Fluvial Processes in Geomorphology

Luna B. Leopold, M. Gordon Wolman, John P. Miller, M. Gordon Wolman, John P. Miller

• Publisher: Dover Publications
• Year: 2012
• Print ISBN: 9780486685885
• eBook ISBN: 9780486139739
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

This excellent text is a pioneering work in the study of landform development under processes associated with running water. Its primary emphasis is on subjects that were the focus of the authors' studies in both field and laboratory. Part I deals with the process of change in the evolving landscape. Part II explores process and form, and Part III, the effects of time.
In Part I, the relation of geomorphology to field problems is analyzed in studies of a mountain block in a semiarid climate, a meandering river cut into bedrock, and benches along a sea coast. Part Two contains studies of weathering, climate, and such denudational processes as flooding and erosion. Here, too, are examinations of the drainage basin as a geomorphic unit, water and sediment in channels, channel form and process, and hillslope characteristics and processes.
In Part III, the authors cover geochronology, drainage pattern evolution, channel changes with time, and the evolution of hillslopes. Two appendixes will help readers convert units and equivalents, and identify symbols and nomenclature. 1964 edition.

Part I

THE EVOLVING LANDSCAPE

Chapter 1

The Changing Scene

When a man makes a pilgrimage to the fields and woods of his boyhood, he does not expect to find the hills and mountains dissolved, or the valleys moved. If other men have not torn up the land to build factories and towns, he expects his children to see the hills and swales as his forefathers saw them. And he is almost right. Probably neither he nor the children will ever notice that in fifty years the surface of the ground has been lowered perhaps a fraction of an inch. Why should they? But they might not be surprised to find that the old mill pond behind the dam is now more mud than water.

Under the action of the force of gravity the land surface is sculptured by water, wind, and ice. This sculpturing produces the landforms with which geomorphology is concerned. Some of these forms owe their origins purely to denudational processes; other forms may be depositional; still others owe their existence to combinations of both processes.

A picture of the dynamics of the earth’s surface is by no means complete, however, if only gradation or leveling is considered. Clearly, if there were no counteracting forces we should expect that the land surface, given sufficient time, would be continuously reduced. Eventually, little or no relief would remain. Geologic history demonstrates, however, that the degradational forces acting on the earth’s surface are opposed by constructional forces. These internal, or endogenous, forces cause the land to rise, and as they do so it is subjected to attack by the external, or exogenous, agents. Geomorphology is primarily concerned with the exogenous processes as they mold the surface of the earth, but the internal forces cannot be disregarded when one considers fundamental concepts of the origin and development of landforms.

Ideally, the basic principles underlying the development of landforms can be considered in simple terms. A given land area is composed of a particular set of rocks, which have particular chemical and mineralogic compositions and specific physical properties. Because these rocks were formed at different temperatures and pressures within the earth, when they are exposed at the surface they are no longer in equilibrium with their environment and thus begin to decompose. Where a gradient is created by gravity, the moving water, earth, air, and ice help in the attack upon the rock and remove the products of weathering. In the process, landforms of various aspects are created. In a given environment the physical and chemical constitution of the rocks determines the way in which they will break down and, in turn, the size and quantity of debris made available to the denudational agencies.

Each denudational agent, depending upon its density, gradient, and mass at a particular place, is capable of applying a given stress on the materials available. A certain amount of work may be performed by the application of this stress, and the results of this work are the landforms that we see developed in various parts of the world. In a given climatic and vegetational environment the shape or form of the landscape will vary, depending upon the character of the rock and the type and available stress of the erosional agents. But as the land surface is reduced—so long as the products of weathering and the applied stress remain constant—the form of the land should remain the same.

If one were able to evaluate properly the properties of the rocks and the present and past capabilities of the denudational agencies, he should have no trouble in developing a rational, even mathematical, equation capable of describing the development history and equilibrium form of any landscape. William Morris Davis said essentially the same thing in 1902 when he observed that any landform is a function of the structure of the rocks (including their composition and structural attitude), the processes acting upon them, and the time over which these processes have been active. Only as we study the interrelations of these three factors are we able to discern which combinations produce which particular landforms and how they do so.

