)jr ^r

Botanical Evidence of 

Floods and Flood-Plain 

Deposition

GEOLOGICAL SURVEY PROFESSIONAL PAPER 485-A

£>.
Botanical Evidence of 

Floods and Flood-Plain 

Deposition

By ROBERT S. SIGAFOOS

VEGETATION AND HYDROLOGIC PHENOMENA

GEOLOGICAL SURVEY PROFESSIONAL PAPER 485-A

A description of botanical methods used to determine 

dates and occurrences of floods and deposition of 

sediment on modern flood plains

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

The U.S. Geological Survey Library has cataloged this publication as follows :

Sigaf oos, Robert Sumner, 1920-

Botanical evidence of floods and flood-plain deposition. 

Washington, U.S. Govt. Print. Off., 1964.

vi, 35 p. illus., map, diagrs. 30 cm. (U.S. Geological Survey. 

Professional Paper 485-A.)

Vegetation and hydrologic phenomena. 

Bibliography: p. 34r-35.

1. Sedimentation and deposition. 2. Floods. I. Title. (Series)

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, B.C., 20402 - Price 35 cents
CONTENTS

Glossary.._ ________________________________________

Abstract.__________________________________________

Introduction. ______________________________________

Acknowledgments.. _ _--________-_-_-_____-___-__

Mechanism of plant growth._____________________

Hydrologic concepts___________________________

Collection of streamflow records._____________

Comparison of floods._______________________

Floodstage-discharge realtionship_____________

Effects of floods upon trees.._________________________

Physical effects of high velocity and debris _________

Effects of inundation upon trees._________________

Growth of trees after flood damage_______-____________

Circumferential growth around trunk scars.________

Growth of sprouts from stumps and water-felled treesGrowth of trees following burial by alluvium during 

floods _______________________________________

Analysis of structure of wood from buried trunk_

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Growth of trees after flood damage Continued

Evidence of floods and deposition in trees along small

streams___ __________________-___--__---_--_ A23

Evidence of sequential deposition and erosion on the

flood plain_________--------_-----_-_--------- 24

Botanical phenomena relating to the formation of the

flood plain._____-_____--_--_--_------------------ 26

Relationship between flood-plain trees and low flow. 27 

Significance of old trees on banks.____-__---_--___ 29

Botanical evidence of current processes affecting the

flood plain__-_-__--_---------_--------------- 31

Economic significance of botanical evidence of floods.-__ 32 

Value of botanical evidence of floods and flood-plain

deposition...-- ___._._-___------_--_--_--_--- 32

List of plants.--.---------------------------------- 34

References cited----------------------------------- 34

ILLUSTRATIONS

FIGURE 1. Map of Potomac River near Washington, D.C_ ______________________---____--_--_---------------------

