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 CONTEKTS Page Page 87 Foiniation of a typical flood plain-Continued 87 Overbank deposition-Continued 88 IIypothetical construction of a flood plain by 91 overbank deposition ____ - - - - - - - - - - - - - - - - - 100 91 Conditions affecting amounts of overbank 91 deposition_________________________-____-.. 100 92 Flood plains in stable, aggrading, and degrading streams. 103 96 Channel pattern and formation of the flood plain - - - - - - - 105 97 Conclusion___-___-_-____________________----------- 106 97 References cited- - - ______ ______ ..................... 106 98 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 FIGURE58. 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, Rockvillc, 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 TABLE1. 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 ____.._________.______._____I. .________ 101 111 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS RIVER FLOOD PLAINS : SOME OBSERVATIONS ON THEIR FORMATION By M. GORDONWOLMAN and LUNAB. LEOPOLD ABSTRACT be considered tentative, as many additional data will On many small rivers and most great rivers, thc flood plain bc needed to verify them adequately. consists of channel and overbank deposits. The proportion of A definition of IL flood 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 treams of different sizes flowing in diversc physiographic and water” (Rice, 1949, p. 139). Valley flats which would limatic regions are subject to flooding about once a year. usually be considered “flood plains” may includc those The uniform frequency OF flooding of the flood-plain surface nd the small amount of deposition observed in great floods formed by different processes such as landslides, t the conclusion that overbank low-angle fans, and perhaps others. Thc distinction a minor part of the material con- between valley flats formed by different processes may he relatively high velocities (1 to 4 not be apparent to the casual observer and, indeed, erbank flows and the reduction in detailed work might be iiecessary to determine the ch often accompanies large floods origin a given feature. However, an important his. Although lateral migration of of s is important in controlling the elevation of the flood process resulting in the formation of valley flats is B rates of migration are extremely variable and alone cannot combination of deposition on the inside of rives curvcs for the uniform relation the flood-plain surface bears to and deposition from overbank flows. This process produced many of the flat areas adjoining river channels ed studies of flood plains in Maryland and in North and probably most of the flood plains of the great a indicate that it is difficult to differentiate between rivers of the world. It is the flood plains produced by hannel and overbank deposits in a stratigraphic section alone. this process with which the present paper is concerncd. ion on the flood plain docs not continue od-plain surface can only be transformed into The data supplied to us by several district engineers by some tectonic or climatic change which of the Geological Survey are essential to the tentative of the river and causes it to entrench itself conclusions reached in this paper. We are iiideb tecl d bed and associated flood plain. A terrace, to H. C. Bolon, L. R. Sawyer, M. T. Wilson, and flood plain by the frequency with E. A. Johnson. We also express our appreciatioii to ch 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 ficld Owing to the extensive use of river flood plains by data in India at our request, and furnished us flood dustry and agriculture, there is practical as well as frequency tabulations for which we are grateful. tellectual interest in the mode of formation, stability We have profited from discussion of the subject with e flood plain, and the frequency of flooding of its J. T. Hack of the Geological Survey. In addition ce. If maximum use is to be made of the broad, to colleagues in the Survey who read earlier drafts, 1 surfaces provided by flood plains, it is important we thank Prof. John C. Geyer of Johns Hopkins know the natural conditions controlling their University, H. T, U. Smith of the University of lopment and preservation. The geologist has long Kansas, and Mr. Cole Fisher of the Virginia Geologi- interested in the characteristics of flood plains cal Survey for their helpful criticisms of the manuscript.. d has written much about them. For the most part, Thanks arc due also to Miss Gladys Braden of Minne- ious papers have dealt primarily with individual apolis, Minn., for a number of references with which ples. The authors believe that the hydrologic as she provided us. 11 as the additional stratigraphic observations We are grateful to thc Viers family of Rockville, ented here may help in developing a more general- Md., and especially to Caroline Viers Mudgett, for picture of the characteristics of river flood plains. permission to study the stream on their property over clusions based upon the data now at hand should a period of years. 87 SS PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS FREQUENCY OF OVERBANK FLOW luced by the process with which we are concerned. Both the narrow valleys of the mountain torrents and On any rivcr there are floods of varying magnitude ,he vsry turbulent flow inay contribute to this condi- which cover a wide range in stage. As successive floods ion. Despite these variations, the data from eastern overtopped the flood plain, one would expect successive md western United States and the examples from India increments of overbank deposition on the flood plain to ndicnte a remarkable uniformity in the recurrence result in a c,oiitinual increase in elevation of this surface iiterval of overbank flooding. rclative to the elevation of the stream bed. As the flood plain became higher and higher, presumably it would I'SBLKI .-Recurrence inierual with which jlood-plain leuel is be flooded less and less frequently. Nevertheless, the attained by annual jlood giaut flood could still be expected to deposit a thin Recur- increment of niaterial on this high surface. As it result rence of such a sequence of events, one would logically expect interval, Drain- Discharge in years to find that the frequency of flooding of flood plains in age at bankjull (from different regions was extremely variable. Some flood area staqe annual (square (cubic feet flood plains woiild be flooded with extreme rarity, and others River and location miles) per second) series) Remarks ______ __ ___ would be flooded frequently. Such considerations Polo Crock ncar Pinedale, 88 514 1. 13 suggested to us that a study of the frequcncy of flooding wyo. Horse Crcek near Daniel, 121 320 I in of flood plains might be useful in understanding their wyo. mode of formation. Studies of a number of flood plains Cottonwood Creek ncar 202 990 15 Terrace(?). Daniel, Wyo. in both the eastern and western Uiiited States and in Little Sandy Creek near Elk- 21 227 4 India indicate that, contrary to the logic of the argu- horn, Wyo. Big Sandy Creek at Leckie 94 573 1 05 incrit above, thc frequency of overbank flow is remark- ranch, Wyo. ably uniform in many rivers flowing in diverse climatic Qreen River Dear Pontenelle. 3, '370 9,170 1 47 and phj-sjographic regions. wyo. Table 1 shows the recurrence interval of the bankfull Hams Fork near Frontier,Wyo. 298 626 1 01 Middle Piney Creek near Big 31 740 200 Mountain torient; fiood or incipient overflow stage at a number of stations, Piney, wyo. plaih poorly defined. most of them in different regions in the United States. Faall Creek near Finedale, 37 1,130 200 DO. wyo.