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

Abstract.____-----__-___-___..______-_--_-__________

Introduction_______________________________________

Frequency of overbank flow.__________________________

Formation of a typical flood plain_____._______________

The characteristics of point bars._________________

Sediments forming a typical point bar_________

Process of formation and elevation of the surface 

of point bars.____________________________

Rate of lateral migration.___________________

Overbank deposition.__^________________________

Nature and amount of overbank deposition 

Natural levees,_____________________________

Page

87

87

88

91

91

91

92

96

97

97

Page

Foimation of a typical flood plain—Continued 

Overbank deposition—Continued

Hypothetical construction of a flood plain by

overbank deposition______________________ 100

Conditions affecting amounts of overbank

deposition. ______________________________ 100

Flood plains in stable, aggrading, and degrading streams _ 103 

Channel pattern and formation of the flood plain.______ 105

Conclusion,_ _______________________________________ 106

References cited--___-__--__-----_--_-_--______----- 106

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

On many small rivers and most great rivers, the flood plain 

consists of channel and overbank deposits. The proportion of 

the latter is generally very small.

Frequency studies indicate that the flood plains of many 

streams of different sizes flowing in diverse physiographic and 

climatic regions are subject to flooding about once a year.

The uniform frequency of flooding of the flood-plain surface 

and the small amount of deposition observed in great floods 

(average 0.07 foot) support the conclusion that overbank 

deposition contributes only a minor part of the material constituting the flood plain. The relatively high velocities (1 to 4 

fps) which can occur in overbank flows and the reduction in 

sediment concentration which often accompanies large floods 

may also help account for this. Although lateral migration of 

channels is important in controlling the elevation of the flood 

plain, rates of migration are extremely variable and alone cannot 

account for the uniform relation the flood-plain surface bears to 

the channel.

Detailed studies of flood plains in Maryland and in North 

Carolina indicate that it is difficult to differentiate between 

channel and overbank deposits in a stratigraphic section alone.

Because deposition on the flood plain does not continue 

indefinitely, the flood-plain surface can only be transformed into 

a terrace surface by some tectonic or climatic change which 

alters the regimen of the river and causes it to entrench itself 

below its established bed and associated flood plain. A terrace, 

then, is distinguished from a flood plain by the frequency with 

which each is overflowed.

INTRODUCTION

Owing to the extensive use of river flood plains by 

industry and agriculture, there is practical as well as 

intellectual interest in the mode of formation, stability 

of the flood plain, and the frequency of flooding of its 

surface. If maximum use is to be made of the broad, 

level surfaces provided by flood plains, it is important 

to know the natural conditions controlling their 

development and preservation. The geologist has long 

been interested in the characteristics of flood plains 

and has written much about them. For the most part, 

previous papers have dealt primarily with individual 

examples. The authors believe that the hydrologic as 

well as the additional stratigraphic observations 

presented here may help in developing a more generalized picture of the characteristics of river flood plains. 

Conclusions based upon the data now at hand should

be considered tentative, as many additional data will 

be needed to verify them adequately.

A definition of allood plain often quoted is "a strip of 

relatively smooth land bordering a stream * * * called 

a living flood plain if it is overflowed in times of high 

water" (Rice, 1949, p. 139). Valley flats which would 

usually be considered "flood plains" may include those 

formed by different processes such as landslides, 

low-angle fans, and perhaps others. The distinction 

between valley flats formed by different processes may 

not be apparent to the casual observer and, indeed, 

detailed work might be necessary to determine the 

origin of a given feature. However, an important 

process resulting in the formation of valley flats is a 

combination of deposition on the inside of river curves 

and deposition from overbank flows. This process 

produced many of the flat areas adjoining river channels 

and probably most of the flood plains of the great 

rivers of the world. It is the flood plains produced by 

this process with which the present paper is concerned.

The data supplied to us by several district engineers 

of the Geological Survey are essential to the tentative 

conclusions reached in this paper. We are indebted 

to H. C. Bolon, L. R. Sawyer, M. T. Wilson, and 

E. A. Johnson. We also express our appreciation to 

D. M. Culbertson for detailed analyses of hydrographs 

of water and sediment in several streams in Kansas. 

Shri B. N. Chatterjea kindly obtained certain field 

data in India at our request, and furnished us flood 

frequency tabulations for which we are grateful.

We have profited from discussion of the subject with 

J. T. Hack of the Geological Survey. In addition 

to colleagues in the Survey who read earlier drafts, 

we thank Prof. John C. Geyer of Johns Hopkins 

University, H. T. U. Smith of the University of 

Kansas, and Mr. Cole Fisher of the Virginia Geological Survey for their helpful criticisms of the manuscript. 

Thanks are due also to Miss Gladys Braden of Minneapolis, Minn., for a number of references with which 

she provided us.

We are grateful to the Viers family of Rockville, 

Md., and especially to Caroline Viers Mudgett, for 

permission to study the stream on their property over 

a period of years.

87
PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

FREQUENCY OF OVERBANK FLOW

On any river there are floods of varying magnitude 

which cover a wide range in stage. As successive floods 

overtopped the flood plain, one would expect successive 

increments of overbank deposition on the flood plain to 

result in a continual increase in elevation of this surface 

relative to the elevation of the stream bed. As the flood 

plain became higher and higher, presumably it would 

be flooded less and less frequently. Nevertheless, the 

giant flood could still be expected to deposit a thin 

increment of material on this high surface. As a result 

of such a sequence of events, one would logically expect 

to find that the frequency of flooding of flood plains in 

different regions was extremely variable. Some flood 

plains would be flooded with extreme rarity, and others 

would be flooded frequently. Such considerations 

suggested to us that a study of the frequency of flooding 

of flood plains might be useful in understanding their 

mode of formation. Studies of a number of flood plains 

in both the eastern and western United States and in 

India indicate that, contrary to the logic of the argument above, the frequency of overbank flow is remarkably uniform in many rivers flowing in diverse climatic 

and physiographic regions.

Table 1 shows the recurrence interval of the bankf ull 

or incipient overflow stage at a number of stations, 

most of them in different regions in the United States. 

The gage height of the bankfull stage was measured at 

gaging stations operated by the U. S. Geological Survey. In mountainous areas where broad flood plains are 

rare, the height of the modern flood plain was considered 

to be the average of the highest surfaces of the channel 

bars. This is in accord with observations to be presented on the process of flood-plain formation. The 

frequency with which this bankfull gage height is 

attained was determined from curves relating discharge, 

or stage, of annual floods to recurrence interval (for 

method, see Langbein, 1949). The annual flood is the 

name given to the highest discharge during a given year.

In most of the rivers listed in table 1, the annual flood 

attains or exceeds the level of the surface of the flood 

plain every year or every other year (recurrence interval=l to 2 years). At those stations where the flood 

plain is clearly defined and its elevation accurately 

known, for example, Seneca Creek near Dawsonville, 

Md., Brandywine Creek at Chadds Ford, Pa., and Hams 

Fork near Frontier, Wyo., the recurrence interval of 

overbank flow lies closer to 1 than 2 years. There are, 

of course, expected variations in any such data from 

natural rivers. Most of the major departures from the 

average, particularly the examples drawn from mountain torrents in Wyoming, may be ascribed to the difficulty of locating representative flood-plain surfaces produced by the process with which we are concerned. 

Both the narrow valleys of the mountain torrents and 

the very turbulent flow may contribute to this condition. Despite these variations, the data from eastern 

and western United States and the examples from India 

indicate a remarkable uniformity in the recurrence 

interval of overbank flooding.

TABLE 1.—Recurrence interval with which flood-plain level is 

attained by annual flood

Recurrence 

interval,

Drain- Discharge in years 

age at bankfull (from 

area stage annual 

(square (cubic fee! flood 

River and location miles) per second) series) Remarks

Pole Creek near Pinedale, 88

Wyo. 

Horse Creek near Daniel, 124

Wyo. 

Cottonwood Creek near 202

Daniel, Wyo. 

Little Sandy Creek near Elk- 21

horn, Wyo. 

Big Sandy Creek at Leckie 94

ranch, Wyo.

Green River near Fontenelle, 3,970

Wyo.

Hams Fork near Frontier, Wyo. 298 

Middle Piney Creek near Big 34

Piney, Wyo. 

Fall Creek near Pinedale, 37

Wyo. 

Middle Fork Powder River 980

near Kaycee, Wyo.

Red Fork near Barnum, Wyo. 

Clear Creek near Buffalo,

Wyo. 

North Fork Clear Creek near

Buffalo, Wyo. 

Little Popo Agie River near

Lander, Wyo. 

Beaver Creek near Daniel,

Wyo.

142

120

29

130

141

Rock Creek near Red Lodge, 100

Mont. 

Yellowstone River at Billings, 11,870

Mont. 

Seneca Creek at Dawsonville, 101

Md. 

Bennett Creek at Park Mills, 63

Md.

Linganore Creek at Frederick, 82

Md. 

Big Pipe Creek at Bruce- 102

ville, Md. 

Piney Run near Sykesville, 11

Md. 

Patuxent River near Unity 35

Md. 

Little Pipe Creek at Avon- 8

dale, Md.

Brandywine Creek at Chadds 287

Ford, Pa. 

