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 miles) per second) series) Remarks ______location __ ___ 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. The gage height of the bankfull stage was measured at Middle Fork Powder River 980 722 1.12 gaging stations operated by the U. S. Geological Sur- near Kaycee, Wyo. vey. In mountainous areas where broad flood plains are Red Fork near Barnum, Wyo. 142 80 1.01 Clear Creek near Buffalo, 120 188 1.01 rare, the height of the modern flood plain was considered wyo. to be the average of the highest surfaces of the channel North Pork Clear Creek near 29 390 2 0 Flood plain poor1 Buffalo, Wyo. defined bars. This is in accord with observations to be pre- Little Popo Agie River near 130 534 1.45 sented on the process of flood-plain formation. The Lander, Wyo. Beaver Creek near Daniel, 141 418 1 36 frequency with which this bankfull gage height is \vyo. attained was determined from curves relating discharge, Rock Creek near Red Lodge, 100 2,000 11 Mountain torrent; fl or stage, of annual floods to recurrence interval (for Mont. plain poorly deked method, see Laiigbein, 1949). The annual flood is the Yellowstone River at Billings, 11,870 21,400 1. 05 Mont. name given to the highest discharge cluring a given year. Seneca Creek at Dawsonville, 101 1,160 1.07 In most of the rivers listed in table 1, the annual flood Md. Bennett Creek at Park Mills. 83 1,510 1. 6 attains or cxcecds the level of the surface of the flood Md. plain every year or every other year (recurrence inter- Linganore Creek at Frederick, 82 2,700 2.7 val=] to 2 years). At those stations where the flood Md. Rig Pipe Creek at Bruce- 102 3,690 1. 25 Only 4 yeam of te plain is clearly defined and its elevation accurately ville, Md. lmown, for cxample, Seneca Creek near Dawsonvillc, Piney Run ncar Sykesvillc, 11 ?585 1.3 Flood plain difRcu Md. de5ne. Md., Brandywine Creek at Chadds Ford, Pa., and Hams Patuxcnt Rivor near Unity 35 1,330 1. 6 Fork near Frontier, Wyo., the recurrence interval of Md. ovcrbnnk flow lies closer to 1 than 2 years. There are, Little Pip0 Crock at Avon- 8 (1)200 (7)l. 25 Gage at distance fro dale, Md. flood plain. of course, expected variations in any such data from natural rivers. Most of the major departures from thc Brandywlne Creek at Chndds 287 4,570 I. 35 Ford, Pa. average, particularly the examples drawn from moun- Biiffalo Creek at Garden- 145 3,000 1.01 tain torrents in Wyoming,*maybe ascribed to the diffi- ville, N. Y. Iknry Fork near l'Ienry River. SO 6,900 3.0 Flood plain surface II culty of locating representative flood-plain surfaces pro- N. C. clear. RIVER FLOOD PLAINS : SOME OBSERVATIONS ON TREIR FORMATION 89
TANLT,1.-Recurrencr Lnterval with which flood-plain level is but usually results from an engineering investigation of attained by nnnunl .flood-Continued flood-control needs and becomes accepted as an “offi- Recur- rence cial” designation. Additional data from records of the interval, Corps of Engineers on the frequency of flood-damage Drain- Discharge in years age at bankfull (from stage are provided in table 2. The median recurrence area atage annual interval in this table is about 2 years, a value which is (square (cubic feet flood River and location miles) per aecond) scr4es) somewhat higher than we have found it in the majority -~-~ of our direct field observations. First Broad River near 1,awn- 198 5,900 1.16 dale, N. C. South Tqgcr River near Reid- 106 3,400 2. 2 TARLE2.-Distribution of JEood-damage stage at 71 river gages in VIllP, E. c. Texas expressed as number of examples in various categories of recurrence interval (Data from Corps of Engineers) South l‘saer Rii crnear Wood- 174 15,000 +Z0 Flood plnin diflienlt to ruff, S. C. define. Categories of recurrence interval, in years (an- Middle Tyger River at 1,200 1. 18 Ly- OS nual flood series) 1.0 1.0-1.5 1.5-2 2-3 3-5 5-8 8 man, S. C. Numbor of eases in each category ...... ___3 7 16 22 12 4 7 Tyger Rircr neat Woodruff, 351 13,500 6 s. c. Willimaiitie Rirer near South 121 1,050 1. 15 In order to establish the relation between the eleva- Coventry, Coiui. Hop Ri\rr near Columbia, 76 730 1.04 Flood plaiii elevation tion of the flood-damage stage and the mean elevation Conn. not aceurntely deter- of the flood plain, an eflort has been made to compare the mined. Pomperaug River at Sonth- 75 1,550 1.05 two at a number of cross sections for which data are bury, Conn available. These stations are listed in table 3. The Buihi Qandak River at Sikan- ____ - (?)24,000 2.05 “Danger stage” as- darpur, Bihar, India. sumed from local flood-damage stage was obtained from records of the information to equal Corps of Engineers (1952) on the Mississippi River bankfull stage; data from series of gage and of the Weather Bureau (1941). The mean height height observations. of the flood plain at each cross section was determined Bagmati Rivrr at Dhang rail- ..___1146,000 2.2 Do. road bridge, Bihar, India from profiles drawn from topographic maps having 5- 1 Estimated miiumum. foot contour intervals. Several of these cross sections are shown in figure 58. Critera for defining a “flood” vary, but in general all The profiles of the Mississippi at Greenville Bridge, dehiitions imply overbank flow. Despite this varia- Natchez, and at Arkansas City in figure 58, clearly 1 definition of a “flood,1Jthere is some consistency show a differcnce in elevation between the defined quency of overbank flows. For example, on the (‘flood stage” and the mean height of the flood plain. rn River in Nebraska the U. S. Corps of Engi- As can be seen on the cross sections, the difference (1949a,p. 18) found evidence “of the occurrence of appears to be due to the presence of natural levees ods on the Elkhorn River and of a greater number adjacent to the river bank which raise the st’age at oods on tributary streams during the 66-year period which damage begins. The differences between the 1881.” Thirty-three flood occurrences in 66 years two stages are given in table 3. In some cases the sent an average frequency of one flood every 2 damage stage may be equal to or below the elevation . On the Yellowstone River the Corps of En- of the mean flood plain. In general, however, the rs (1949b1 p. 22) reports that “records indicate damage stage is considerably above the level of the at least 48 floods have caused serious inundation natural flood plain. arm lands and damage to property along the main The recurrence interval of the flood damage stage, in the 64-year period from 1882 to 1945.” That is, if known, is given in the last column in table 3. These s have occurred with a frequency of approximately values are in