2. Sprouts that strated to grow from ash stem after flood damaged first new growth-___________--__-_---------

3. View of flood-plain forest showing form of trees___-_-_____-__________________-_-_---------------------

4. Floating ice overriding flood plain scars trees________________________-_________-___-------------------

5. Ice jam on Potomac River, Washington, D.C., February 16, 1948_--___---__-----------------------------

6. Potomac River, low flow, Washington, B.C. July 1956_________-_____-______-_----___-------------------

7. Diagrams representing circumferential growth of trunk-_______--____-_________--_-_---------------------

8. Parts of a stem of an ash tree damaged by minor ice jam on January 15, 1958________________-__--_------

9. Fragments of ice deposited on flood plain of Potomac River on January 15, 1958.--------------------------

10. Cross sections through stem of damaged ash tree shown in figure 8___________-___-___-_-----_-----------

11. Potomac River in flood on August 20, 1955------__--_______-_-__-__----_-__-----_---------------------

12. Potomac River at low flow, June 1956_________________________________________________-__-------------

13. Sprouts from stump cut off by an ice jam on February 16, 1948- -________-___--------_-------------------

14. Vertical sprout growing from inclined trunk that was felled by 1948 ice jam.__________-____-_-_------_---

15. Trunk of tree suggesting origin as a sprout which started to grow in 1861____________--_--_-------------_

16. Ash sprouts of four ages growing from a stump______________________________________-__-_-__-_---------

17. Diagrams showing events that produced the form of the ash tree illustrated in figure 16_--_-_---------------

18. Diagram of tree bases postulated as result of different rates of deposition_________________-_---__----------

19. Ash tree part_y uprooted prior to growing season of 1936 and subsequently buried______________ _______

20. Inclined buried ash tree.____________________________________________________________________________

21. Drawing showing basal sprout, location of samples, and approximate level of flood plain prior to 1936-------

22. Photomicrograph of wood from buried trunk of ash tree.______________________-______--_----------------

23. Ratio of difference between mean of early wood and late wood vessel diameters to the mean of early wood vessel 

diameters. _ ______________________________________________________________________________________

24. Scar on channel side of river birch on Difficult Run, Va_______________________-------_-_----------------

25. Alder sprout growing from parent stem knocked over in 1956, Scott Run, Va________________-__--_--__----

26. Ash sprout growing from inclined trunk knocked over prior to 1928, Scott Run, Va___________-_____----_---

27. Ash from flood plain, Tripps Run, Va., showing flood-damage form.___________________---__-__--_--------

28. Drawing of buried part of ash tree in figure 19_ ______________________________________-_-----_--_-_-----

29. Ash tree knocked over by one flood, partly buried by another, and exposed by a third______-__-___------_-

30. Silver maple trees on bank flooded at least 60 times between 1931 and 1960________________-_____--------

31. Diagrams representing establishment of trees on flood plain__-_________-______-_-_-__-__-------_---------

32. Cut bank of Potomac River showing newly exposed roots and recently fallen trees.______-____-_------------

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GLOSSARY

Annual ring. A layer of wood resulting from one season's growth of a tree; appears as a ring when seen 

in a cross section of a tree trunk.

Apical dominance. The inhibiting effect of hormones produced in buds at the apex of a stem which, as 

long as this growing tip remains intact, prevents the development of buds farther down on the stem.

Bark. A general term for the tissues outside the vascular cambium in tree trunks and stems.

Callus. A tissue of thin-walled cells that grows over a scar.

Cambium. A layer of meristematic cells which by dividing forms new tissues. Vascular cambium divides, 

producing xylem toward inside and phloem toward outside beneath bark in woody plants. Cork cambium lies outside the phloem and produces layers of cork that constitute most of the outer bark.

Coniferous trees. Trees that produce naked seeds in cones, as opposed to fruits, and are typically needleleaved evergreens. Pine, hemlock, and Douglas-fir are examples.

Crest. The highest elevation of a flood.

Deciduous trees. Trees whose leaves turn brown and die or fall in the autumn. Baldcypress (Taxodium 

distichum) and larch (Larix laricina) are examples of deciduous coniferous trees.

Diffuse porous. Wood in which vessels are all about the same size across an annual ring, as in maple or 

birch.

Discharge. In this report, discharge refers to the amount of flow in a stream or river. It is measured in 

cubic feet per second (cfs). One cfs is equivalent to 7.48 gallons per second, about 646,000 gallons per 

day, or 28.32 liters per second.

Evergreen trees. Trees that retain green leaves all winter. Live oak (Quercus virginiana) and American 

holly (Ilex opaca] are examples of broad-leaved, evergreen trees. Most coniferous trees are evergreen.

Flood. Any flow of a stream or river that overtops the banks of the channel and spreads across the flood 

plain.

Flood damage. Generally considered to mean the economic loss resulting from floods. In this report, the 

physical effects of floods upon trees and other plants are also considered as flood damage.

Growth. The process of adding new cells or cellular material by the conversion of organic and inorganic 

materials.

Growth increment. The amount of new material added as a result of growth. One season's increment is 

the annual increment, which approaches the shape of a hollow cone in a tree trunk.

Hardwood trees. A general term, used mostly in the lumber industry, referring to broad-leaved deciduous 

trees. Wood of some hardwood trees is softer than that of some softwood trees.

Hormone. A substance, produced in a plant, that influences specific physiological processes and is often 

called a growth regulator.

Meristem. Living tissue in which new cells form by division and grow by the addition of synthesized protoplasm. Vascular cambium is a lateral meristem.

Parenchyma. Tissue composed of thin-walled, nearly spherical cells. Pith in the center of corn cobs is 

parenchyma.