Buffalo Creek at Garden- 145

ville, N. Y. 

Henry Fork near Henry River, 80

N. C.

544

320

990

227

573

526

740 200

1,130 200

722 1.12

80

188

390

534

418

2,000

21,400

1,160

1,510

2,700

3,690

?585

1,330

1.13 

1.10

15 Terrace (?). 

4 

1.05

1.47

1.01

Mountain torrent; flood 

plain poorly defined. 

Do.

1.01

1.01

2.0 Flood plain poorly

denned. 

1.48

1.36

Mountain torrent; flood 

plain poorly defined.

11

1.05

1.07

1.6

2.7

1.25 Only 4 years of record.

1.3 Flood plain difficult to

define. 

1.6

(?)260 (?)1.25 Gage at distance from 

flood plain.

4,570 1.35

3,000 1.01

6,900 3.0 Flood plain surface not 

clear.
RIVER FLOOD PLAINS: SOME OBSERVATIONS ON THEIR FORMATION 89

TABLE 1.—Recurrence interval with which flood-plain level is 

attained by annual flood—Continued

River and location

First Broad River near Lawndale, N. C. 

South Tyger River near Reidville, S. C.

Drainage 

area 

(square 

miles)

198 

106

Discharge 

at ban kf nil 

stage 

(cubic feet 

per second)

5,900 

3,400

Recurrence 

interval, 

in years 

(from 

annual 

flood 

series) Remarks

1.16 

2.2

South Tyger River near Woodruff, S. C.

Middle Tyger River at Lyman, S. C.

Tyger River near Woodruff, 

S. C.

Willimantic River near South 

Coventry, Conn.

Hop River near Columbia, 

Conn.

Pomperaug River at Southbury, Conn.

Burhi Gandak River at Sikandarpur, Bihar, India.

Bagmati River at Dhang railroad bridge, Bihar, India 

1 Estimated maximum.

174

68

351

121

76

15,000 +20 Flood plain difficult to

define. 

1,200 1.18

13,500

1,050

730

75 1,550 

.... (?)24,000

—... '146,000

5

1.15

1.04 Flood plain elevation 

not accurately determined.

1.05

2.05 "Danger stage" assumed from local 

information to equal 

bankfull stage; data 

from series of gage 

height observations.

2.2 Do.

Critera for defining a "flood" vary, but in general all 

definitions imply overbank flow. Despite this variation in definition of a "flood," there is some consistency 

in frequency of overbank flows. For example, on the 

Elkhorn Kiver in Nebraska the U. S. Corps of Engineers (1949a, p. 18) found evidence "of the occurrence of 

33 floods on the Elkhorn River and of a greater number 

of floods on tributary streams during the 66-year period 

since 1881." Thirty-three flood occurrences in 66 years 

represent an average frequency of one flood every 2 

years. On the Yellowstone River the Corps of Engineers (1949b, p. 22) reports that "records indicate 

that at least 48 floods have caused serious inundation 

of farm lands and damage to property along the main 

stem in the 64-year period from 1882 to 1945." That is, 

floods have occurred with a frequency of approximately 

one in 1.3 years.

Flood-damage stage, as used by the U. S. Weather 

Bureau, refers to the gage elevation at which overflow 

begins to cause damage. In an unpublished study of the 

U. S. Weather Bureau data on flood-damage stage in 

rivers, W. B. Langbein found that the median recurrence interval of the damage stage at 140 stations was 

2.1 years, based on annual flood series. "Flood stage" 

in most instances is a term used synonymously with 

"flood-damage stage." Its designation at any particular place is sometimes a reflection of local experience

but usually results from an engineering investigation of 

flood-control needs and becomes accepted as an "official" designation. Additional data from records of the 

Corps of Engineers on the frequency of flood-damage 

stage are provided in table 2. The median recurrence 

interval in this table is about 2 years, a value which is 

somewhat higher than we have found it in the majority 

of our direct field observations.

TABLE 2.—Distribution of flood-damage stage at 71 river gages in 

Texas expressed as number of examples in various categories of 

recurrence interval (Data from Corps of Engineers')

Categories of recurrence interval, in years (annual flood series) __—— ——— - —— 

Number of cases in each category.-.--..

1.0 1.0-1.5 1.5-2 2-3 3-5 5-8 8 

3 7 16 22 12 4 7

In order to establish the relation between the elevation of the flood-damage stage and the mean elevation 

of the flood plain, an effort has been made to compare the 

two at a number of cross sections for which data are 

available. These stations are listed in table 3. The 

flood-damage stage was obtained from records of the 

Corps of Engineers (1952) on the Mississippi River 

and of the Weather Bureau (1941). The mean height 

of the flood plain at each cross section was determined 

from profiles drawn from topographic maps having 5- 

foot contour intervals. Several of these cross sections 

are shown in figure 58.

The profiles of the Mississippi at Greenville Bridge, 

Natchez, and at Arkansas City in figure 58, clearly 

show a difference in elevation between the defined 

"flood stage" and the mean height of the flood plain. 

As can be seen on the cross sections, the difference 

appears to be due to the presence of natural levees 

adjacent to the river bank which raise the stage at 

which damage begins. The differences between the 

two stages are given in table 3. In some cases the 

damage stage may be equal to or below the elevation 

of the mean flood plain. In general, however, the 

damage stage is considerably above the level of the 

natural flood plain.

The recurrence interval of the flood damage stage, 

if known, is given in the last column in table 3. These 

values are in general accord with those in the preceding 

tables 1 and 2. It is clear, however, from the comparisons of damage stage and average flood-plain elevation 

that the stage equal to the elevation of the flood plain 

will occur more frequently because it is lower. If the 

frequency of flooding, as determined from the analysis 

of the damage stage, is adjusted to allow for the difference between the elevation of damage-stage and average 

elevation of the flood plain, the resultant frequency is 

close to the value obtained from the analysis of those 

locations where the flood-plain elevation was studied by 

us in the field. In summary, the annual flood (highest
90

Left bank

230- 

220- 

210-

200-

190-

100- 

90- 

80-

70-

60-

150-

140- 

130- 

120- 

110-

\

-L £y^ n

\ \ i i1 r* °*

~~

PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

MISSISSIPPI RIVER AT MEMPHIS, TENN.

Damage stage

Damage stage

1 0 3 Miles

I i , i i_______I_______J_ __I

MISSISSIPPI RIVER AT VICKSBURG, MISS.

MISSISSIPPI RIVER AT ARKANSAS CITY, ARK.

V

Damage stage

Right bank

MISSISSIPPI RIVER AT GREENVILLE BRIDGE, MISS.

>

>

g

130-

110-

100-

^-— ^

1 0 3

1 , , , 1 I 1 1

Damage stage

MISSISSIPPI RIVER AT NATCHEZ, MISS.

70-1

60-

50- 

40-

\

\ -Damage stage

\ ~^ /^N

^ ' Xx.

1 0

30-J 1 , , , 1 1

1( V

3 Miles

1 |

290

280-

270-

260-

250-

240-

SACRAMENTO RIVER AT RED BLUFF, CALIF.

Damage stage

FIGURE 58.—Kelation of flood-damage stage to elevation of flood plain at several river cross sections.
RIVER FLOOD PLAINS! SOME OBSERVATIONS ON THEIR FORMATION 91

TABLE 3.—Differences between elevation of flood-damage stage and average elevation of natural flood plain

River and location

Memphis, Tenn___ .__ ____ _ _ ___ _

Cairo, 111. _ _____.________... ______________ _ _____

Mouth White River __ _ _____ _____ __ _

Arkansas City, Ark_ . _ _

Greenville Bridge, Miss _____ __ _ ____

Natchez, Miss _ _ _____ __

Sacramento River, near Red Bluff, Calif. _ _ ______ _ ____

1 List of floods and period of record from U. S. Weather Bureau, 1941.

Drainage area 

(square miles)

171,500

932, 800

203, 940

970, 100

1, 130, 700

1, 130/800

1, 130, 900

1, 144, 500

1, 148, 900

1, 149, 400

150, 218

9, 300

Elevation 

of gage 

datum 

(feet)

395. 5 

183.9 

270.6 

108. 7 

96. 7 

74.9 

69. 7 

46. 2 

33. 1 

17.3 

372.4 

248. 2

Flooddamage 

stage 

(feet)

21 

34 

44 

44 

44 

48 

37 

43 

40 

48 

22 

23

Difference Recurrence 

Flood- Mean in feet interval of 

damage flood-plain (-^-indicates flood-darnelevation elevation damage age stage 1 

at cross at cross stage is (annual 

section section above flood flood 

(feet) (feet) plain) series)

416.5 

217.9 

314.6 

152. 7 

140.7 

122.9 

106. 7 

89.2 

73. 1 

65.3 

394.4 

271. 2

425 

214 

315 

145 

132 

118 

98 

84 

71 

53 

400 

265

Qo 

+ 4 

0 

+ 8 

+ 9 

+ 5 

+ 9 

+ 5 

+ 2 

+ 12 

-6 

+ 6

1. 2 

1. 5

1.3

1. 5 

1.7

discharge in each year) will equal or exceed the elevation of the flood plain nearly every year.

