boom of the great river. One finds at times he has forgotten the ever-present roar of the rapids and, as if suddenly awakened, he hears it again. So persistent is the sound that I often wonder how the mind can put away the noise into some recess, even momentarily.

The river's boom is associated with a pervasive uneasiness which never leaves a man while he is clamped within the cliffs of the canyon. This uneasiness is not the reflection of a queasy stomach for, in fact, the dry air, the sun-dappled water, and the intense color tend to give a sense of exhilaration. Rather, the uneasiness is a subdued but undeniable cold fear which never departs.

To anyone who has been down the big river, the words in Powell's journals convey clearly the fact that even those courageous men had the same constant unrest. They had more reason than we for a deep and troubled fear. On that first trip, no one knew whether high and vertical waterfalls might block completely any passage by boat. Clearly, there was no return upstream.

Powell (1875, p. 62), halfway through his trip, expressed his feelings this way: "* * * there are great descents yet to be made, but, if they are distributed in rapids and short falls, as they have been heretofore, we will be able to overcome them. But, may be, we shall come to a fall in these canyons which we cannot pass, where the walls rise from the water's edge, so that we cannot land, and where the water is so swift that we cannot return. Such places have been found, except that the falls were not so great but that we could run them with safety. How will it be in the future!"

In the hundreds of miles through which the river flows in a canyon section, the channel consists of an alternation of flat pools and steep rapids. Yet there are no waterfalls in the usual sense of the word. What John Wesley Powell feared the most does not exist. Why not? This seems a simple enough question, yet the answer is neither simple nor obvious.

This chapter is an attempt to explain, albeit incompletely, why rivers characteristically develop a uniform profile downstream, gradually decreasing in steepness. Despite this progressive flattening of slope, they tend to maintain an alternation of low-gradient deep pools and higher gradient riffles or rapid reaches. The general explanation will then be applied to the Colorado River in the Grand Canyon section to inquire in what ways, if any, a canyon alters a river's characteristic bed profile.