general accord with those in the preceding tables 1 and 2. It is clear, however, from the compari- Flood-damage stage, as used by the U. S. Weather sons of damage stage and average flood-plain elevation ureau, refers to the gage elevation at which overflow that the stage equal to the elevation of the flood plain ins to cause damage. In an unpublished study of the will occur more frequently because it is lower. If the . Weather Bureau data on flood-damage stage in frequency of flooding, as determined from the analysis rs, TV. B. Langbein found that the median recur- of the damage stage, is adjusted to allow for the differ- e iiiterval of the damage stage at 140 stations was ence between the elevation of damage-stage and average 1 years, based on annual flood series. “Flood stage” elevation of the flood plain, the resultant frequency is most instances is a term used synonymously with close to the value obtained from the analysis of those od-damage stage.” Its designation at any partic- locations where the flood-plain elevation was studied by place is sometimes a reflection of local experience us in the field. In summary, the annual flood (highest PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS
Left bank
230
220
210-
200
190
\ 1w - i MISSISSIPPI RIVER AT VICKSBURG, MISS
90 -
80
70-
60-
MISSISSIPPI RIVER AT ARKANSAS CITY, ARK
1 -1
1 0 3 Miles ILUL 1 I 1 1
w_I
MISSISSIPPI RIVER AT NATCHU. MISS \ 70 \ Damage Itale -- LL- - /+ - - - ,z<__ 60
50 -
40j30
290 SACRAMENTO RIVER AT RED BLUFF. CALIF 1 - I I 280 ~ I __ --Damage stage I
-iJ-\ 7’ ,’ _- 270-260- Jqr;r:7-\ I 250- 1000 0 3000 Feel Ir‘LL- I I -J 740- RIVER FLOOD PLAINS: SOME OBSERVATIONS ON THEIR FORMATION 91
TABLE 3.-&fferences between elevation of jood-damage stage and average elevation of natural $ood plain Difference Recurrence Flood- Mcnn In eet interval of damage poodqloin (+indiefa flood-darn- Elevation Flood- elevation elevation damuge age stage 1 of gage damage at cross at cr08.3 stwe ie (annual Drainage area datum atage section section above flood PTd River and location (quare miles) (feet) (feet) (feet) (feet) &in) series) --.__--- Mississippi River, at Alton, I11 ______---_---171,500 395. 5 21 416.5 425 -8 1. 2
Memphis, Tenn ___-______L______932,800 183.9 34 217.9 214 $4 1. 5 Cairo, I11 ______--203,940 270. 6 44 314.6 315 0 -__ Mouth White River ______.______970,100 108. 7 44 152. 7 145 +8 _-- Arkansas City, Ark- 1,130,700 96.7 44 140.7 132 +9 __- Greenville Bridge, Miss ______I___1,130,800 74.9 48 122.9 118 +5 _-- Lake Providence, La ___.______1,130,900 69. 7 37 106. 7 98 +9 _-- Vicksburg, Miss- ______-______1,144, 500 46. 2 43 89. 2 84 +5 1. 3 St. Joseph, La ______1,148,900 33. 1 40 73. 1 71 +2 --_ Natchez, Miss ______-______1,149,400 17.3 48 65. 3 53 +12 _-- Arkansas River, at Van Buren, Ark- ______-______150,218 372.4 22 394.4 400 -6 1. 5
Sacramento River, near Red Bluff, Calif L______-__--- 9, 300 248. 2 23 271. 2 265 $6 1. 7 3 List 01fl00ds and period of record from U. 8. Weather Bureau, 1941. discharge in each year) will equal or exceed the eleva- often difficult to distinguish between them in the field. tion of,t,h
A, Point bar deposite and flood-plain snrface adjacent to channel of Seneca Creek near Prathertown. Md. The surfaces of point bars occur at varioun elevations up to the general level of the flood-plain surface
B. Flat-topped mavel bar in the New Fork River, 3 miles below Pinedale, Wyo. This bar occupies a position near the center of the channel of a braided river and emerge. as an ieland at low Bow. Bar surface is at name elevation as wooded flood plain men in background. RIVER FLOOD PLAINS : .SOME OBSERVATIONS ON THEIR FORMATION 93 94 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS
Feet
3-1
1
0-
f 0' oh og 0 oe Od
0' b OP 0
I 0
PICUBE60.-Distribution ot materials making up point bar in flood plain of Watts Branch near Rockville, Md. Map and photograph of this locatlon appear in flgurc 59 and plate 1. RIVER FLOOD PLAINS: SOME OBSE RVATIONS ON THEIR FORMATION 95
The accordance of elevation of a point bar and an adjacent broad, flat flood plain was observed on the Burhi Gandak River about 2 miles downstream from 1 Patna in the State of Bihar, India. A bend in the river had moved laterally about 600 feet in 2 years. This rapid movement had bcen carefully noted because the levee being constructed in 1955 was close enough to the bend for local engineers to be anxious lest continued rnovcmcut endanger the levee. The point bar built on the convex shore of this bend was level with the ad- jacent flood plain, and near the channel its profile dropped off in a smooth curve, convex upward, to the edge of the low water surface. Plate 2B shows a gravel bar deposited in the New Fork River near Pincdale, Wyo. This exceedingly flat- topped bar has a composition identical to that found in the flood plain which borders the stream in this reach. Furthermore, its surface is at the same level 5 6 as the adjacent flood plain. Laboratory observations (Leopold and Wolman, 1957) suggest that such a bar develops by deposition within the channel, often as a linear deposit roughly along the center-line of the chan- nel. As the upper surface of the submerged bar builds up near the water surface, the flat top is developed as a result of planation by currents and eddies when the depth of flow over the bar becomes relatively small. 8 This same accordance of level and similarity of compo- sition of bar and flood plain are also found on the Pop0 Agie River near Hudson, Wyo. The flood plains of these streams appear to be composed of coalescent bars MILLIMETERS MILLIMETERS each deposited originally within the stream channel. Overflow across these coalescent bars tends to leave a GVRE 61 .