In view of the obvious differences in the amounts of 

runoff in different areas, it may seem illogical that there 

should be a uniform frequency at which overbank flow 

occurs. Some evidence suggests, however, that where 

the runoff is high, the channel is larger than where it is 

small. In other words, the larger channel is adjusted 

to carry the larger bankfull flows, and thus the frequency 

with which the flood-plain stage is attained may be the 

same in regions of diverse runoff.

The uniform frequency of overbank flow suggests that 

overbank deposits may constitute a small part of the 

flood plain. If overbank deposition were a significant 

feature of flood-plain formation, because of the frequent 

intervals at which flooding takes place one would logically expect that present flood-plain surfaces would be 

built to much higher levels and would be flooded much 

less often than they are. Too, there would be no reason 

to expect any uniformity in the frequency with which 

flood plains in diverse regions are flooded.

It is necessary to consider now the nature of the 

deposits making up the flood plain and some possible 

explanations of the inferred lack of importance of overbank deposition in flood-plain formation.

FORMATION OF A TYPICAL FLOOD PLAIN

The two fundamental types of deposits which make 

up a flood plain have often been described in the literature (Fenneman, 1906; Melton, 1936; Mackin, 1937; 

Happ, Kittenhouse and Dobson, 1940; Challinor, 1946; 

Fisk, 1947; Jahns, 1947). The first type is the point 

bar; the second is the overbank deposit previously referred to. The two types may respectively be described 

as deposits of lateral and vertical accretion. Despite 

the distinction between the two in the literature, it is

often difficult to distinguish between them in the field. 

Fenneman (1906) describes an example in Wisconsin of a 

flood plain composed solely of point-bar deposits, or 

deposits of lateral accretion. He attributes the absence 

of overbank deposition to a lake that controls the flow 

in the stream and prevents flooding of the flood plain 

downstream. Such an ideal example, however, is rare. 

In his study of the Mississippi River, Fisk (1947) was 

able to develop criteria for distinguishing various types 

of deposits by lithology, texture, and morphology. In 

this way he could distinguish channel, natural levee, and 

back-swamp deposits in the flood plain. Similar distinctive deposits were described by Lorens and Thronson (1955) in the Sacramento Valley. On other rivers 

where these criteria are less recognizable, it becomes 

more difficult to distinguish these types. As we shall 

attempt to show, point bars may be composed of material of a great variety of sizes and textures. To a 

lesser extent, there is also variability in deposits from 

overbank flows.

The modes of deposition are, nevertheless, different. 

Basically, overbank deposits consist of material deposited from high water flowing or standing outside of 

the channel. Point bars originate within the confines 

of the channel. Despite the difficulty involved in distinguishing between them, for purposes of discussion the 

distinction continues to be a useful one.

THE CHARACTERISTICS OF POINT BARS

SEDIMENTS FORMING A TYPICAL POINT BAR

A point bar is a deposit formed on the inside, or convex side, of a river bend by lateral accretion. Deposition is related to the existence of circulatory motion or 

helicoidal flow associated with the channel bend. Deposition on the convex bank and the concomitant erosion 

of the concave bank both tend to be the greatest just
92 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

downstream from the position of maximum curvature 

(see Mackin, 1937, p. 827; Eardley, 1938; Fisk, 1947, 

p. 32; Dietz, 1952). Together the processes of erosion 

and deposition tend to maintain a constant channel 

width during lateral shifts of the channel.

Figure 59 is a detailed map and plate 1 a photograph 

of a point bar on a meander of Watts Branch near 

Rockville, Md. At this point the stream drains about 

4 square miles and its flood plain is about 270 feet wide. 

This flood plain is typical of many rivers in the eastern 

United States and illustrates the type of deposition and 

stratigraphy commonly found in this area.

The meandering channel which can be seen in figure 

59 contains the flow at low and moderate discharges. 

At flows large enough to recur only 2 or 3 times a year, 

water passes directly from A to B across the point bar. 

In this meander we have measured velocities of 3 fps in 

water flowing over the point bar, and this velocity is 

sufficient tq scour and to move coarse sand across the 

bar. Some swale-and-ridge topography on the surface 

of the bar is produced by these flows. During periods 

of high flow when rapidly moving water crosses the 

channel from the convex point toward a position on the 

concave bank downstream from the position of maximum 

curvature (from C to D in fig. 59) an eddy usually 

forms close to the convex bank (E), and is a locus of 

deposition of relatively fine material. We have often 

noticed another eddy on the outer, or concave, bank 

just upstream from the point of maximum curvature of 

the bend (F). The alternation of high and low flow 

and the concomitant shifts in the velocity and streamline pattern at any given place give rise to considerable 

heterogeneity in the point-bar deposits.

The map of Watts Branch and data in figure 60 

demonstrate this diversity. Our measurements show 

the stream is gradually altering its position in the flood 

plain (see fig. 62). A local resident says that about 30 

years ago there was a hole at point B in figure 59 deep 

enough for small children to swim in. As the map 

shows, the stream channel has moved at least 10 feet 

from that place. As the main thread of the current in 

Watts Branch has moved from one position to another 

at different stages of flow and at different times, localized eddies formed in slack water adjacent to these currents. At such points as, for example, in the old swimming hole, exceedingly fine material has been deposited 

in juxtaposition to rather coarse material carried by the 

main current. This is shown in the auger holes in 

figure 60.

The nature of the deposits in the point bar is also 

illustrated by the histograms in figure 61, showing size 

distributions of samples taken from the stream bed and 

from the surface of the adjacent point bar. The 

locality of these samples is shown in figure 60. Finegrained material was deposited at high flow near the 

downstream edge of the point bar (sample 1) as a result 

of small eddies to the left of the main current. The 

samples at points 2, 4, and 6 include material deposited 

in slack water adjacent to the main thread of the current. The bimodal character of these deposits appears 

to indicate that each sample contained material from 

more than a single sedimentation unit. Most of the 

stream bed is covered with coarse gravel, and comparable material appears in some of the samples in 

figure 61. As can be seen in figure 60, gravel occurs 

interbedded with sand and with mixtures of clay to fine 

sand. It is believed that these fine materials were 

deposited by low or moderate flows. Unless a special 

effort is made to sample each period of flow separately, 

a sample will include deposits from two periods. Just 

downstream from sample 1, the sloping margin of the 

point bar was covered with piles of leaves and other 

organic material and generally surrounded by fine mud. 

These examples from active depositional environments illustrate the wide variety of materials constituting the point bar and indicate that, contrary to the 

common supposition, the point bar is not necessarily 

composed of material coarser than that which is found 

in the overbank deposits.

PROCESS OF FORMATION AND ELEVATION OF THE SURFACE OF POINT BARS

During 3 years of observation and measurement of 

this point bar on Watts Branch, we have recorded as 

much as 6 feet of lateral movement of the channel. 

Cross sections presented in figure 62 show that erosion 

of the steep or outside bank was accompanied by deposition on the opposite point bar. At cross section X-Xr 

the amount of material deposited is roughly equivalent 

to the amount eroded. At Y-Y' erosion has exceeded 

deposition, while further downstream at Z-Z' during 

the period of observation deposition has exceeded erosion. We would not expect precise equivalence of 

erosion and deposition at an individual cross section 

during a short period of time. It is clear, however, 

that the eroding flood plain is being replaced by lateral 

or within-channel deposits.

The map of Watts Branch presented in figure 59 

might alone suggest that the surface of the point bar is 

generally lower than the overall flood plain. Our field 

observations on many rivers lead us to conclude that 

the point bar is actually built up to the level of the flood 

plain; that is, the so-called lateral deposit can be found 

at all elevations from the water surface to the floodplain surface. The distinction between overbank and 

lateral deposits becomes quite vague at stages of flow 

during which water covers the entire point bar. Overbank flow, as used in this study, refers to stages above 

the level of the cut bank of the existing flood plain.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 282 PLATE 1

View downstream across the flood plain of Watts Branch near Rockville, Md. Point 

bar to the right of the channel in middle distance is shown in figure 59.
GEOLOGICAL, SURVEY PROFESSIONAL PAPER 282 PLATE 2

A, Point bar deposits and flood-plain surface adjacent to channel of Seneca Creek 

near Pralhertown, Md. The surfaces of point bars occur at various elevations up 

to the general level of the flood-plain surface.

B, Flat-topped gravel 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 emerges 

as an island at low flow. Bar surface ie at same elevation as wooded flood plain seen 

in background.
RIVER FLOOD PLAINS! SOME OBSERVATIONS ON THEIR FORMATION

View in plate 1

EXPLANATION 

Edge of low-water surface

60 Feet

Contour interval 0.5 foot 

Arbitrary datum Steep or vertical bank 

FIGURE 59.—Map and cross section of a typical point bar of the flood plain of Watts Branch, 1 mile northwest of Rockville, Md.
94 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

Feet

3-

2-J

0 J

X

II i

«uger hole

Locality of sample described 

in figure 61

View in plate 1

FIGUBE 60.—Distribution of materials making up point bar in flood plain of Watts Branch near Rockville, Md. Map and photograph

of this location appear in figure 59 and plate 1.
RIVER FLOOD PLAINS I SOME OBSERVATIONS ON THEIR FORMATION 95

w

1*0

55 20

I 

o

s

FIGURE Gl.—Size distribution of samples from bed of Watts Branch, Rockville, Md. 

and from adjacent surface near stream margin. Localities of samples shown in 

figure 60.

