Letters From Grand Canyon

Most of us don't stay awake much worrying about drainage networks—the systems of interconnected rivers, streams, washes, gullies, ponds and lakes that collectively drain water from their watershed. If we think about it at all, our thoughts are more likely to turn to issues such as beauty, transportation, environment, river running, or, deliciously, fishing. In other words, we take river systems for granted. But wait. Perhaps there is more to it than we might think, perhaps we can learn once again to see the world in a new, surprising, and richer way.
As a pleasant example, think of the Rhine, river of song and ancient legend peopled by the Maidens, a silver thread linking the exploits of runic heroes and gods. The legends take us far back in human history, but not far enough to reach the times before humans strode onto the stage. What did the place look like then? Was the river there? Has it always been there? If it has not, then when was it born, and how? And what was there before the birth?
Perhaps there was a time when the landscape was devoid of drainages—no rills, washes, streams, or rivers. Can we envision such a landscape? Has anyone ever seen one? The answer is no. Even the Saharan sand seas bury ancient water courses well known to Paleolithic hunters, and so do glaciers. Emerging coastal plains are criss-crossed by tidal channels, and the beds of lakes quickly develop drainage networks. Only where land is covered by water—seas, lakes—can one speak of an absence of drainage systems. This means that the question, “How old is the Rhine, or the Colorado” leads us back inexorably to the time when the land was last covered by water. That is when these rivers were born. But they probably did not have a course anything like the present one. If one could play the movie of geologic time at a greatly accelerated pace, one would see drainage networks whose configurations, whose connections, would constantly change in response to events such as deformation of the earth's surface, advance and retreat of ice sheets, and the like. Only gradually, and probably rather recently, would the configuration of the drainage networks evolve towards something resembling the modern Rhine, or Colorado. So, the question of how old a river is resolves itself in how much departure from the present configuration one is willing to accept before one is forced to say “This drainage system just does not resemble in any way the Colorado that we know today, therefore I will not call it by that name.” As we shall see presently, the evolution of drainage systems is a question of ruthless Darwinian-type competition in which the strongest and fittest rivers battle it out with lesser ones and acquire territory at their expense.
The weapons that rivers use in their wars are also the tools that they use in their daily work. In either case, the driving force behind it all is gravity. Gravity is what causes the water of rivers to move relentlessly toward the center of the earth, gravity is what causes all objects to seek that center. Of course, the water of a river cannot flow directly toward the center of the earth, but is constrained to move in a nearly horizontal direction by its channel. Consequently, we can think of water as sliding down a gently inclined plane in response to gravity. For most rivers of any size, the inclination of the plane is measured in feet per mile to a few tens of feet per mile, which is a very shallow slope angle of only a small percentage of one degree.
If the movement of water down the slope were not affected by friction, we could easily calculate the velocity attained by the water. But the water is indeed affected by friction—friction between the water and the river channel, and between the water and the air above it. Many people are surprised by the concept of friction between water and air, yet waves in oceans and lakes are formed precisely by the drag exercised by moving air on the water.
Friction, then, controls the velocity of water moving down a river channel, primarily in these ways:
• The shallower the water (i.e. the smaller the discharge), the slower the flow. This happens because friction occurs at the interface between water and everything else, that is, along the perimeter of the water body. This perimeter is greater, relative to the volume of water, when the water is shallow than when it is deep. River runners are well acquainted with the silvery white noise made by gravel bars, where the water is shallow, flows slowly, and dissipates a lot of energy (thus making noise). In contrast, deep, swift water is silent.

• The rougher the channel, the slower the water. For the same discharge and gradient, water will move much faster in a smooth concrete canal than in a bouldery streambed.
• The greater the velocity of the water, the greater the frictional drag, all else being equal.
For our purposes, the most important thing to remember is that the greater the discharge of a river, the greater the water depth and the greater the velocity. The velocity of the water, together with the quantity of water, that is, the discharge, is the engine that enables a river to do its work.
As is the case with any moving object, moving water has energy, a fact not unknown to anyone trying to row a raft out of a backeddy. This type of energy is known as energy of motion, or kinetic energy. Long ago, people determined that the quantity of this energy (Ek) is related to the amount of stuff that is moving (m), and to its velocity (v), by the equation
Ek= 2mv2
“Aha!” says the astute reader, “I see that this energy depends on the amount of stuff moving, and also, and much more strongly, on its velocity.” Precisely. That's why the velocity of a river is so important in understanding what a river can do and when it can do it.
Because velocity increases with discharge, and energy increases not only with discharge but also and especially with the square of the velocity, it is no great trick to understand that energy increases very rapidly as discharge increases. This is illustrated nicely by data which relates discharge, velocity, and other river properties as measured at the Lees Ferry gauging station, and reported many years ago by the great hydrologist Luna Leopold. Leopold's data for 1948 show that a 13-fold increase in discharge of the Colorado River (from 7,000 to 91,000 cubic feet per second) resulted in a more than 65-fold increase in energy value (from 56 to 3,686).
And that, my friends, is the great secret of How Rivers Work. Specifically, most work done by a river—transporting sediment, clearing boulder jams, cutting down—is done in highwater (flood) stages, when discharge and velocity are large. Very little happens during low-water stages.
As a consequence of the above, drainages really do not need permanent flow (base flow) to function perfectly well as agents of transport and erosion. This is something that at times is overlooked even by professional earth scientists, yet desert areas are full of washes that function just fine even though they are totally dry most of the time—their work is done entirely during infrequent floods.
Floods, then, are the key, and floods have an interesting relation to the drainage basin of a river: the greater the basin, the greater the river's discharge is likely to be—that's obvious. What is less obvious is that increasing the drainage basin also increases the chance of floods for that basin. All of which means that big rivers with big drainage basins are more likely to have big discharges than little rivers with small drainage basins.
One more thing. Rivers need energy to do their work, whether transporting material or cutting down, and the source of the energy is explained above. The relation between the amount of material supplied to a river and the energy available to the river determines not only the kind of floodplain that the river has, but also the level, or grade, of this floodplain. If more material is brought to a river than it can carry, the river dumps the material, becoming a braided stream and raising its bed. Examples are common in Alaska and British Columbia, where rivers issue from glaciers. If the amount of material brought to a river is just what it can carry, there will be no net accumulation or erosion. If the material brought in is less than the river can carry, the river will have energy to spare, which it uses by picking up whatever it can and by cutting down; the river will be in a highly erosive state. The Colorado River downstream from Glen Canyon Dam is a good example of this last type of river.
Understanding the basic physical controls at work on all rivers helps clarify the ways of Grand Canyon's Colorado River. The ways of the Colorado have influenced Pueblo farmers, the building of riverine beaches, the effects of Glen Canyon Dam, and more, as will be seen in subsequent Letters from Grand Canyon.
Dr. Ivo Lucchitta
This is the third in a series of “Letters from Grand Canyon” by Ivo Lucchitta that will appear in future issues of the bqr.