--Size distribution of samples from bed of Watts Branch, Rockville, Md. from adjacent surface near stream margin. Localities of samples shown in veneer of fine material on the gravel surface. This re 60. veneer, however, makes up a very small part of the total flood plain. In our study of point bars on Little times deposition probably takes place in the rela- Pipe Creek near Avondale, Md., the intimate relation deep water over the slc 3ping nose of the point bar between what is clearly a point bar and the level flood- n eddies adjacent to 1Lhreads of rapidly flowing plain surface was shown by a gravel lens that was con- Such deposition thus occurs well within the tinuous from the point bar into the adjacent flood plain. el. The water flowing over the higher parts or In summary, as a stream shifts laterally, deposition e of the point bar may scour or deposit, and we on the convex bank keeps pace with erosion of the bserved both at various times during flood flow opposite concave bank. Our data indicate that the the point bar pictured in figure 59. Examples surface of the material deposited approaches the eleva- several rivers are presented below which illustrat,e tion of the older part of the flood plain, and thus the ccordance of elevation of the point bar and eroding volume of material in the point bar is about equal to the quantity of material eroded. Ultimately, all of the Watts Branch stations p, m, and n, on the larger debris passing the mouth of a river consists of material e two point bars in figure 60, are at an elevation eroded from the land of the drainage basin above but close to that of the flood-plain surface. Plate 2A only part of this eroded material moves continuously reach of Seneca Creek near Prathertown, Md., and directly from its source to the river mouth. Another different point bars composed of a wide variety part is stored temporarily in point bars and in the flood re found to have surface elevations ranging plain at various places in the channel system. Bank from several feet below the flood-plain erosion of the flood plain consists of removal of this e height of the surface itself. material from temporary storage. Point-bar building 96 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS
X X
z Z’
EXPLANATION
Feet 1
04 .- T. 0 5 Feet
FIGUREBZ.--Progressive erosion and deposition in measured cross sections of Watts Branch near Rockville, Md., irom 1853 to 1856. consists of placing in storage a similar quantity of mate- their valleys. Table 4 gives a list of measured and es rial derived either from storage elsewhere in the system mated rates of lateral migration from a number or directly from the drainage basin. sources. These are at best crude. They demonstra’ Those examples in the West are in accord with the more than anything else, the variability of lateral mot previous observations in the eastern United Statcs. ment. Although the larger streams tend to have t Both illustrate similar principles of flood-plain forma- more rapid rate of migration, the data show no ca t,ion. These observations indicate that as much as 80 sistent rates of lateral movement related to thc size to 90 percent of a normal flood plain may be composed position of the channel. In many instances a stre of deposits of lateral accretion, and the remaining 10 channel may maintain a reasonably stable position fll or 20 percent consists of thc overbank deposit. have but little lateral movement over a long period time. The same site may experience very rapid mol RATE OF LATERAL MIGRATION ment during a succeeding period. Considerable eri The more sensational changes in river coursc fre- might result from extrapolation of these rapid sma quently attract the attention of geologists as well as scale shifts in position to long-term movements ia newspaper reporters. When, for example, a slice of constant direction across an entire flood-plain surf&( Rrkansas is contributed to the State of Mississippi by Even the slower rates shown in table 4 when cons a change in the position of the channel of the Missis- over periods of 500 to 1,000 gears permit the ri sippi River we hear about it. Yet, it is extremely dif- move readily from one side of its valley to the ficult to get reliable data on the normal rate of lateral The very existence of the broad valley indicate migration, in rivers; that is, movement laterally across it has done so in the past. RIVER FLOOD PLAINS: SOME OBSERVATIONS ON THEIR FORMATION 97
TABLE 4.-Some data on rates of lateral migration of rivers across valleys Approxi- mate size of Amount Rate of drainage of move- area moue- ment (square mat Period of (Jeet per Remarks bearing on measurement and Rwer and locution milea) (feet) meaaurement year) amount of movement Source of inJarmalion -- - -
Tidal creeks in Massachusetts ...... ~~ 0 6&75yr.-.. 0 ...... Goldthwait, 1937. Normal Brook near Terre Haute, Ind ...... *l 30 1897-1010..- 2.3 Average movement downvalley ...... Dryer and Davis, 1910.
Watts Branch near Rockville, Md- ._._.~ __ __ 4 0-10 1915-55 ..... 0.25 ...... Topographic map and ground survey. Do ...... 4 6 1953-66 ..... 2 Mnxixnum movement; loenlly in Successive planetable surveys. doivnvalley direction. Rock Creek near Washington, D. C...... 7-60 0-20 1915-55..-.- 0.50 ...... Topographic map and ground survey. Middle River near Bethlehem Church, near 18 25 10-15 yr---- 2.5 ...... Local observer. Staunton. Va. Tributary to Minnasota Rivcr near New Ulm, 10-15 250 1910- 38-...- 9 Tributarynearrailroad ...... U. S. Corps Engrs. map and acrial Mlnn. photographs. North River, Parnmus quadrangle, Vn--. .... 50 410 1834-84.-.-- 8 ...... Account by local observers. Seneca Cmk at Dawsonville, hfd ...... 101 0-10 50-100yr..- 0.20 ...... Maximum age of trcw on flood plain is 100 years. Laramie River near Fort Laramle, Wgo...... 4,W 100 1851-1954--. 1 Average movement in 55 mile reach; Bleb, 1954. old map. Minnesota River new New Ulm, Minn ...... 10, ooo 0 1910-38 ..... 0 Most of IC-mile reach ...... U. S. Corps Engrs. map and aerial photographs. Ramganga Rive! near Shnhnhabad, Indin...... l00,OOO 2,900 1795-1806... 264 Movement to west; drainage arcii Central Board of Irrigation, 1947. only approximate. Do ...... 100, OOO 1,050 1806-1883.-. 14 Movement to east; =me bend as Do. above. Do ...... 100,oOO 790 1883-1945.-- 13 Movement to west; %me bend as Do. above. Colorado River near Needles, Calif ...... 170,600 20.000 1858-83 _____ 800 One bend (maxlmum movement in hleans.l short period). Do ...... 170,600 3,000 1883-1903.-. 150 ___..do...... Do. Do ...... 170,600 4,000 1903-1952.-. 82 __.__do...... Do. Do ...... 170,600 100 1942-5 Z-.... 10 Minimum movement in short period- Do. Do...... 170,600 3,800 190342_____ 98 Median total movement ...... Do. Yukon River at Kayukuk River Alaska ...... 320,000 5,500 170yr ___.._32 From evidence furnished by forest Eardley, 1938. succession. Yukon River at Holy Cross, Alaska ...... 320,OOO 2.400 18961916-.- 120 Local observer- ...... Do. Eosi River. North Bihar. India______.__...... 369.000 150~...... 2.460 ...... Qhosh. 1942. Missouri River near Peru, Nebr ______- ______350, OOO 5,000 1883-1903.-. 250 Rate varied from 60-500 feet annually. Dunason, 1909. Mississippi River near Rosedale, Miss ...... 1,100, WO 2,380 193M5..... 158 About average movement; move- Fisk, 1951, fig. 9. ment varlable because of variations In bank material. Do ...... _1,100, oOO 9,500 1881-1913.-- 630 Maximum movement; cutoe channel Do. not included.
I Menns, T.H.. 1953. The Colorado River in Mohave V;rlley-xiicaiiderings of the stream in historic times: U. S. Bur. Reclamation uupub. rept.