At times deposition probably takes place in the relatively deep water over the sloping nose of the point bar 

and in eddies adjacent to threads of rapidly flowing 

water. Such deposition thus occurs well within the 

channel. The water flowing over the higher parts or 

surface of the point bar may scour or deposit, and we 

have observed both at various times during flood flow 

over the point bar pictured in figure 59. Examples 

from several rivers are presented below which illustrate 

the accordance of elevation of the point bar and eroding 

flood plain.

At Watts Branch stations p, m, and n, on the larger 

of the two point bars in figure 60, are at an elevation 

very close to that of the flood-plain surface. Plate 2A 

is a reach of Seneca Creek near Prathertown, Md., 

where different point bars composed of a wide variety 

of material are found to have surface elevations ranging 

successively from several feet below the flood-plain 

surface to the height of the surface itself.

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 

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 been carefully noted because the 

levee being constructed in 1955 was close enough to the 

bend for local engineers to be anxious lest continued 

movement endanger the levee. The point bar built on 

the convex shore of this bend was level with the adjacent 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 25 shows a gravel bar deposited in the New 

Fork River near Pinedale, Wyo. This exceedingly flattopped bar has a composition identical to that found 

in the flood plain wT hich borders the stream in this 

reach. Furthermore, its surface is at the same level 

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 channel. 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. 

This same accordance of level and similarity of composition of bar and flood plain are also found on the Popo 

Agie River near Hudson, Wyo. The flood plains of 

these streams appear to be composed of coalescent bars 

each deposited originally within the stream channel. 

Overflow across these coalescent bars tends to leave a 

veneer of fine material on the gravel surface. This 

veneer, however, makes up a very small part of the 

total flood plain. In our study of point bars on Little 

Pipe Creek near Avondale, Md., the intimate relation 

between what is clearly a point bar and the level floodplain surface was shown by a gravel lens that was continuous from the point, bar into the adjacent flood plain.

In summary, as a stream shifts laterally, deposition 

on the convex bank keeps pace with erosion of the 

opposite concave bank. Our data indicate that the 

surface of the material deposited approaches the elevation of the older part of the flood plain, and thus the 

volume of material in the point bar is about equal to 

the quantity of material eroded. Ultimately, all of the 

debris passing the mouth of a river consists of material 

eroded from the land of the drainage basin above but 

only part of this eroded material moves continuously 

and directly from its source to the river mouth. Another 

part is stored temporarily in point bars and in the flood 

plain at various places in the channel system. Bank 

erosion of the flood plain consists of removal of this 

material from temporary storage. Point-bar building
96 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

x'

EXPLANATION

1953 

' 19*55

FIGUKE 62.—Progressive erosion and deposition in measured cross sections of Watts Branch near Rockville, Md., from 1953 to 1956.

consists of placing in storage a similar quantity of material derived either from storage elsewhere in the system 

or directly from the drainage basin.

Those examples in the West are in accord with the 

previous observations in the eastern United States. 

Both illustrate similar principles of flood-plain formation. These observations indicate that as much as 80 

to 90 percent of a normal flood plain may be composed 

of deposits of lateral accretion, and the remaining 10 

or 20 percent consists of the overbank deposit.

KATE OF LATERAL MIGRATION

The more sensational changes in river course frequently attract the attention of geologists as well as 

newspaper reporters. When, for example, a slice of 

Arkansas is contributed to the State of Mississippi by 

a change in the position of the channel of the Mississippi River we hear about it. Yet, it is extremely difficult to get reliable data on the normal rate of lateral 

migration, in rivers; that is, movement laterally across

their valleys. Table 4 gives a list of measured and estimated rates of lateral migration from a number of 

sources. These are at best crude. They demonstrate, 

more than anything else, the variability of lateral movement. Although the larger streams tend to have the 

more rapid rate of migration, the data show no consistent rates of lateral movement related to the size or 

position of the channel. In many instances a stream 

channel may maintain a reasonably stable position and 

have but little lateral movement over a long period of 

time. The same site may experience very rapid movement during a succeeding period. Considerable error 

might result from extrapolation of these rapid smallscale shifts in position to long-term movements in a 

constant direction across an entire flood-plain surface. 

Even the slower rates shown in table 4 when considered 

over periods of 500 to 1,000 years permit the river to 

move readily from one side of its valley to the other. 

The very existence of the broad valley indicates that 

it has done so in the past.
RIVER FLOOD PLAINS! SOME OBSERVATIONS ON THEIR FORMATION 97

River and location

TABLE 4.—Some data on rates of lateral migration of rivers across valleys

Approximate

size of Amount Rate of 

drainage of movearea move- ment

(square ment Period of (feet per Remarks bearing on measurement and 

miles) (feet) measurement year) amount of movement Source of information

Tidal creeks in Massachusetts..— — —

Normal Brook near Terre Haute, Ind.

Watts Branch near Rockville, Md...

Do.—............................

±1 

4

4

7-60 

18

10-15

50

101

Rock Creek near Washington, D. C——..__ 

Middle River near Bethlehem Church, near

Staunton, Va. 

Tributary to Minnesota River near New Ulm,

Minn.

North River, Parnassus quadrangle, Va__... 

Seneca Creek at Dawsonville, Md___-....

Laramie River near Fort Laramie, Wyo....._

Minnesota River near New Ulm, Minn._-_... 

Ramganga Rivet near Shahabad, India........

Do........................................ 100,000

Do.

Colorado River near Needles, Calif—. — — ..

Do——————.————————————. 

Do——— —— ————— — — ———— .

Do-—————————————————.

Do.—— —— ———— ———————— —— .

Yukon River at Kayukuk River Alaska__.. 320,000

Yukon River at Holy Cross, Alaska——....... 320,000 2,400 1896-1916... 120

Kosi River, North Bihar, India............_ .....__ 369,000 ISOyr...... 2,460

Missouri River near Peru, Nebr..—————— 350,000 5,000 1883-1903— 250

Mississippi River near Rosedale, Miss..——— 1,100,000 2,380 1930-45—— 158

0 60-75 yr.... 0 —————————————————— Qoldthwait, 1937.

30 1897-1910_ 2.3 Average movement downvalley___ Dryer and Davis, 1910.

0-10 1915-55__ 0-0.25 ___________________... Topographic map and ground survey.

6 1953-56__ 2 Maximum movement; locally in Successive planetable surveys.

	downvalley direction.

0-20 1915-55__ 0-0.50 _________________-__ Topographic map and ground survey.

25 10-15 yr__ 2.5 __________________ __.. Local observer.

250 1910-38__ 9 Tributary near railroad..______ U. S. Corps Engrs. map and aerial

photographs.

410 1834-84-— 8 ———————.—.————————— Account by local observers. 

0-10 50-100yr_ 0-0.20 ____________________- Maximum age of trees on flood plain

is 100 years. 

Average movement in }•§ mile reach; Hieb, 1954.

old map. 

Most of 10-mile reach______.... U. S. Corps Engrs. map and aerial

photographs. 

Movement to west; drainage area Central Board of Irrigation, 1947.

only approximate. 

Movement to east; same bend as Do.

above. 

Movement to west; same bend as Do

above. 

One bend (maximum movement in Means.*

short period). 

._—do._—————— —————-- Do.

__.do.._.___———...——.— Do. 

Minimum movement in short period. Do. 

Median total movement———— ——— Do. 

From evidence furnished by forest Eardley, 1938.

succession. 

Local observer-_____.....———— Do.

——....................._...—— Qhosh, 1942.

Rate varied from 50-500 feet annually. Duncanson, 1909. 

About average movement; move- Fisk, 1951, fig. 9. 

ment variable because of variations 

in bank material.

Maximum movement; cutoff channel Do. 

not included.

4,600

10,000

100,000

100,000

100,000

170,600

170,600 

170,600 

170,600 

170,600 

320,000

100

0

2,900

1,050

790

20,000

3,000 

4,000 

100 

3,800 

5,500

1851-1954—

1910-38——

1795-1806—

1806-1883—

1883-1945...

1858-83——

1883-1903— 

1903-1952— 

1942-52——— 

1903-42——— 

170 yr—— ..

1

0

264

14

13

800

150 

82 

10 

98 

32

Do—————————————— —— —— —— 1,100,000 9,500 1881-1913— 630

1 Means, T. H., 1953, The Colorado River in Mohave Valley—meanderings of the stream in historic times: U. S. Bur. Reclamation unpub. rept

OVEBBANK DEPOSITION

NATURE AND AMOUNT

There is adequate evidence to show that in some 

places and at some times significant amounts of material 

are deposited on flood plains by overbank flow. We 

shall consider some specific examples, as well as some 

reasons that might account for the small amount of 

deposition which appears to occur at many localities.

At Prentiss Landing on the Mississippi River opposite the mouth of the Arkansas River, the Mississippi 

River exposed in 1955 a section containing an old 

court house buried by the river in 1865. The flood 

plain at this site contains a basal section of crossbedded 

coarse sand and silt constituting channel or point-bar 

deposits about 50 feet thick. This is capped by about 

4 feet of finer, banded overbank deposits. Some of 

this overbank material is probably natural levee deposit, and thus the overbank section may be somewhat 

thicker than the normal. In either event, the proportion of the total section made up of overbank deposits 

is small.