Some landforms, such as volcanoes, which may have been unaffected by denudational processes, may be considered purely constructional forms. As soon, however, as they are modified by external agencies, their form begins to represent the resultant of an interaction between the constructional forces, the rock substrate, and the applied stress.

The application of such an ideal concept to any actual landform at the present time is fraught with problems. The natural world is highly variable and the mechanics of uplift, weathering, and erosion are for the most part poorly understood. As will be seen, climate itself is a complex factor, and in most regions of the world inorganic processes are inseparable from the complex organic processes carried on by plants and animals. Although it is frequently convenient and helpful to construct a simplified synthetic picture of the natural environment, we should not lose sight of the fact that a given landscape must be the result of a complex set of factors which encompass the behavior of materials and processes over varying periods of time.

It is important to note that whether one refers to the effect on landforms of different rock types, or to the effect of different rates of uplift, such differences or changes must manifest themselves in the environment of the landform in simple physical terms. A normal fault whose strike is perpendicular to the direction of flow of a river, with downthrown block in the downstream direction, constitutes to the river a merely local increase in gradient. A similar increase in gradient might be effected by local changes in lithology, an abrupt shortening in channel length, or by an abrupt change in discharge downstream. The same physical principles determine the river’s subsequent response in each case. The permanence or impermanence of the change, as well as its possible propagation either upstream or down, will depend upon the type and amount of material available and the distribution and quantity of flow. Any true principle enunciated to explain one of the cases must be applicable to the others as well.

Thus, although the application of the principle to any one example may be fraught with difficulty, an understanding of the principle at least reduces the burden of innumerable “unique” cases. Geomorphologists have always sought such unifying concepts, and for a proper view of the field as a whole one must turn initially to the classical concepts of landform evolution.

The influence of William Morris Davis on geomorphology was without doubt greater and longer-lasting than that of any other individual. His major contribution was a genetic system of landform description. Beginning in 1899, Davis developed the concept that during erosion of a highland the landscape evolves systematically through distinctive stages, to which he gave the names, youth, maturity, and old age. This entire sequence of stages he called an erosion cycle (or geomorphic cycle), and the end product was supposed to be a surface of low relief, or peneplain. He elaborated the effects of interruptions in the cycle and argued that the principal factors controlling the character of landforms are geologic structure, geomorphic processes, and the stage of development. Davis’ genetic concept of landform development was a brilliant synthesis, which grew directly out of the work by Powell, Gilbert, and Dutton and also from the controversial ideas on organic evolution which were prevalent at the time.

The concept of the erosion cycle was never accepted in Europe to the same degree as in North America. The most serious challenge came during the 1920’s from Walther Penck, who attempted to show a direct causal relation between tectonics and the properties of landforms. Many of his conclusions about the trends and ultimate results of tectonics and erosion processes differed only slightly from those of Davis. Penck, however, emphasized slope development, and his theory of slope development is a major contribution that is still being tested and debated.

The principal alternative to the Davisian conception differs mainly in the view of the effect of time, the third of the three fundamental elements, on landforms. Restating and extending the work of Gilbert, Hack (1960) emphasizes the concept of a dynamic equilibrium in the landscape which is quickly established and which responds to changes that occur during the passage of time. This view postulates that there is at all times an approximate balance between work done and imposed load and that as the landscape is lowered by erosion and solution, or is uplifted, or as processes alter with changing climate, adjustments occur that maintain this approximate balance.

More will be said about these different views in subsequent chapters, as various aspects of the landscape are considered in greater detail. Paralleling developments in other phases of geology, the past decade has witnessed a remarkable increase in the application of analytical and experimental techniques to geomorphic problems. These investigations have taken two principal directions: (1) efforts to describe landforms more precisely through the use of statistics and other analytical techniques, (2) application of physical and chemical principles to field and laboratory studies of geomorphic processes. Although a few geologists—G. K. Gilbert, and later W. W. Rubey—helped to pave the way for this current trend, developments in other fields of science, especially in engineering and physics, were more directly responsible for it. One outstanding example is the field and experimental work on sand transport by R. A. Bagnold during the 1930’s. Another is the contribution of fundamental ideas on the development of stream networks by R. E. Horton. Recently many developments in hydraulics and in the application of soil mechanics have attracted the attention of geomorphologists. At present there is greatly increased interest in the use of more precise tools for studying landforms. The pace of research seems to be quickening and there is reason to hope that a new era of discovery is under way.