OVERBANK DEPOSITION tion of the total section made up of overbank deposits is small. NATURE AND AMOUNT Table 5 presents some data on the average thickness There is adequate evidence to show that in some of sediment deposited on flood plains by great A oods. places and at some times significant amounts of material TABLE5.-&les of amounts of deposition on flood plains are deposited on flood plains by ovcrbank flow. We during major jloods shall consider some spccific examples, as well as some AOerage reasons that might account for the small amount of thiekneaa 01deposi- deposition which appears to occur at many localities. Rioer basin Date ofpood tim (feet) Source of data At Prentiss Landing on the Mississippi River oppo- Ohio River ...... Jan.-Feb. 1937.-- 0.008 MensReld, 1939. site the mouth of the Arkansas River, the Mississippi Connecticut River ... Mar. 1936 ...... 114 Jahns, 1947. Do ...... Sept. 1938 ...... 073 Do. River exposed in 1955 a sectioii containing an old Kansns River ...... July 1051 ...... OB8 Carlson md Runncls, 1952. court house buried by the river in 1865. The flood plain at this site contains a basal section of crossbedded Because of the magnitude of these record floods, it coarse sand and silt constituting channel or point-bar is impossible to specify exactly their recurrence inter- deposits about 50 feet thick. This is capped by about val. From the known record, the Ohio flood of 1937 4 feet of finer, banded overbank deposits. Some of certainly exceeded a 100-year flood. It must be recog- this overbank material is probably natural levee de- nized, of course, that the figures in table 5 are averages. posit, and thus the overbank section may be somewhat Under special conditions a foot or several feet of ma- thicker than the normal. In either event, the propor- terial were deposited. In other places scour rather 98 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS
than deposition occurred. From the data presented Eiler, 1957). The local nature of thick deposits, the by Mansfield (1939, p. 700) the rccord flood of 1937 large variation in thickness of deposits even within a on the Ohio River deposited material which would small area, and the occurrence of areas of scour suggest amount to about y8 inch if spread uniformly over the that widespread deposition of sediment by major floods area flooded. However, certain areas were subject to is not so well established a fact as a glance at table 5 scour, not deposition, and the amount removed was might imply. These observations suggest that, com- approximately one-quarter as much as the total of the pared with the overall volume of the flood plain, incre- material deposited. ments added by overbank deposition are on the average In his study of the Connecticut Valley, Jahns (1947) quite small. describes deposits of the record floods of 1936 and 1938 An economic study of effects of a great flood add (table 5) and Pleistocene terrace sequences composed collateral evidence on the magnitude of deposition by of flood-plain materials. For the most part the over- overbank flow. In a study of the flood of 1951 on the bank deposits of the modern floods ranged in thickness Kansas River, it was found that, in general, damage from 6 feet near the stream, to a thin veneer at the resulting from deposition on agricultural lands during margins. Jahns (p. 85) estimated that “* * * a floods was extremely low (Wolman, Howson, and blanket of sediment was deposited over the entire Veatch, 1952). Locally, overbank deposits may be flooded area to an average depth of 1%inches during significant, but such occurrences appear to be anomalies March, 1936. About inch was added during the rather than usual. hurricane flood of September, 1938.” The stratigraphy If successive increments of overbank deposition were of the terraces described by Jahns consists of channel responsible for building a flood plain, we might expect and overbank deposits, although he uses different terms to find minute laminations representing these deposits. for these features. He (p. 49) identified overbank de- The data in table 5 indicate that these laminations posits in these sections primarily by their stratification would be so fine that they could hardly be found, much ,and finer texture. less sampled. Although we have observed short The thicknesses of channel and overbank deposits stringers and very thin layers of sand, or even pebbles, respectively, in the Connecticut Valley appear in the in exposures of the flood plain, they have little or no ratio of about 2 to 1; this is a somewhat larger propor- lateral extent. They are certainly lenses rather than tion of material derived from overbank flows than we extensive overbank layers. have found elsewhere. The difficulty of distinguishing Because material is distributed in the vertical of a point-bar from overbank deposits may account, in stream of flowing water according to size or settling part at least, for the difference. velocity, one would expect the deposits in a flood plain Happ, Rittenhouse, and Dobson (1940) and many to be successively finer from bottom to top. Examples others have noted large amounts of overbank deposi- from Brandywine Creek (Wolman, 1955, p. IS), Watts tion from individual floods. Occurrences of thick local Branch, Seneca Creek, and elsewhere indicate that such deposits, even as coarse as gravel, are not uncommon. a gradation in size is virtually impossible to find in an For example, Harrod and Suter (1881, p. 136) reported individual section. Available data show primarily that in the Missouri River flood of 1881 the “immediate that flood plains may include materials of quite different banks were raised for long distances from 4 to 6 feet” sizes. Figure 63 gives examples of flood deposits and between Sioux City ancl Glasgow, Iowa. Although flood-plain sediments from several regions and shows they also noted deposits from 6 to 12 feet in width that the greatest diversity in size is in the basin for within the channel in areas which were “sheltered from which most data are available, the Connecticut River the fierce current,” even this report makes no mention basin. Here the major differences in size are due prin- of widespread deposits sufficient to raise the general cipally to differences in source material within the basin. level of the flood plain. NATURAL LEVEES Obviously, it is difficult to estimate accurately the thickness of sediment deposited over large areas. Al- The report of Harrod and Suter on deposition along though there are examples of thick local deposits from the “immediate banks” leads to a consideration of the overbank flows, there are also contrasting examples of importance of natural levees in the formation of flood local scour (see Davis and Carlson, 1952, p. 232; Breed- plains. Many observers have pointed out that when ing ancl Montgomery, 1954, p. 6). Observations in the flow leaves the stream channel its veloci Connecticut following the disastrous record-breaking checked, and as a result the stream is unable to flood of August, 1955 indicate that deposition was its load and deposits material adjacent to the extremely irregular and cannot be considered as if it Natural levees are prominent along the Mississ were uniform over the entire area flooded (Wolman and and Sacramento Rivers where they are particul RIVEll FLOOD PLAINS: SOME OBSE:RVATIONS ON THEIR Ff3HMATION 99