Table 5 presents some data on the average thickness 

of sediment deposited on flood plains by great floods.

TABLE 5.—Examples of amounts of deposition on flood plains 

during major floods

Average 

thickness

Piver basin Date of flood tion (feet) Source of data

Ohio River————. Jan.-Feb. 1937...

Connecticut River— Mar. 1936—— ...

Do_.———... Sept. 1938———

Kansas River._.— July 1951...„„..

0.008 Mansfield, 1939.

. 114 Johns, 1947.

. 073 Do.

. 098 Carlson and Runnels, 1952.

Because of the magnitude of these record floods, it 

is impossible to specify exactly their recurrence interval. From the known record, the Ohio flood of 1937 

certainly exceeded a 100-year flood. It must be recognized, of course, that the figures in table 5 are averages. 

Under special conditions a foot or several feet of material were deposited. In other places scour rather
98 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

than deposition occurred. From the data presented 

by Mansfield (1939, p. 700) the record flood of 1937 

on the Ohio River deposited material which would 

amount to about % inch if spread uniformly over the 

area flooded. However, certain areas were subject to 

scour, not deposition, and the amount removed was 

approximately one-quarter as much as the total of the 

material deposited.

In his study of the Connecticut Valley, Jahns (1947) 

describes deposits of the record floods of 1936 and 1938 

(table 5) and Pleistocene terrace sequences composed 

of flood-plain materials. For the most part the overbank deposits of the modern floods ranged in thickness 

from 6 feet near the stream, to a thin veneer at the 

margins. Jahns (p. 85) estimated that "* * * a 

blanket of sediment was deposited over the entire 

flooded area to an average depth of 1% inches during 

March, 1936. About % inch was added during the 

hurricane flood of September, 1938." The stratigraphy 

of the terraces described by Jahns consists of channel 

and overbank deposits, although he uses different terms 

for these features. He (p. 49) identified overbank deposits in these sections primarily by their stratification 

and finer texture.

The thicknesses of channel and overbank deposits 

respectively, in the Connecticut Valley appear in the 

ratio of about 2 to 1; this is a somewhat larger proportion of material derived from overbank flows than we 

have found elsewhere. The difficulty of distinguishing 

point-bar from overbank deposits may account, in 

part at least, for the difference.

Happ, Rittenhouse, and Dobson (1940) and many 

others have noted large amounts of overbank deposition from individual floods. Occurrences of thick local 

deposits, even as coarse as gravel, are not uncommon. 

For example, Harrod and Suter (1881, p. 136) reported 

that in the Missouri River flood of 1881 the "immediate 

banks were raised for long distances from 4 to 6 feet" 

between Sioux City and Glasgow, Iowa. Although 

they also noted deposits from 6 to 12 feet in width 

within the channel in areas which were "sheltered from 

the fierce current," even this report makes no mention 

of widespread deposits sufficient to raise the general 

level of the flood plain.

Obviously, it is difficult to estimate accurately the 

thickness of sediment deposited over large areas. Although there are examples of thick local deposits from 

overbank flows, there are also contrasting examples of 

local scour (see Davis and Carlson, 1952, p. 232; Breeding and Montgomery, 1954, p. 6). Observations in 

Connecticut following the disastrous record-breaking 

flood of August, 1955 indicate that deposition was 

extremely irregular and cannot be considered as if it 

were uniform over the entire area flooded (Wolman and

Eiler, 1957). The local nature of thick deposits, the 

large variation in thickness of deposits even within a 

small area, and the occurrence of areas of scour suggest 

that widespread deposition of sediment by major floods 

is not so well established a fact as a glance at table 5 

might imply. These observations suggest that, compared with the overall volume of the flood plain, increments added by overbank deposition are on the average 

quite small.

An economic study of effects of a great flood add 

collateral evidence on the magnitude of deposition by 

overbank flow. In a study of the flood of 1951 on the 

Kansas River, it was found that, in general, damage 

resulting from deposition on agricultural lands during 

floods was extremely low (Wolman, Howson, and 

Veatch, 1952). Locally, overbank deposits may be 

significant, but such occurrences appear to be anomalies 

rather than usual.

If successive increments of overbank deposition were 

responsible for building a flood plain, we might expect 

to find minute laminations representing these deposits. 

The data in table 5 indicate that these laminations 

would be so fine that they could hardly be found, much 

less sampled. Although we have observed short 

stringers and very thin layers of sand, or even pebbles, 

in exposures of the flood plain, they have little or no 

lateral extent. They are certainly lenses rather than 

extensive overbank layers.

Because material is distributed in the vertical of a 

stream of flowing water according to size or settling 

velocity, one would expect the deposits in a flood plain 

to be successively finer from bottom to top. Examples 

from Brandywine Creek (Wolman, 1955, p. 18), Watts 

Branch, Seneca Creek, and elsewhere indicate that such 

a gradation in size is virtually impossible to find in an 

individual section. Available data show primarily 

that flood plains may include materials of quite different 

sizes. Figure 63 gives examples of flood deposits and 

flood-plain sediments from several regions and shows 

that the greatest diversity in size is in the basin for 

which most data are available, the Connecticut River 

basin. Here the major differences in size are due principally to differences in source material within the basin.

NATURAL LEVEES

The report of Harrod and Suter on deposition along 

the "immediate banks" leads to a consideration of the 

importance of natural levees in the formation of flood 

plains. Many observers have pointed out that when 

the flow leaves the stream channel its velocity is 

checked, and as a result the stream is unable to carry 

its load and deposits material adjacent to the bank. 

Natural levees are prominent along the Mississippi 

and Sacramento Rivers where they are particularly
RIVER FLOOD PLAINS I SOME OBSERVATIONS ON THEIR FORMATION 99

Sand 

(>100 microns)

Very fine sand 

(50-100 microns)

Silt 

(<50 microns)

!;;;;;i:::::| Connecticut River flood deposits of September 

li^illllJil 1938 (Jahns, 1947) 

o Ohio Valley flood of 1937, composite of 12

analyses, coarser deposits (Mansfield, 1939,

in Jahns, 1947) 

• Ohio Valley flood of 1937, composite of 159

analyses, finer textured deposits (Mansfield,

1939, in Jahns, 1947)

9 Potomac River deposits, Maryland and Virginia, composite of 4 analyses, (Mansfield,

1939, in Jahns, 1947) 

o Brandywine Creek, Pennsylvania,- flood-plain

materials (Wolman, 1955) 

X Kansas River, 1951 flood deposit on flood

plain (Carlson and Runnels, 1952) 

y Kansas River flood-plain surface (Carlson

and Runnels, 1952) 

j Seneca Creek, Maryland flood plain,

composite of 4 analyses 

^ 0 Watts Branch, Maryland flood plain, 2

composites of 4 analyses

FIGUKE 63.—Size distribution of flood deposits and flood-plain sediments (original 

diagram from Jahns, 1947).

well formed along the concave bank at channel bends. 

Natural levees occur along the Nile River, and are 

conspicuous on cross profiles of many rivers on the 

Gangetic plain in northeast India. Hilgard O'Reilly 

Sternberg (1955, personal communication) noted that 

many large alluvial islands in the Amazon River are 

rimmed by natural levees. Water overflowing the 

island during flood is held in the basin formed by the 

surrounding levee when the flood recedes. As a result, 

the center of such alluvial islands is a lake during 

periods of low flow.

Along the rivers of small and moderate size which 

exist in Maryland, Virginia, and Pennsylvania, we 

have found few recognizable levees. (See Wolman, 

1955, figure 16, p. 16 for examples of cross sections of

typical flood plains.) Minor ridges occasionally parallel the stream but not infrequently these are the 

result of cultivation, or more accurately, the absence 

of cultivation. Farmers may plow close to a stream 

but leave 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 been 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 related to man's activities has much to do with their 

formation. Such a possibility is supported by the 

existence of prominent natural levees on Little Falls 

Branch and on parts of Rock Creek near Washington, 

D. C. The levees on these small streams (drainage 

areas 4-50 square miles) stand in marked contrast 

to the absence of such levees on most of the streams in 

the region. Both of these small creeks drain suburban 

areas where municipal construction has altered the 

basin characteristics.

Little Falls Branch not only has natural levees but 

differs in another way from nearby basins less affected 

by man's activities. It appears to have experienced 

a change in frequency of overbank flow. A competent 

observer, W. W. Rubey (personal communication) has 

lived for more than 15 years in sight of this creek and 

states that the frequency of overbank flooding has 

increased from about once a year to at least 10 times 

a year during the period of his observation. This 

increase in frequency can probably be attributed to 

street construction, paving, and home construction on 

the watershed. It is logical to suppose that open cuts 

and unpaved streets during the boom in suburban 

construction have greatly increased the amount of 

sediment carried b\ the stream during the past decade. 

The combination of increased sediment supply and 

frequency of overbank flow may account for the prominent natural levees there in contrast to absence of 

levees on other streams in the area.

These illustrations are in accord with Malott's 

(1928, p. 27) observations that the increase in height 

of the flood plain adjacent to channels is "* * * 

usually small and noticeable chiefly during low flood 

stages when this area is the last to be submerged." 