Geomorphology in North America has gone through a phase during which extensive description of the landscape in terms of the erosion cycle has been carried out. It was apparently believed that the processes were known or could be inferred, and that form could be assessed by eye.

Similarly, one current earth-history view of geomorphology assumes that enough is now known to interpret landforms and deposits in terms of processes that operated in times past. In the most qualitative way this is probably true. However, we believe that the genetic system breaks down when it is subjected to close scrutiny involving quantitative data. At present deductions are subject to considerable doubt, for the detailed properties of landform have not been studied carefully enough and the fundamental aspects of most geomorphic processes are still poorly understood. So long as this is true, the interpretation of geomorphic history rests on an exceedingly unstable base.

Accordingly, we plan to concentrate on geomorphic processes. The emphasis is primarily upon river and slope processes; river processes will receive greatest attention, since the greatest volume of information available is on rivers. Our objective is to synthesize the material on these subjects in an attempt to assess the current status of knowledge and at the same time to draw attention to its shortcomings.

Process implies mechanics—that is, the explanation of the inner workings of a process through the application of physical and chemical principles. We realize that some readers may be more interested in descriptions of landforms than in the detailed analysis of the processes that formed them. So far as possible, we attempt to relate the processes discussed to specific types of landforms. Unfortunately, the gap between our understanding of specific processes in microcosm and the explanation of major large-scale landforms is still wide. It is interesting to note that geomorphologists seem to have a better understanding of depositional than of erosional forms. This may be because the formation of depositional features such as sand dunes, deltas, and flood plains is more easily seen in the field, or because many erosional features retain less clear evidence of their mode of formation.

Detailed understanding of geomorphic processes is not a substitute for the application of basic geologic and stratigraphic principles. Rather, such understanding should help to narrow the range of possible hypotheses applicable to the explanation of different geomorphic forms and surficial earth processes and deposits.

Our approach involves some use of mathematics. We are aware that the feelings of professional geomorphologists about numbers, graphs, and formulas range from acceptance and enthusiasm to bewilderment and forthright hostility. We have not gone out of our way to be mathematical, but wherever we felt that mathematics contributed either clarity or brevity to the discussion, it has been used. Some fundamental principles of mechanics and statistics are introduced in the text where they are appropriate and necessary to an understanding of the subject at hand. Because fundamental principles of geomorphology are drawn from both mechanics and geology, some readers—depending on their backgrounds—will find specific explanations oversimplified to suit their taste, while others will find the same material wanting in simplicity. Although we have attempted to achieve balance in this regard, the wide spectrum of readers’ interests and background in the subject suggests. that a perfectly happy medium is not likely to be attained at this time.

With those readers who have a conditioned reflex against “quantitative geomorphology” we agree that numerical descriptions can be used to give misleading and even erroneous impressions of erudition. However, the fact remains that one’s senses, especially sight, when coupled with a conscious or unconscious bias, sometimes play strange tricks. Thus, a property which seems perfectly apparent, or an “obvious” relation of cause and effect, may upon careful measurement and analysis prove to be exactly the reverse of the “apparent” or the “obvious.” Some examples will be cited in the text. From a scientific standpoint, most students agree that numerical data are superior to subjective adjectives—such as big, little, high, low, steep, and gentle—in objective analyses and comparisons.

We recognize that the decision to concern ourselves primarily with the dynamics of processes has some serious pitfalls. The most critical is the fact that field investigations of modern process cannot be segregated completely from historical aspects of landform development. The same statement applies to geologic structure. Each element of the landscape has evolved through a long period to its present configuration, and this heritage doubtless influences the processes now acting upon it. Sequential observations, comparative studies with statistical controls, and perhap...

Table of contents

• DOVER SCIENCE BOOKS
• Title Page
• Copyright Page
• Dedication
• Preface
• Table of Contents
• Part I - THE EVOLVING LANDSCAPE
• Part II - PROCESS AND FORM
• Part III - THE EFFECTS OF TIME
• Appendix A - Conversion of Units and Equivalents
• Appendix B - Symbols and Nomenclature
• Author Index
• Subject Index