River Flood Plains: Some Observations On Their Formation GEOLOGICAL SURVEY PROFESSIONAL PAPER 282-C River Flood Plains: Some Observations On Their Formation By M. GORDON WOLMAN and LUNA B. LEOPOLD PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS GEOLOGICAL SURVEY PROFESSIONAL PAPER 282-C UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director First Printing 1957 Second Printing 1959 Third Printing 1965 Fourth Printing 1967 CONTENTS Page Page Abstract.____-----__-___-___..______-_--_-__________ 87 Foimation of a typical flood plain—Continued Introduction_______________________________________ 87 Overbank deposition—Continued Frequency of overbank flow.__________________________ 88 Hypothetical construction of a flood plain by Formation of a typical flood plain_____._______________ 91 overbank deposition______________________ 100 The characteristics of point bars._________________ 91 Conditions affecting amounts of overbank Sediments forming a typical point bar_________ 91 deposition. ______________________________ 100 Process of formation and elevation of the surface of point bars.____________________________ 92 Flood plains in stable, aggrading, and degrading streams _ 103 Rate of lateral migration.___________________ 96 Channel pattern and formation of the flood plain.______ 105 Overbank deposition.__^________________________ 97 Conclusion,_ _______________________________________ 106 Nature and amount of overbank deposition 97 References cited--___-__--__-----_--_-_--______----- 106 Natural levees,_____________________________ Index. ______...______________________ 109 ILLUSTRATIONS PLATE 1. View of meanders on Watts Branch, Md______.___________-______________-_---______-_-_-_-___-Facing 92 2. A, View of successive elevations of surfaces of point bar on Seneca Creek, Prathertown, Md.; B, Flat-topped gravel bar, New Fork River near Pinedale, Wyo____-____-__-__--_----_-_-__---__--_------_-----Facing 93 FIGURE 58. Relation of flood-damage stage to elevation of flood plain._______________________________________________ 90 59. Map and cross section of point bar, Watts Branch, Rockville, Md_ _______________________________________ 93 60. Distribution of materials in point bar, Watts Branch, Rockville, Md___________________--_-__-__-__-_--__- 94 61. Size distribution of samples from bed of Watts Branch, Rockville, Md., and adjacent bar__________________ 95 62. Erosion and deposition in cross sections of Watts Branch, Rockville, Md_ _________________________________ 96 63. Size distribution of flood deposits and flood-plain sediments.______________________-__--__--__--_-_-_-____ 99 64. Hypothetical rate of increase in elevation of flood plain by overbank deposition____________________________ 100 65. Sediment concentration and discharge in several Kansas rivers__.._-________-___-_-_-__---__-----__------- 102 66. Cross sections of river flood plains in North and South Carolina._________________________________________ 104 67. Relation of distance from headwater divide to thickness of alluvium and to mean depth of average discharge.._ 104 TABLES TABLE 1. Recurrence interval with which flood-plain level is attained by annual flood_________________________________ 88 2. Distribution of flood-damage stage at 71 river gage locations in Texas expressed as number of examples in various categories of recurrence interval_________________________.____________________--_--__-_---_-_---_---- 89 3. Differences between elevation of flood-damage stage and average elevation of natural flood plain_______________ 91 4. Some data on rates of lateral migration of rivers across valleys___-_-__-_---_------------------------------- 97 5. Examples of amounts of deposition on flood plains during major floods.________--_-__---____---------_------ 97 6. Velocity and depth of flow in overbank sections of river flood plains.__________________-_-__-____--_--_-_--- 101 HI PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS RIVER FLOOD PLAINS: SOME OBSERVATIONS ON THEIR FORMATION By M. GORDON WOLMAN and LUNA B. LEOPOLD ABSTRACT be considered tentative, as many additional data will On many small rivers and most great rivers, the flood plain be needed to verify them adequately. consists of channel and overbank deposits. The proportion of A definition of allood plain often quoted is "a strip of the latter is generally very small. relatively smooth land bordering a stream * * * called Frequency studies indicate that the flood plains of many a living flood plain if it is overflowed in times of high streams of different sizes flowing in diverse physiographic and water" (Rice, 1949, p. 139). Valley flats which would climatic regions are subject to flooding about once a year. usually be considered "flood plains" may include those The uniform frequency of flooding of the flood-plain surface and the small amount of deposition observed in great floods formed by different processes such as landslides, (average 0.07 foot) support the conclusion that overbank low-angle fans, and perhaps others. The distinction deposition contributes only a minor part of the material con­ between valley flats formed by different processes may stituting the flood plain. The relatively high velocities (1 to 4 not be apparent to the casual observer and, indeed, fps) which can occur in overbank flows and the reduction in detailed work might be necessary to determine the sediment concentration which often accompanies large floods may also help account for this. Although lateral migration of origin of a given feature. However, an important channels is important in controlling the elevation of the flood process resulting in the formation of valley flats is a plain, rates of migration are extremely variable and alone cannot combination of deposition on the inside of river curves account for the uniform relation the flood-plain surface bears to and deposition from overbank flows. This process the channel. produced many of the flat areas adjoining river channels Detailed studies of flood plains in Maryland and in North and probably most of the flood plains of the great Carolina indicate that it is difficult to differentiate between channel and overbank deposits in a stratigraphic section alone. rivers of the world. It is the flood plains produced by this process with which the present paper is concerned. Because deposition on the flood plain does not continue indefinitely, the flood-plain surface can only be transformed into The data supplied to us by several district engineers a terrace surface by some tectonic or climatic change which of the Geological Survey are essential to the tentative alters the regimen of the river and causes it to entrench itself conclusions reached in this paper. We are indebted below its established bed and associated flood plain. A terrace, to H. C. Bolon, L. R. Sawyer, M. T. Wilson, and then, is distinguished from a flood plain by the frequency with E. A. Johnson. We also express our appreciation to which each is overflowed. D. M. Culbertson for detailed analyses of hydrographs INTRODUCTION of water and sediment in several streams in Kansas. Shri B. N. Chatterjea kindly obtained certain field Owing to the extensive use of river flood plains by data in India at our request, and furnished us flood industry and agriculture, there is practical as well as frequency tabulations for which we are grateful. intellectual interest in the mode of formation, stability We have profited from discussion of the subject with of the flood plain, and the frequency of flooding of its J. T. Hack of the Geological Survey. In addition surface. If maximum use is to be made of the broad, to colleagues in the Survey who read earlier drafts, level surfaces provided by flood plains, it is important we thank Prof. John C. Geyer of Johns Hopkins to know the natural conditions controlling their University, H. T. U. Smith of the University of development and preservation. The geologist has long Kansas, and Mr. Cole Fisher of the Virginia Geologi­ been interested in the characteristics of flood plains cal Survey for their helpful criticisms of the manuscript. and has written much about them. For the most part, Thanks are due also to Miss Gladys Braden of Minne­ previous papers have dealt primarily with individual apolis, Minn., for a number of references with which examples. The authors believe that the hydrologic as she provided us. well as the additional stratigraphic observations We are grateful to the Viers family of Rockville, presented here may help in developing a more general­ Md., and especially to Caroline Viers Mudgett, for ized picture of the characteristics of river flood plains. permission to study the stream on their property over Conclusions based upon the data now at hand should a period of years. 87 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS FREQUENCY OF OVERBANK FLOW duced by the process with which we are concerned. Both the narrow valleys of the mountain torrents and On any river there are floods of varying magnitude the very turbulent flow may contribute to this condi­ which cover a wide range in stage. As successive floods tion. Despite these variations, the data from eastern overtopped the flood plain, one would expect successive and western United States and the examples from India increments of overbank deposition on the flood plain to indicate a remarkable uniformity in the recurrence result in a continual increase in elevation of this surface interval of overbank