Although the natural levee is a feature which has 

received considerable mention in the literature as a 

type of overbank deposit, it appears to constitute a 

relatively small proportion of the normal flood plain.
100 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS

HYPOTHETICAL CONSTRUCTION OF A FLOOD PLAIN BY 

OVERBANK DEPOSITION

If a specific thickness of material were deposited on 

the flood plain every time a river overflowed its banks, 

it would be possible to compute the rate of rise of the 

flood-plain surface by the use of flood frequency data. 

Figure 64 is a plot of hypothetical flood-plain elevation 

against time on Brandywine Creek at Chadds Ford, 

Pa. It was constructed in the following way: The 

average number of days per year on which a given 

stage is equaled or exceeded was computed from the 

gaging station records. Assuming that each time the 

stream overflows a given level it deposits a specific 

thickness of material, the time required for the surface 

of a flood plain to reach a given elevation can be computed. In this example it was assumed that each 

increment consisted of a layer of sediment 0.005 foot 

thick (see table 5 for reasonableness of this assumption).

The most conspicuous feature in figure 64 is the 

rapid increase in elevation in the first 10 years. Although the rate decreases somewhat as the figure 

shows, in 170 years it would have been possible to 

construct a flood plain to its present height at Chadds 

Ford. From 80 to 90 percent of this construction 

would have taken place in the first 50 years.

1000 

TIME, IN YEARS

FIGUEE 64.—Hypothetical formation and rate of increase in flood-plain elevation by 

overbank deposition. Graph shows increase in elevation with time on Brandywine Creek at Chadds Ford, Pa.

Not far from Chadds Ford is Buck Run, a tributary 

of Brandywine Creek, near Coatesville, Pa. In the 

bank of this tributary channel, a log was found at the 

base of a flood plain. The log lay just below the low 

water level, 0.5 foot above the bed of the present 

stream (see Wolman, 1955, p. 18, fig. 20). Bedrock 

lies approximately 3 to 5 feet below the log and crops 

out in the channel about 50 feet downstream. A 

carbon-14 analysis dated the log as approximately 

1,450 ±200 years old (Rubin and Suess, 1955, p. 487). 

The vertical section above the log comprises only 3.1

feet of deposition. The absence of any discernible 

stratigraphic break in the section allows the possibility 

that the base of the section is as old as the log. If the 

log has been in place during this period, then the elevation of the surface of the flood plain has risen so little 

that it must be considered, in effect, stable.

Assuming that the flood plain at Chadds Ford is 

about 1,450 years old, we should expect from figure 64 

that the present flood plain would actually be 4 feet 

above its present position. The fact that it is not 

suggests that overbank deposition is not the primary 

mechanism by which a flood plain is formed. Further, 

if overbank deposition is not a major factor in raising 

the elevation of the surface of the flood plain, it follows 

that in a normal channel the bed will not become progressively farther below its own flood plain as a result 

of continued overbank deposition.

The data presented thus far indicate that despite 

frequent flooding, the elevation of the surface of a 

flood plain remains stable relative to the level of the 

channel bed. In the following section we shall consider some possible explanations for the absence of 

continued increase in elevation.

CONDITIONS AFFECTING AMOUNTS OF OVERBANK 

DEPOSITION

Three lines of evidence may help to explain the 

relative unimportance of overbank deposition in floodplain formation which we postulate. First, periodic 

removal of the flood plain by lateral erosion helps to 

control its height. Second, the highest discharges are 

often characterized by lower concentrations of suspended sediment than discharges of intermediate sizes. 

Third, velocities of water on the flood plain during 

periods of overbank flow may be high and competent 

to move sediment of small and medium size.

The extent to which periodic removal and replacement is effective in limiting the height of the flood-plain 

surface depends on the relative rates of lateral swinging 

and overbank accretion. The available data do not 

provide satisfactory comparison of these rates. Some 

rough relations, however, can be inferred from data 

presented earlier in this paper.

Table 4 indicates that it is likely that specific areas 

of any flood plain may not be eroded by the stream for 

periods as long as 200 or more years. This figure 

would be even greater if one considered the fact that 

the movement of the channel across the valley floor 

more often than not consists of many reversals of direction rather than continuous movement from one side of 

the valley to the other. Those areas of the flood plain 

which have not been reached by the river channel for a 

long time should presumably have higher elevations as
RIVER FLOOD PLAINS I SOME OBSERVATIONS ON THEIR FORMATION 101

a result of continued overbank deposition than the more 

recently constructed areas of the flood plain. Figure 

64 suggests that in 200 to 400 years one could expect 

1 to 2 feet of deposition above present flood-plain level 

as a result of overbank flows. The relief on any flood 

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 

elevation of the flood plain is partially controlled by 

lateral migration, additional factors also control the 

amount of overbank deposition.

Contrary to some expectations, high discharges are 

often associated with lower concentrations of suspended 

load than are more moderate floods. The curves of 

sediment concentration and discharge presented in 

figure 65 show, for example, that in several Kansas 

streams concentration reaches a maximum and declines 

while discharge is continuing to increase. In addition, 

during individual floods the peak of the sediment concentration often precedes or follows the peak discharge.

It has been suggested that the decrease in concentration may result from deposition on the flood plain as a 

stream goes out of its banks. In the South Fork 

Solomon River at Alton, Kans., 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 deposition on the flood 

plain. If we consider the fact that the load is primarily 

a function of the physiographic and climatic conditions 

in the contributing drainage area and, although correlated with discharge, is not a function of the discharge 

itself, it is by no means necessary that the highest concentrations be associated with the highest flows. The 

fact that high flows may not be associated with the 

highest sediment concentration may help to account 

for a paucity of deposition during these periods.

Another possible mechanism contributing to small 

amounts of overbank deposition relates to the ability 

of overbank flows to transport material across the flood 

plain. There are many streams transporting material 

having a moderate range of sizes that do not have 

natural levees, nor do they display a major amount of 

overbankod position. Only the finer particles are carried near the water surface or in the upper part of the 

flow, and these sizes are not likely to drop out along the 

edge of the channel in accord with the simple naturallevee explanation, nor need they be deposited to any 

great extent on the surface of the flood plain. Water 

which leaves the channel and flows over the flood plain 

tends to move directly downvalley rather than to 

follow the longer distance along the more sinuous

TABLE 6.—Velocity and depth of flow in overbank sections of river

flood plains

[Data from current-meter discharge measurements by U. S. Geological Survey] 

Approximate Total flood Mean Mean veloedrainage discharge Depth of velocity ity of overarea (square (cubic feet overflow (feet per flow l(feet 

miles) Date per second) (feet) second) per second)

1.2

OSAGE RIVER NEAR ST.

14,500 July 17,1951 116,590

THOMAS,

6.7

3.0

7.0

4.8

4.8

4.7

MO.

1.5

.5

2.2

.8

2.1

.2

MISSOURI RIVER AT JEFFERSON CITY, MO

507,525 July 18,1951 519,800

MERAMEC RIVER NEAR

3,788 June 11,1945 114,000

5.0

8.5

7.7

7.5

6.0

10.3

7.0

EUREKA,

4.0

6.0

6.0

8.0

5.0

3.0

1.0 '

5.4

2.8

2.0

3.4

2.0

4.7 , MO.

.9

2.4

.2

1.5

2.4

3.2

CHIPPEWA RIVER NEAR NELSON, WIS.

5, 600 Mar. 25,1935 58,200 5.0-7. 0 1. 3-3.4

Mar. 30,1935 ....... — . 2.3-4.0 1.1-1.9

Apr. 1,1935 ..... — ... 1.6-2.3 0.3-0.7

MISSISSIPPI RIVER AT PRESCOTT, WIS.

44,800 Apr. 14,1939 30,800 8.4 1.1

7.5 

10.5 

WHITEWATER RIVER AT BEAVER,

288 Mar. 30,1940

Mar. 25,1943

Jan. 6,1946

1,530

1,930

2,470

1.2

1.3

1.7 

1.2

.7

.5

MINN.

1.1

1.3

2.0

1.7

.04

1.2

.5

.11

1.96

ROOT RIVER NEAR HOUSTON, MINN. 

1,300 Apr. 1,1952 29,970 3.3 2.6

5.3 3.4 

7.8 3. 2 

9. 2 3.9 

SENECA CREEK NEAR DAWSONVILLE, MD.

101 Sept. 1,1952 3,300 .48 

.62 

.77 

1.68 

1.71 

2.44 

2.30 

1.80 

NEV

.10 

.12 

.16 

3.01 

3.43 

HUMBOLDT RIVER NEAR IMLAY, NEV.

1.69 

2.14 

1.56 

2.02 

1.13

1 Discharge in overflow divided by cross-sectional area of overflow.

1.4

1.5 

1.7 

1.6 

2.0 

2.2 

2.0 

1.2 

HUMBOLDT RIVER NEAR ROSE CREEK.

13,000 May 9,1952 5,790 .8

1.5 

2.6 

4.3 

5.5

13,500 May 10,1952 5, 900 3.2 

1. 7 

1.8 

2. 2 

2. 8

2.4

1.6

1.5

1.5
102 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OP RIVERS

10,000

• • • -. •• ^ • •• *•• • • .»••_ *••

1000

10,000

1000

SOUTH FORK SOLOMON RIVER 

ALTON, KANS.

