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

A river is both the route and the transporting agent by which rock and soil eroded off the continent are carried to the sea. The necessity for such movement lies merely in the energy possessed by any object as a result of its elevation. Water falling on mountains as precipitation will flow downhill because of the pull of gravity, and in the course of its movement it will carry along bits of rock and soil. The water moving downhill is constantly replenished by more falling as precipitation, and therefore, through the action of the hydrologic cycle, the continents are gradually worn down. Though the water falls over a widespread area, it does not long remain so dispersed and gathers in the well-defined ribbons of a channel network.

No aspect of the work of rivers can be discussed without some reference to the concept of quasi-equilibrium and least work. The pool-and-rapid sequence, which is the major concern of this essay is integrally related to the concept.

Power is expended—that is, energy is mechanically converted into heat—throughout the natural world. Water converts its energy of elevation into heat as it flows downhill. A rolling rock does the same as it moves down a slope. Wind dissipates its energy as it blows from a high-pressure area to one of low pressure. The work done during such energy conversion tends to be uniformly distributed because any nonuniformity causes a concentration of work on the dissident or anomalous feature.

For example, a carpenter sawing a board strikes a nail. All the work of the saw is concentrated on the nail and little on the wood until the nail is eliminated. So also in planing a board. Any slight prominence or bump on the surface is reduced by the plane faster than the surrounding uniform surface.

These examples are analogous to the work done by flowing water in a river channel. The channel bed—considered over some miles of length—tends toward a uniform downchannel slope. If some unusual feature exists, such as a ledge of especially hard rock, a very large boulder, or a waterfall, the flowing water being locally blocked will flow over and around the obstacle with higher than usual velocity, undercutting the downstream edge and eroding the sides of the obstacle. Therefore, in accord with the general tendency referred to above, energy expenditure concentrates on the bumps of the streambed, tending to reduce them and to make the whole streambed uniform.

Such a tendency toward uniformity is, in the physical world, usually counterbalanced by other tendencies arising from other conditions that must be met. The tendency of the flowing water to erode and lower the stream-bed is counteracted first by the resistance of the rock or other riverbed materials. This is one of the simpler balances operating in the river system. There are others more complex. The river derives from tributaries and from its bed and banks a debris load of silt, sand, or gravel. This debris will accumulate anywhere along the river where the flow conditions make the capacity to transport less than the load brought in from upstream. The factors governing transport capacity, especially width, depth, velocity, and slope, adjust among themselves to keep in balance the transport capacity and the load to be carried. The ubiquitous form of the river profile—steep in the headwaters and gradually decreasing in gradient downstream—results from the internal adjustments among the hydraulic factors as tributaries introduce additional water and their debris load.

There is another constraint on the tendency for uniform river gradient which is of controlling importance in the present discussion of pools and rapids. Coarse debris, especially gravel, will not move downstream in a uniform sheet but will tend to bunch up in mounds separated by troughs. This concentration of coarse particles at some places on the riverbed, separated by zones of relative scarcity of similar rocks, results from the effect of one rock on another in close proximity. The closer rocks are spaced, the greater is the water flow required to move them. Gravel bars in rivers, then, are the result of the tendency for rocks to accumulate in groups. The phenomenon is strikingly similar to the tendency for automobiles on a highway to accumulate in groups separated by stretches of open road nearly devoid of cars, even though the highway is free of obstructions or causes for local slowdown.

The river channel, then, is a result of complicated interactions among many factors that tend to reinforce or oppose each other. The net result of their interaction is a more or less stable and self-adjusting system, having overall characteristics of uniformity, and, within restraints, of minimization of work. This stable but self-adjusting condition is often described as quasi-equilibrium.