Sand typical flood plains.) Minor ridges occasionally paral- microns) (1100 lel the stream but not infrequently these are the result of cultivation, or more accurateIy, the absence of cultivation. Farmers may plow close to a stream but lcave a margin of vegetation which may trap some sediment from the flow. This same area is also built up by plowed material. These might best be termed “plowshare” levees. Natural levees have becn described by Happ, Rittenhouse, and Dobson (1940) in the Southeast. We have also observed very extensive natural levees in Georgia. Because of the coarseness of the material in them, and because of their restriction to certain areas, it is very possible that accelerated erosion re- *om) lated to man’s activities has much to do with their formation. Such a possibility is supported by the Connecticut River flood deposits of September existence of prominent natural levees on Little Falls Ohio Valley flood of 1937, composite of 12 Branch and on parts of Rock Creek near Washington, analyses, coar ser deposits (Mansf ield, 19 39, D. C. The levees on these small streams (drainage in Jahns, 1947) areas 4-50 square miles) stand in marked contrast 0 Ohio Valley flood of 1937, composite of 159 to the absence of such levees on most of the streams in analyses, finer textured deposits (Mansfield, the region. Both of these small creeks drain suburban 1939, in Jahns, 1947) areas where municipal construction has altered the 0 Potomac River deposits, Maryland and Vir- ginia, composite of 4 analyses, (Mansfield, basin characteristics. 1939, in Jahns, 1947) Little Falls Branch not only has natural levees but Br andyw ine Creek, Pennsylvania,. flood -p lain differs in another way from nearby basins less affected materials (Wolman, 1955) by man’s activities. It appears to have experienced x Kansas River, 1951 flood deposit on flood a change in frequency of overbank flow. A competent plain (Carlson and Runnels, 1952) observer, W. W. Rubey (personal communicatjoD) has Kansas River flood-plain surface .(Carlson and Runnels, 1952) lived for more than 15 years in sight of this creek and Seneca Creek, Maryland flood plain, states that the frequency of overbank flooding has composite of 4 analyses increased from about once a year to at least 10 times $ 6 Watts Branch, Maryland flood plain, 2 a year during the period of his observation. This composites of 4 analyses increase in frequency can probably be attributed to
IGWRE G.-Siae distrlbution of flood deposits and flood-plain sediments (original street construction, paving, and home construction on diagram from Jahns, 1947). the watershed. It is logical to suppose that open cuts and unpavcd streets during the boom in suburban formed along the concave bank at channel bends. construction have greatly increased the amount of ural levees occur along the Nile River, and are sediment carried by the stream during the past decade. nspicuous on cross profiles of many rivers on the The combination of increased sediment supply and ngetic plain in northeast India. Hilgard O’Reilly frequency of overbank flow may account for the promi- ernberg (1955, personal communication) noted that nent natural levees there in contrast to absence of ny large alluvial islands in the Amazon River are levees on other streams in the area. ed by natural levees. Water overflowing the These illustrations are in accord with Malott’s and during flood is held in the basin formed by the (1928, p. 27) observations that the increase in height ounding levee when the flood recedes. As a result, of the flood plain adjacent to channels is ‘I* * * center of such alluvial islands is a lake during usually small and noticeable chiefly during low flood riods of low flow. stages when this area is the last to be submerged.” ong the rivers of small and moderate size which Although the natural levee is a feature which has in Maryland, Virginia, and Pennsylvania, we received considerable mention in the literature as a ve found few recognizable levees. (See Wolman, type of overbank deposit, it appears to constitute a 55, figure 16, p. 16 for examples of cross sections of relatively small proportion of the normal flood plain. 9. 100 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS
HYPOTHETICAL CONSTRUCTION OF A FLOOD PLAIN BY feet of deposition. The absence of any discernible OVERBANK DEPOSITION 3tratigraphic break in the section allows the possibility If a specific thickness of material were deposited on that the base of the section is as old as the log. If the the flood plain every time a river overflowed its banks, log has been in place during this period, then the eleva- it would be possible to compute the rate of rise of the tion of the surface of the flood plain has risen so little flood-plain surface by -the use of flood frequency data. that it must be considered, in effect, stable. Figure 64 is a plot of hypothetical flood-plain elevation Assuming that the flood plain at Chadds Ford is against time on Brandywine Creek at Chadds Ford, about 1,450 years old, we should expect from figure 64 Pa. It was constructed in the following way: The that the present flood plain would actually be 4 feet average number of days per year on which a given above its present position. The fact that it is not stage is equaled or exceeded was computed from the suggests that overbank deposition is not the primary gaging station records. Assuming that each time the mechanism by which a flood plain is formed. Further stream overflows a given level it deposits a specific if overbank deposition is not a major factor in ra thickness of material, the time required for the surface the elevation of the surface of the flood plain, it fo of a flood plain to reach a given elevation can be com- that in a normal channel the bed will not become puted. In this example it was assumed that each gressively farther below its own flood plain as a result increment consisted of a layer of sediment 0.005 foot of continued overbank deposition. thick (see table 5 for reasonableness of this assumption). The data presented thus far indicate that despite The most conspicuous feature in figure 64 is the frequent flooding, the elevation of the surface of a rapid increase in elevation in the first 10 years. Al- flood plain remains stable relative to the level of the though the rate decreases somewhat as the figure channel bed. In the following section we shall con- shows, in 170 years it would have been possible to sider some possible explanations for the absence of construct a flood plain to its present height at Chadds continued increase in elevation. Ford. From 80 to 90 percent of this construction would have taken place in the first 50 years. CONDITIONS AFFECTINU AMOUNTS OF OVERBANE DEPOSITION
r w Three lines of evidence may help to explain the relative unimportance of overbank deposition in flood- plain formation which we postulate. First, periodic removal of the flood plain by lateral erosion helps to control its height. Second, the highest discharges are oftcn characterized by lower concentrations of sus- pended sediment than discharges of intermediate sizes. Third, velocities of water on tho flood plain during periods of overbank flow may be high and competen to move sediment of small and medium size. The extent to which periodic removal and replace- mcnt is effective in limiting the height of the flood-plai TIME, IN YURS surface depends on the relative rates of lateral swingin FIGURE64.-Hypothetical formation and rate of increase in flood-plainelevation bJ and overbank accretion. The available data do overbank deposition. Graph shows increase in elevation with time1oon Brandy wine Creek at Chadds Ford, Pa. provide satisfactory comparison of these rates. So rough relations, however, can be inferred from d Not far from Chadcls Ford is Buck Run, a tributag presented earlier in this paper. of Brandywine Creek, near Coatesville, Pa. In thc Table 4 indicates that it is likely that specific areas banli of this tributary channel, a log was found at tht of any flood plain may not be eroded by the stream for basc of a flood plain. The log lay just below the lon periods as long as 200 or more years. This figure water level, 0.5 foot above the bed of the presenl would be even greater if one considered the fact tha stream (see Wolman, 1955, p. 18, fig. 20). Bedrock the movement of the channel across the valley floo lies approximately 3 to 5 feet below the log and crops more often than not consists of many reversals of direc- out in the channel about 50 feet downstream. A tion rather than continuous movement from one side o carbon-14 analysis dated the log as approximatelj the valley to the other. Those areas of thefloodpla 1,450 f200 years old (Rubin and Suess, 1955, p. 487) which have not been reached by the river channel for The vertical section above the log comprises only 3.1 long time should presumably have higher elevations RIVER FLOOD PLAINS: SOME OBSERVATIONS ON THEIR FORMATION 101 a result of continued overbank deposition than the more TaBLE B.