10,000

1000

50 100 500 1000 

DISCHARGE, IN CUBIC FEET PER SECOND

5000

FIGURE 65.—Relation of sediment concentration to discharge in several rivers in Kansas.
RIVER FLOOD PLAINS I SOME OBSERVATIONS ON THEIR FORMATION 103

path of the channel. The downvalley slope may be 

considerably greater than that of the channel, and this 

higher gradient tends to keep the velocity high and 

reduces the probability of deposition.

Some data on the conditions of overbank flow are 

available in measurements of velocity made by the 

Geological Survey. Although such measurements on 

truly natural, unobstructed flood plains are difficult to 

find, table 6 shows measured depths and velocities from 

10 overbank sections. Some of the observed velocities 

are rather high, even as great as mean velocities observed in natural channels themselves. These results 

are plausible if one considers that the slope may be 

greater and the roughness of the flood-plain section may 

be less than the roughness of the main channel. These 

observed velocities exceed the velocities competent to 

transport silts and sands (Inman, 1949). Because silt 

and sand are the predominant sizes carried in the overbank section where such velocities occur, it is perhaps 

to be expected that much of the material can be carried 

downvalley and not deposited on the flood plain.

The data from the Humboldt River in table 6 represent velocities measured in overflow sections which are 

former channels of the river (M. T. Wilson, personal 

communication). As in the main channel, depending 

upon the depth and proximity to the banks, great variation is to be expected in the velocity. In a flood plain 

constructed by lateral migration accompanied by 

abandonment of old channels, the remnants of these 

channels may serve at high flows as avenues of transport or as places for deposition, depending upon their 

orientation in relation to the direction of overbank 

flows. On many streams where flood plains are several 

hundred to several thousand feet wide, channels are 

often found at the margin of the flood plain adjacent 

to the valley wall. These may represent courses of 

former channels but their position suggests that they 

are probably drainage channels on the surface of the 

flood plain created and maintained by overbank flows. 

Only where oxbows or distinct relics of a channel pattern exist elsewhere on the flood plain, is it possible to 

distinguish between remnants of abandoned channels 

and surficial drainage channels produced by floods.

It has been pointed out to us that conditions on the 

present flood plain may not be representative of the 

period of flood-plain formation. In particular, the 

removal of trees from flood plains in the East and in the 

Middle West may have markedly reduced the likelihood 

of deposition (H. C. Bolon, 1954, personal communication). In some areas, however, including many in the

flood-frequency study previously mentioned, there has 

probably been little or no change in the plant cover and 

there is no apparent difference between these and the 

other flood plains studied.

In summary, we suggest that one possible reason for 

the general lack of deposition by overbank flow may be 

the ability of the overbank section itself to transport 

sediment.

FLOOD PLAINS IN STABLE, AGGRADING, AND DEGRADING STREAMS

Thus far the "typical" flood plain considered here is 

one in which the relative position of the bed of the channel to the surface of the flood plain has remained stable 

during the formation of the flood plain. The relatively 

constant frequency of overbank flooding described 

earlier could apply even if the entire valley were being 

aggraded at a constant rate, with channel bed and flood 

plain rising uniformly. In such an instance there would 

be the usual difficulty in differentiating overbank from 

point-bar deposits, and there would be added the difficulty of differentiating deposits of different ages in the 

aggrading sections. The difficulty is illustrated in the 

following analysis of several flood plains in North and 

South Carolina and in Georgia. The description of 

these flood plains is based on numerous borings (P. K. 

Theobald, Jr., and W. C. Overstreet, 1954-55, personal 

communication), and examples are presented in crosssection form in figure 66.

The flood plains rest upon bedrock and have surfaces 

which are overtopped by the annual flood approximately once each year or once every 2 years (table 1). 

They are composed of silt, sand, and clay in an infinite 

variety of combinations. These are commonly underlain by pebble and cobble gravels, as figure 66 illustrates. From the stratigraphy alone it is impossible to 

tell how much of this material is overbank and how 

much is point bar. A log found within a similar floodplain section in North Carolina is 2,370 ±200 years old, 

according to a carbon-14 determination.

At first glance it might seem as if these sections are 

actually aggraded ones rather than stable, alluvial 

sequences. The following analysis, however, indicates 

first, that the distinction between the two is not easily 

made, and second, that the flood plains in figure 66 may 

well represent a single stable deposit.

Figure 67A is a graph of the depth of fill in valleys 

in the North and South Carolina region, plotted against 

the length of the valley from its headwaters to the 

given cross section. Although there is considerable
104 PHYSIOGRAPHIC AND HYDRAULIC STUDIES OP RIVERS

100 Feet BUCK CREEK

500 Feet

L 0

13

SECOND BROAD RIVER

50 Feet

SOUTH TYGER RIVER

500 Feet

NORTH TYGER RIVER

%^%%%^^%^

500 Feet

EXPLANATION

Bedrock

Silt and clay

Sand

o 

Gravel

FiouRE1 66.—Cross sections of river flood plains in North Carolina and South

Carolina.

scatter on this graph, the average relation can be 

described by the equation

mean depth of filloclength-45 (1) 

That is, the depth of fill is proportional to the .45 power 

of the valley length. Figure 67J5 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 

average discharge, as well as the depth of fill, is proportional to the valley length. This relation is expressed by the equation

mean depth of flow oc length-64 (2) 

The progressive increase downstream of both the depth 

of fill and the mean depth of flow suggests that the fill

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 

scour and depth above mean bed level) are attained 

during floods which recur on the average, once each 5 

to 8 years. Lacey's studies (Inglis, 1949, p. 327) of 

irrigation canals indicate that the maximum depth of 

scour in a regime or equilibrium channel may be on the 

order of 1.75 to 2 times the regime depth. Such scour 

is associated with a dominant flow in the regime channel 

which probably is somewhat lower than a flood discharge having a recurrence interval of about 5 to 8 

years. Such flows, however, do not represent extremely 

unusual conditions. Assuming that conditions in the 

regime channel are akin to those in natural channels, it 

is reasonable to suppose that depths of scour IK to 2 

times the depths attained during these flows are both 

reasonable and possible. These conditions make it 

seem probable that the Carolina streams represented in 

figures 66 and 67 actually scour to bedrock at infrequent 

intervals. Even if they do so only once every 50 years, 

this still allows for many scourings during the period of 

2,300 years available for formation of flood plains in 

this area.

O iu

5£

Z Ul

g z <n ?

Tl I

„>*:

MM

-tf*5

t=

B —

1 5 10 50 100 500 

DISTANCE FROM HEADWATER, IN MILES

FIGURE 67.—Relation of distance from headwater divide to mean depth of floodplain alluvium and to mean depth of flow at average discharge at selected localities 

in North Carolina and South Carolina.
RIVER FLOOD PLAINS: SOME OBSERVATIONS ON THEIR FORMATION 105

Evidence of this scour is also provided by observations of the flood of 1903(?), which occurred on the 

Pacolet River in North Carolina (P. K. Theobald, Jr., 

1954-55, personal communication). Observers reported that "the entire alluvial fill was removed to bedrock" by this flood. Allowing for possible exaggerations, it is probably safe to say that scour to bedrock 

occurred even if "the entire alluvial fill" was not removed. Because the bedrock is occasionally reached by 

the flowing water, one must suppose that slow degradation of the valley is taking place; but the rate of degradation is extremely slow and, in relation to flood-plain 

formation, the elevation of the river is in effect stable. 

Both the frequency of flooding and the calculated 

potentialities for scour imply that these flood plains are 

being continuously constructed and reconstructed by 

the streams which now flow within them in accord with 

the processes described earlier.

Although the active flood plain associated with the 

present regimen of the river has been emphasized here, 

it has already been pointed out that flood plains can 

exist in aggrading channels. During the process of 

aggradation the discharge regimen of a stream may be 

maintained; that is, floods occur and the stream as a 

rule experiences a certain range in discharge. During 

the process of aggradation both the bed of the channel 

and the surface of the flood plain continue to rise. 

Although the channel may be choked with material at 

times (Happ, 1950), there is no reason to believe that 

a flood plain will not continue to be constructed during 

the rise.

In those cases where continual aggradation produced 

the valley fill, it is difficult to explain how the relative 

position of the channel to the flood plain remained fixed 

during aggradation if overbank deposition is considered 

the principal mechanism of laying down the valley fill. 

Rather, concomitant rise of both stream bed and 

flood-plain surface appears to be best explained by 

attributing the bulk of the deposited material to the 

process of point bar formation.

The uniform frequency of flooding of flood plains does 

not rule out the possibility that both the surface of the 

flood plain and the bed of the channel are being built 

simultaneously. Gages on the Nile River, which provide the longest periods of record of any river in the 

world, indicate that both the bed and the flood plain of 

the Nile are being raised at a rate of about 3 to 4 feet in 

1,000 years (Lyons, 1906, p. 313-317). Such rates are 

unmeasurable in the brief periods of record with which 

we are dealing. The 1,500 to 2,300-year-old logs beneath the flood plain in Pennsylvania and North Carolina, respectively, provide the only fairly good evidence 

of the age of the modern flood plain. These dates are in 

general in accord with dates derived from terrace

sequences found in the West (Leopold and Miller, 1954).

