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

The Colorado River flows several hundreds of miles through a series of canyons, some of which are of very hard and resistant rock. The river seems encased in a vise so confining and limiting that any freedom of action or movement seems to be foreclosed. In fact, however, the river has nearly all the characteristics of an unconfined channel flowing in a broad flood plain, save one, the tendency to move laterally. The Colorado adjusts its depth and velocity by scour and fill of the bed in response to changes of debris load. It formed and maintains bed alternations of deep pool and shallow rapid by the construction of gravel bars, which maintain their size and position despite the trading of rocks on the bar surface. The river profile, except for the alternation of pool and rapid, is smooth and nearly straight.

Only in the lack of lateral migration as a result of the confining rock walls does the canyon river seem markedly different from a free or unconfined one. Yet the perfect form of some meanders entrenched in hard rock indicates that the river has no proclivity for lateral movement, for it has cut nearly vertically hundreds of feet, at least in some places, for periods of several millions of years. Why the canyon river does not erode laterally more than it does is simply not known.

The Grand Canyon section of the Colorado River, despite its impressive rapids, has the characteristics of a river in balance, maintaining its quasi-equilibrium poise by self-adjustment.

The great age and stability of the rapids do not result in all rapids being equal in size or declivity. Their magnitude ranges from small to great. Random variation alone might well have produced one or more rapids so steep or so nearly approaching a real waterfall that the Powell party would have been blocked. Powell took a long chance and was lucky as well as capable.