-Velocitly and depth of $ow in overbank sections of river jlood plains recently constructed areas of the flood plain. Figure [Data from current-meter discharge measurements by U. 9. Oeologicsl Survey] 64 suggests that in 200 to 400 years one could expect Approximate Total flood Mean Mean vcloc- dralnage dlscharge Deplh of vclocltg itu of over- 1 t802 feet of deposition above present flood-plain level area (aquare (cdkfeet overflow flow t(feet miles) Date per second) (feet) (!:?% per second) as a result of overbank flows. The relief on any flood ~ -- __I_ -- plain would then be a function of rate of migration or of difference in age. When a flood plain is flat as many are, the low relief suggests that although the 1.2 elevation of the flood plain is partially controlled by lateral migration, additional factors also control the amount of overbank deposition. 507,525 July 18,1951 519,800 5.0 1.0 ’ Contrary to some expectations, high discharges are 8.5 5.4 7.7 2.8 often associated with lower concentrations of suspended 7.5 2.0 I 2.4 load than are more moderate floods. The curves of 6.0 3.4 10.3 2.0 sediment concentration and discharge presented in 7.0 4.7 I figure 65 show, for example, that in several Kansas streams concentration reaches a maximum and declines 3,788 June 11,1945 114,000 4.0 .9 ‘ while discharge is continuing to increase. In addition, 6.0 2.4 6.0 .2 ’ 1.6 during individual floods the peak of the sediment con- 8.0 1.5 centration often precedes or follows the peak discharge. 5.0 2.4 3.0 3.2 , It has been suggested that the decrease in concentra- tion may result from deposition on the flood plain as a stream goes out of its banks. In the South Fork Solomon River at Alton, Icans., and in the Solomon River at Beloit, Kans. (fig. 65), the maximum sediment concentration occurs in discharges well below bankfull stage, and thus the decrease of concentration at high flow cannot be attributed to dep65tion on the flood plain. If we consider the fact that the load is primarily a funct,ion of the physiographic and climatic conditions in the contributing drainage area and, although corre- lated with discharge, is not a function of the discharge y no means necessary that the highest con- be associated with the highest flows. The igh flows may rlot be associated with the irnent concentration may help to account y of deposition during these periods. possible mechanism contributing to small 101 Sept. 1,1952 3.m 1.4 .48 ’ overbank deposition relates to the ability 1.5 .62 1.7 .77 bank flows to transport material across the flood 1.6 1.68 , 1.5 There are many streams transporting material 2.0 1.71 2.2 2.44 ng a moderate range of sizes that do not have 2.0 2.30 , nor do they display a major amount of 1.2 1.80 , osition. Only the finer particles are car-
e water surface or in the upper part of the .15 and these sizes are not likely to drop out along the of the channel in accord with the simple natural- 2. 7 explanation, nor need they be deposited to any extent on the surface of the flood plain. Water leaves the channel and flows over the flood plain 1.5 to move directly downvalley rather than to the longer distance along the more sinuous 102 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS
PRAIRIE DOG CREEK NORTON, KANS.
n aW
LL 0
l 10 50 100 500 loo0 5000 DISCHARGE, IN CUBIC FEET PER SECOND
FIQURE 65.-Relation of sediment concentration to discharge in several rivers in hans3s. RIVER FLOOD PLAINS: SOME OBSERVATIONS ON THEIR FORMATION 103 path of the channel. The downvalley slope may be flood-frequency study previously mentioned, there has considerably greater than that of the channel, and this probably been little or no change in the plant cover and higher gradient tends to keep the velocity high and there is no apparent difference between thesc and the reduces the probability of deposition. other flood plains studied. Some data on the conditions of overbank flow are In summary, we suggest that one possible reason for available in measurements of velocity made by the the general lack of deposition by overbank flow may be Geological Survey. Although such measurements on the ability of the overbank section itself to transport truly natural, unobstructed flood plains are difficult to sediment . find, table 6 shows measured depths and velocities from FLOOD PLAINS IN STABLE, AGGRADING, AND DEGRAD- 10 overbank sections. Some of the observed velocitics ING STREAMS are rather high, even as great as mean velocities ob- served in natural channels themselves. These results Thus far the “typical” flood plain considered here is are plausible if one considers that the slope may be one in which the relative position of the bed of the chan- greater and the roughness of the flood-plain section may nel to the surface of the flood plain has remained stable be less than the roughness of the main channel. These during the formation of the flood plain. The relatively observed velocities exceed the velocities competent to constant frequency of overbank flooding described transport silts and sands (Inman, 1949). Because silt earlier could apply even if the entire valley were being and sand are the predominant sizes carried in the over- aggraded at a constant rate, with channel bed and flood bank section where such velocities occur, it is perhaps plain rising uniformly. In such an instancc there would to be expected that much of the material can be carried be the usual difficulty in differentiating overbank from downvalley and not deposited on the flood plain. point-bar deposits, and there would bc added the diffi- The data from the Humboldt River in table 6 repre- culty of differentiating deposits of different ages in the sent velocities measured in overflow sections which are aggrading sections. The difficulty is illustrated in tlie former channels of the river (M. T. Wilson, personal following analysis of several flood plains in North and communication). As in thc main channel, depending South Carolina and in Georgia. The description of upon the depth and proximity to the banks, great vari- thesc flood plains is based on numerous borings (P. IC. ation is to be expected in the velocity. In a flood plain Theobald, Jr., and W. C. Overstreet, 1954-55, personal constructed by lateral migration accompanied by communication), and examples are presented in cross- abandonment of old ch&nnels, the remnants of these section form in figure 66. channels may serve at high flows as avenucs of trans- The flood plains rest upon bedrock and have surfaccs ort or as places for deposition, depending upon their which are overtopped by the annual flood approxi- entation in relation to the direction of overbank mately once each year or once every 2 ycars (table 1). ows. On many streams where flood plains are several They are composed of silt, sand, and clay in an infinite undred to several thousand feet wide, channels are variety of combinations. These are commonly under- ten found at the margin of the flood plain adjacent lain by pebble and cobble gravels, as figtirc 66 illus- the valley wall. These may represent courses of trates. Prom the stratigraphy alone it is impossible to €ormcr channels but their position suggests that they tell how much of this material is overbank and how probably drainage channels on the surface of the much is point bar. A log found within a similar flood- d plain created and maintained by overbank flows. plain section in North Carolina is 2,370 rt 200 years old, where oxbows or distinct relics of a channel pat-% according to a carbon-14 determination. xist elsewhere on the flood plain, is it possible to At first glance it might seem as if these sections are guish between remnants of abandoned channels actually aggraded ones rather than stable, alluvial rficial drainage channels produced by floods. sequences. The following analysis, however, indicates It has becn pointed out to us that conditions cmt6e first, that the distinction bctwcen the two is not easily esent flood plain may not be representative of tlie made, and second, that the flood plains in figure 66 may riod of flood-plain formation. In particular, thc well represent a single stable deposit. a1 of trees from flood plains in the East and in the Figure 67A is a graph of thc depth of fill in valleys e West may have markedly reduced the likelihood in the North and South Carolina region, plotted against deposition (H. C. Bolon, 1954, personal communica- the length of the valley from its headwaters to the on). In some areas, lion-ever, including many in the given cross section. Although there is considcrablc 104