It must be admitted that at this time the stability of 

the absolute elevation of the surfaces of most flood 

plains cannot be proven. The evidence demonstrates, 

however, that even during aggradation the difference in 

elevation between the river bed and the surface of its 

flood plain does in many instances remain constant 

over long periods of time. That the same may be true 

during periods of rapid degradation as well as the slow 

degradation previously mentioned (p. 33) is indicated 

by observations of the Ukak River in Alaska. In 1912 

volcanic ash from the eruption of Katmai filled the river 

valley. In the 40-year period since the eruption, the 

river has cut down 10 to 40 feet and is continuing to do 

so (H. Coulter, 1955, personal communication). At 

the same time the Ukak has and is continuing to construct a flood plain. These observations of a variety of 

rivers indicate that a channel may have a flood plain 

associated with it when it is stable and flowing on bedrock, gradually eroding a valley, or gradually depositing 

a fill.

When aggraded valley fill or any flood plain is incised 

after its formation, the former flood plain becomes a 

terrace. An alluvial terrace is an abandoned flood 

plain whose surface no longer bears the normal relation 

to the stream bed. This study indicates that the 

normal relation between the surface of the active flood 

plain and the stream bed on many streams can be defined by the frequency of flooding. Where such is the 

case a flood plain becomes a terrace when the channel 

incises itself to the point where the former active flood 

plaiii is no longer overtopped by that annual flood, 

which on the average occurs less than once every 2 3Tears.

CHANNEL PATTERN AND FORMATION OF THE FLOOD

PLAIN

Point bars have been described in the literature 

primarily in relation to meandering streams. Many of 

the examples given above are from such streams. We 

have shown (Leopold and Wolman, 1957), however, 

that there are only small differences in the fundamental 

characteristics of so-called straight channels, meanders, 

and braids. Very few straight channels are actually 

straight. An inspection of their thalwegs reveals bars 

and pools analogous to those found in meanders. The 

wandering straight channel deposits material in the 

same way as does a meander and hence, is equally 

capable of forming a true flood plain even though the 

process of formation may be somewhat more difficult to 

recognize. Braided channels also build islands and bars 

to elevations correlative with the flood-plain surface. 

The New Fork River example (pi. 2B) shows one such 

bar. The data also show that such incipient flood 

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 summary, flood plains may be constructed by channels of 

any type or size.

CONCLUSION

This study supports the views of several authors 

that the flood plain is composed of channel deposits, 

or point bars, and some overbank deposits. The 

relative amounts of each vary, but on the average, the 

proportion of overbank material appears to be small. 

This conclusion is supported by the uniform frequency 

of flooding and by the small amount of deposition observed in great floods. Lateral migration, relatively 

high velocities which can occur in overbank flows, and 

the decrease in sediment concentration at high flows 

contribute to this result.

In many instances it is difficult to differentiate in 

stratigraphic section alone the two fundamental types 

of deposits. Point bars are extremely heterogeneous, 

and where overbank deposition does take place the 

small amount of material deposited and its irregular 

distribution usually make it difficult to identify.

Frequency studies indicate that many flood plains 

are subject to flooding at approximately yearly intervals. 

These studies, as well as stratigraphic observations, 

indicate that the flood plain is also related to the present 

regimen of the stream flowing within it.

If neither natural nor man-induced changes take 

place in the structural, climatic, or physiographic conditions which control the regimen of a natural channel, 

the channel will not form terraces by gradually building 

up its own flood plain until flooding no longer occurs. 

The flood plain can only be transformed into a terrace 

by some tectonic, climatic, or man-induced change 

which alters the regimen of the river, causing it to 

intrench itself below its established bed and associated 

flood plain.

Lateral migration of a stream across its flood plain 

takes place with almost no change in channel width. 

The volume of material deposited tends to be about 

equal to the volume eroded. Material eroded from the 

drainage basin is only temporarily stored in the flood 

plain. Only when the stream erodes laterally into 

terraces or hillsides higher than the flood plain does the 

volume eroded exceed the volume deposited. Only in 

this case can stream-bank protection be expected to 

reduce the total sediment yield from a drainage basin.

Although we recognize the conclusions presented here 

require much additional supporting data, we give them 

forthrightly in the hope that they will stimulate further observation and study.

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RIVER FLOOD PLAINS! SOME OBSERVATIONS ON THEIR FORMATION 107

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INDEX

Page 

Acknowledgments_..___.-.-._._____..---.__.--______ 87

Age of flood-plain deposits___._____________...___ 100,103,104,105 

Aggrading streams, flood plains in.__________..._________ 103,105 

Annual flood, defined-.—.____________________________ 88

Bank erosion, as source of sediment_______......___..____ 106

rate measured...-...-...-.___.____._____________... 96

Bankful stage, recurrence interval..._.._____.._____________ 88-89 

Brandywine Creek, Chadds Ford, Pa.__._______._____._____ 100 

Braided ^channels, flood plains on........_____...._______ 95,105, pi. 2B

Buck Run, Coatesville, Pa..-..---...-...__........_..................... 100

Channel patterns, relation of flood plains to..__.........__..._...._ 105-106

Conclusions..___-...._._.._____...____.__________ 106

Connecticut Valley, flood-plain deposits______._______________ 98,99

Degrading streams, flood plains in.___...--___....._____...... 105

Depth of fill, variation with valley length_____-.-_._________. 104

Depth of flow, in overbank sections...__.————...__________ 101 (table)

variation with valley length at average discharge.._........__.._. 104

Equilibrium channels, scour and depth of flow...._____________.. 104

Elkhorn River, Nebr., flood frequency ________....._...______ 89

Erosion and deposition in bends, measured______......_______.. 96

Flood, annual, defined_..._______________________ 88

Flood, defined-________________. _______.__.______ 89

Flood flow. See Overbank flow.

Flood plains, cross sections in North and South Carolina...__________ 104 

defined.._________________.______.__._______ 87 

formation...________________________.___________ 91-103

Flood-damage stage, defined-___________________________ 89 

frequency distribution table.________________________ 89 

relation to height of flood plain...__...__________...__._ 89-91 

table showing differences from natural flood-plain elevations______.. 91

Flow, relation of sediment load to rate of....__.______.._.___ 101,102 

velocity and depth in overbank sections.._________________ 101

Jahns, R. H., quoted..—_____________________________ 98,99

Lateral accretion deposits, amount in flood plains__.____________ 96 

defined....................——......——........................__.. 91

See Point-bar deposits;

Page 

Lateral migration of channels, rate. --------------—...... —..--—— 96,97 (table)

Little Falls Branch, near Washington, D. C-—.........__...-.-........-- 99

Little Pipe Creek, Avondale, Md—----———-„.--—--—.......—.... 95

Mississippi River, at Arkansas City, cioss section showing flood-damage stage. 90

at Greenville bridge, Miss., cross section showing flood-damage stage..— 90

at Memphis, cross section showing flood-damage stage—————————— 90

at Natchez, cross section showing flood-damage stage...——————_— 90

at Vicksburg, cross section showing flood-damage stage... — -. —....—— 90

Natural levees, effect on flood and flood-damage stages...--..--....-..__... 89 

formation and distribution__---------- ——— ---------- — -.---.....—— 98-99

New Fork River, PInedale, Wyo., flat-topped gravel bar.... ——— —— —— pi. 2B

Overbank and point-bar deposits, distinguishing-—....——. ——. —....—— 91

Overbank deposition, construction of flood plains by_—-... — -.--...——— 100 

nature---.--..------..--—--..---————.-—--—--——------—— 97-98

Overbank deposits, amount......—————————————————————— 96.97-98

conditions affecting amounts.—... —.————.. — -..————————... 100-103 

natural levees.__.__..-________.__.___———.—————.—— 98-99 

size-distribution diagram.---------—._ — — --—.-- — — ———————.... 99

Overbank flow, frequency...___..-.__._..__-... —— ———...__— 88-91 

transport ability— — ———.——..— —————————..— 101, 103

Point-bar and overbank deposits, distinguishing___....____.___— 91

Point-bar deposits, deposition and sediments._-...---.—-—.-— ——.-.— 91-92 

distribution of materials in....———————.——————————————— 94

elevation..--.—_————— —— ———————————— - 92-93,95-96

Popo Agie River, Hudson, Wyo______________-———————————— 95

Sacramento River, at Red Bluff, Calif., cross section showing flood-damage

stage.......——.——....——„——— —— „..——....—..._—— 90

Sediment load, time of maximum concentration...._—————.........— 101

Seneca Creek, Prathertown, Md., views of elevations.. ————...._...... pi. 2A

Stable streams, flood plains in.....————————— — ——— — ——— 103-105

Stream length, relation to mean depths of fill and flow...........—.......— 104

Velocity of flow in overbank sections.------- —...——————.. 101 (table), 103

Watts Branch, Rockville, Md., point bar in.....__——...——...— 92-94, piYellowstone River, flood frequency———.--. ——.—————————————— 89

109

•;VU.S. GOVERNMENT PRINTING OFFICE: 1967 O—256-926