BUCK CREEK rlooFeet may be related to the development of the present channel. The interrelation of the flood plain and the present channel is suggested further by computations of the potential depth of scour in these channels. Analysis of the flood records at 8 gaging stations indicates that mean depths of flow from 10 to 15 feet (includes both Io scour and depth above mean bed level) are attain SECOND BROAD RIVER during floods which recur on the average, once each to 8 years. Lacey's studies (Inglis, 1949, p. 327) irrigation canals indicate that the maximum dept
0 50 Feet xour in a regime or equilibrium channel may be on order of 1.75 to 2 times the regime depth. Such sc is associated with a dominant flow in the regime cha which probably is somewhat lower than a flood SOUTH TYGER RIVER
regime channel are akin to those in natural channel is reasonable to suppose that depths of scour 1%to 2 0 500 Fee( times the depths attained during these flows are both reasonable and possible. These conditions make it jeem probable that the Carolina streams represented in Iigures 66 and 67 actually scour to bedrock at infrequent NORTH TYGER RIVER
o-_, o-_, 500 Feet this area. EXPLANATION X Silt and clay Flood plain Sand
0 mBedrock Gravel
FIG~JHEGG.-Cross sections of river flood plains In North Carolina and South Carollna. scatter on this graph, the average relation can be described by the equation mean depth of fill~clength.~~ (1) That is, the depth of fill is proportional to the .45 power of the valley length. Figure 67B is a graph relating the mean depth of the water corresponding to average discharge (mean annual discharge) in channels of different sizes to the valley length. In many rivers the mean annual discharge is equaled or exceeded about 25 percent of the time. This second curve, derived from gaging stations in the same region as the locations in figure 67A, indicates that the mean depth at the nvcrage discharge, as well as the dcptli of fill, is pro- portional to the valley length. This relation is ex- pressed by the equation mean depth of flow~clcngth~~~ (2) The progressive increase downstream of both the depth FWJREt37.-Relat1ou of dlstauce from Iieedwnter divide to meeu depth of fl plaln alluvium and to mean depth of flow nt avei~gedischarge at selectcd local of fill and the mean depth of flow suggests that the fill In North Carolina and South Carolina. RIVER FLOOD PLAINS: SOME OBSERVATIONS ON THEIR FORMATION 105
Evidence of this scour is also provided by observa- sequences found in the West (Leopold and Miller, 1954). tions of the flood of 1903(?), which occurred on the It must be admitted that at this time the stability of Pacolet River in North Carolina (P. K. Theobald, Jr., the absolute elevation of the surfaces of most flood 1954-55, personal communication). Observers re- plains cannot be proven. The evidence demonstrates, ported that “the entire alluvial fill was removed to bed- however, that even during aggradation the difference in rock” by this flood. Allowing for possible exaggera- elevation between the river bed and the surface of its tions, it is probably safe to say that scour to bedrock flood plain does in many instances remain constant occurred even if “the entire alluvial fill” was not re- over long periods of time. That the same may be true moved. Because the bedrock is occasionally reached by during periods of rapid degradation as well as the slow the flowing water, one must suppose that slow degrada- degradation previously mentioned (p. 33) is indicated tion of the valley is taking place; but the rate of degra- by observations of the Ukak River in Alaska. In 1912 dation is extremely slow and, in relation to flood-plain volcanic ash from the eruption of Katmai filled the river formation, the elevation of t,he river is in effect stable. valley. In the 40-year period since the eruption, the Both the frequency of flooding and the calculated river has cut down 10 to 40 feet and is continuing to do potentialities for scour imply that these flood plains are so (H. Coulter, 1955, personal communication). At being continuously constructed and reconstructed by the same time the Ukak has and is continuing to con- the streams which now flow within them in accord with struct a flood pla.in. These observations of a variety of the processes described earlier. rivers indicate that a channel may have a flood plain Although the active flood plain associated with the associated with it when it is stable and flowing on bed- present regimen of the river has been emphasized here, rock, gradually eroding a valley, or gradually depositing it has already been pointed out that flood plains can a fill. exist in aggrading channels. During thc process of When aggraded valley fill or any flood plain is incised aggradation the discharge regimen of a stream may be after its formation, the former flood plain becomes a maintained; that is, floods occur and the stream as a terrace. An alluvial terrace is an abandoned flood rule experiences a certain range in discharge. During plain whose surface no longer bears the normal relation the process of aggradation both the bed of the channel to the stream bed. This study indicates that the and the surface of the floocl plain continue to rise. normal relation between the surface of the active flood Although the channel may be choked with material at plain and the stream bed on many streams can be de- times (Happ, 1950). there is no reason to believe that fined by the frequency of flooding. Where such is the a flood plain will not coiitinuc to be constructed during case a flood plain becomes a terrace when the channel incises itself to the point where the former active flood In those cases where continual aggradation produced plah is no longer overtopped by that annual flood, the valley fill, it is difficult to explain how the relative which on the average occurs less than once every 2 years, position of the channel to the flood plain remained fixed during aggradation if overbank deposition is considered CHANNEL PATTERN AND FORMATION OF THE FLOOD PLAIN the principal mechanism of laying down the valley a. Rather, concomitant rise of both stream bed and Point bars have been described in the literature flood-plain surface appears to be best explained by primarily in relation to meandering streams. Many of buting the bulk of the deposited material to the the examples given above are from such streams. We ess of point bar formation. have shown (Leopold and Wolman, 1957), however, The uniform frequency of flooding of flood plains does that there are only small differences in the fundamental ot rule out the possibility that both the surface of the characteristics of so-called straight channels, meanders, ood plain and the bed of the channel are being built and braids. Very few straight channels are actually ultaneously. Gages on the Nile River, which pro- straight. An inspection of their thaIwegs reveals bars e longest periods of record of any river in the and pools analogous to those found in meanders. Thc dicate that both the bed and the flood plain of wandering straight channel deposits material in the re being raised at a rate of about 3 to 4 feet in same way as does a meander and hence, is equally years (Lyons, 1906, p. 313-317). Such rates are capable of forming a true flood plain even though thc asurable in the brief periods of record with which process of formation may be somewhat more difficult to e are dealing. The 1,500 to 2,300-year-old logs be- recognize. Braided channels also build islands and bars neath the flood plain in Pennsylvania and North Caro- to elevations correlative with the flood-plain surface. lina, respectively, provide the only fairly good evidence The New Fork River example (pl. 2B) shows one such of the age of the modern flood plain. These dates are in bar. The data also show that such incipient flood general in accord with dates derived from terrace plains are overtopped at a frequency similar to that of 106 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS flood plains elsewhere in the United States. In sum- REFERENCES CITED mary, flood plains may be constructed by channels of Breeding, S. D., and Montgomery, J. H.. 1954, Floods of Septem- any type or size. ber 1952 in the Colorado and Guadalupe River basins, CONCLUSION central Texas: U. S. Geol. Survey Water-Supply Paper, 1260-A, 46 p. This study supports the views of several authors Carlson, W. A. and Runnels, R. T., 1952, A study of silt de- that the flood plain is composed of channel deposits, posited by the July 1951 flood, central Kansas River valley: or point bars, and some overbank deposits. The Kans. Acad. Sci. Trans., v. 55, p. 209-213. relative amounts of each vary, but on the average, the Central Board of Irrigation, India, 1947; Annual report (techni- cal), part 11, p. 552. proportion of overbank material appears to be small. Challinor, J., 1946, Two contrasted types of alluvial deposits: This conclusion is supported by the uniform frequency Geol. Mag., v. 83, p. 162-164. of flooding and by the small amount of deposition ob- Davis, S. N., anc Carlson, W. A., 1952, Geological and ground served in great floods. Lateral migration, relatively water resources of the Kansas River valley between Law- rence and Topeka, Kans.: Kans. Geol. Survey Bull. 96, high velocities which can occur in overbank flows, and p. 201-276. the decrease in sediment concentration at high flows Dietz, R. A., 1952, The evolution of a gravel bar: Mo. Bot. contribute to this result. Garden Ann,, v. 39, p. 249-254 In many instances it is difficult to differentiate in Dryer, C. R., and Davis, M. K., 1910, The work done by Normal stratigraphic section alone the two fundamental types Brook in 13 years: Ind. Acad. Sei. Proc., p. 147-152. Duncanson, H. H., 1909, Observations on the shifting of the of deposits. Point bars are extremely heterogeneous, channel of the Missouri River since 1883: Science, v. 29, and mherc ovcrbank deposition does take place the p. 869-871. small amount of material cleposited and its irregular Eardley, A. J., 1938, Yukon channel shifting: Geol. SOC.Am. distribution usually make it difficult to identify. Bull., V. 49, 343-358. Frequency studies indicate that many flood plains Fenneman, N. M., 1906, Floodplairls produced without floods: Am. Geog. SOC.Bull. 38, p. 89-91. are subject to flooding at approximately yearly intervals. Fisk, H. N., 1947, Fine grained alluvial deposits and their effects These studies, as well as stratigraphic observations, on Mississippi River activity: U. S. Waterways Exp. Sta., indicate that the flood plain is also related to the present 2 v., 82 p. regimen of the stream flowing within it. 1951, Mississippi River Valley geology, relation to river If neither natural nor man-induced changes take regime: Am. SOC.Civil Eng. Trans., v. 117, p. 667-689. Ghosh, B. P. C., 1942, A comprehensive treatise on North place in the structural, climatic, or physiographic con- Bihar flood problems: Govt. Printing, Patna, Bihar ditions which control the regimen of a natural channel, (India), 200 p. the channel will not form terraces by gradually building Goldthwait, J. W., 1937, Unchanging meanders of tidal creeks, up its own flood plain until flooding no longer occurs. Massachusetts [abs.]: Geol. SOC. America Proc., 1936, p. The flood plain can only be transformed into a terrace 73-74. Goldthwait, R. P., 1941, Changes on the intervales of Connecticut by some tectonic, climatic, or man-induced change and Merrimac Rivers: N. H. Acad. Sci. Proc., v. I, p. 17. which alters the regimen of the river, causing it to Happ, S. C., 1950, Geological classification of alluvial soils labs.]: intrench itself below its established bed and associated Geol SOC.America Bull., v. 61, p. 1568. flood plain. Happ, 8. C., Rittenhouse, G., and Dobson, G. C., 1940, Some Lateral migration of a stream across its flood plain principles of accelerated stream and valley sedimentation: U. S. Dept. Agricultural Tech. Bull. 695, 133 p. takes place with almost no change in channel width. Harrod, B. M., and Suter, C. R., 1881, Report of the Committee The volume of material deposited tends to be about on Outlets and Levees upon the floods of the Missouri equal to the volume eroded. Material eroded from the River in the spring of 1881: 47th Cong., 1st sess., S. EX. drainage basin is only temporarily stored in the flood Doc. no. 10, pt. I, p. 135-139 (app. H.). plain. Only whcn the stream erodes laterally into Hieb, D. L., 1954, Fort Laramie National R/Ionument, Wyo., U. S. Dept. Interior, Natl. Park Service Hist. Handbook, terraces or hillsides higher than the flood plain does the series 20, 43 p. volume eroded exceed the volume deposited. Only in Inglis, C. C., 1949, The behavior and control of rivers and canals, this case can stream-bank protection be expected to Central Water-power Irrigation and Navigation Res. Sta., reduce the total sediment yield from a drainage basin. Poona: Res. Pub. 13, 2 v., 486 p. Although we recognize the conclusions presented here Inman, D. L., 1949, Sorting of sediments in the light of fluid mechanics: Jour. Sed. Petrology, v. 19, p. 51-70. require much additional supporting data, we give them Jahns, R. H., 1947, Geologic features of the Connecticut Valley, forthrightly in the hope that they will stimulate fur- Mass., as related to recent floods: U. S. Geol. Survey ther observation and study. Water-Supply Paper, 996, 158 p. RIVER FLOOD PLAINS: SOME OBS lRVATIONS ON THEIR FORMATION 107
angbein, 11’. B., 1949, Annual floods and the partial-duration Rice, C. M., 1949, DictionaryIof geological terms: Ann Arbor, flood series: Am. Geophys. Union Trans., v. 30, p. 879-881. Mich., Edwards Bros. eopold, I,. B., and Miller, J. P., 1954, .4 postglacial chronology Rubin, Meyer and Suess, €I. E., 1955, U. S. Geological Survey for some alluvial valleys in Wyoming: U. S. Geol. Siirvey radiocarbon dates 11: Science, v. 121, p. 481-488. Water-Supply Paper 1261, 90 p. U. S. Army Corps of Engineers, 1949a, Examination Elkhorn eopold, L. B., and Wolman h4. G., 1957, River channel pat- River and tributarics, Nebr.: 81st Cong., 1st sess., H. Doc. terns-braided, meandering, mid straight: U. 8. Geol. no. 215, 60 p. Survey Prof. Paper 282-B. orens, P. J., and Thronson, R. E., 1955, Geology of the fine- --_ 194910, Examination Yellowstone River and t,ributarics: grained alluvial deposits in Sacramento Valley and their 81st Cong., 1st sess., H. Doc. no. 216, 71 p. relationship to seepage, p. Al-A26, in Seepage conditions --- 1952, Stages and discharges of the Mississippi River and in Sacramento Valley: Rept. to Calif. Water Project tributaries in the Vicksburg district: Vicksburg Ilist., 281 p. Authority by Dept. Public Works, Div. Water Res. yons, W. G., 1906, The physiography of the River Nile and its U. S. Weather Bureau, 1941, The river and flood forecasting basin: Survey Dept., Egypt, Cairo, 411 p. service of the Weather Bureau, Washington, p. 16. ackin, J. H., 1937, Erosional history of the Big Horn Basin, Wolman, hbel, IIowson, L. R., and Veatch, IC. T., 1953, Report Wyo.: Geol. SOC.Am. Bull., v. 48, p. 813-894. on flood protection, Kansas River basin, I