Peak. The highest discharge attained during a flood. Crest and peak stage are synonomous. Peak pertains 

also to the maximum discharge.

Phloem. Food-conducting tissue of most plants; forms on outside of vascular cambium and constitutes most 

of inner bark.

Pith. Tissue in the center of stems composed of a mass of parenchyma cells.

Rays. A layer of cells extending radially in the wood and inner bark of tree trunks.

Ring-porous. Wood in which vessels in earlywood are larger than those in latewood, as in a hickory or oak.

Root. As used in this report, the underground parts of plants on which, as opposed to stems, there are no 

buds, leaves, or leaf scars. Most roots of woody plants do not have pith in the center.
VI GLOSSARY

Sedimentation. The complete series of processes that includes the removal of particles from bedrock, their

transportation, deposition, and consolidation as another rock. 

Softwood trees. A general term used mostly in lumber industry referring to coniferous trees. Wood of

some softwood trees is harder than that of some hardwood trees. 

Stage. The elevation of the surface of a body of water. 

Stem. Plant organ which has buds, leaves, or leaf scars. Most stems grow above ground, but some are

subterranean.

Streamflow. Flow of water, or discharge, in a natural channel. 

Tissue. Group of cells that have a similar structure or function or both. 

Trunk. A general term referring to a large, single, woody stem of a tree. 

Vessels. Specialized tissue in wood or xylem of plants, consisting of cells of different shape and size in

different species. Vessel cells are alined vertically, and end walls are absent or are perforated by holes

or slits. Water or sap flows in vessels. 

Xylem. Water-conducting tissue of most plants and supporting tissue (wood) of trees. Develops from

cells formed toward inside of cambium.
VEGETATION AND HYDROLOGIC PHENOMENA

BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION

BY EGBERT S. SIGAFOOS

ABSTRACT

Methods of identifying past floods have been developed from 

a study of the form and age of parts of trees growing on the 

flood plain of the Potomac River near Washington, D.C. The 

date of deposition of sediment can be learned from the study of 

the structure of wood in the buried part of t»ee trunks, and 

the approximate thickness of the deposit can be determined 

by measuring to the level of the original tree base. Anatomical characteristics of the wood of roots exposed by 

erosion of the banks and the flood plain provide data for 

ascertaining the year that the roots were first exposed. The 

methodsi can be used to determine the occurrence of floods and 

flood-plain deposition on streams draining areas at least as 

small as 4 square miles.

The establishment and maturation of trees on a flood plain 

are the result of an interrelated sequence in the timing of seed 

dissemination and germination, suitable environmental conditions, and the flow regime of the river. The channelward limit 

of perennial woody plants tree species in the humid Eastern 

United States represents either the level of the maximum 

discharge during extended periods of low flow or the edge of 

the channel that is encroaching on a flood plain as the result of 

lateral corrasion. This line, separating the flood-plain forest 

from the channel, can be regarded as the bank of the river or the 

channelward limit of the flood plain, because it is related to the 

complete range of river discharges and can be readily seen and 

mapped.

Erosion and deposition on densely tree-covered surfaces away 

from the bank are characteristic processes and thus are effective 

in the formation and modification of flood plains in the humid 

Eastern United States. Although evidence of lateral corrosion 

and accretion has been found along the Potomac River, the 

presence of many old trees on the vertical face of banks 

indicates that some banks have been stable for more than 100 

years. The botanical evidence shows that on forested flood 

plains, certainly on those of rivers which are moving laterally 

only slowly, alluvium is interchanged rapidly by local erosion 

during overbank flows and by subsequent deposition either 

during the same flood or during following floods. This process 

of vertical trading of flood-plain sediments without intervention of channel movement may affect large amounts of 

material.

In the absence of streamflow data, engineers must rely on 

indirect methods of analysis to determine the magnitude and 

frequency of floods. Like most hydrologic data, these analyses 

are strange to many who are directly concerned with floodplain phenomena. Evidence of floods and flood-plain deposition 

as seen in trees is easy to comprehend and presents an

irrefutable record of past floods. The methods can be used, 

therefore, along reaches for which no discharge records exist. 

Because the methods have proved to be valid on small streams 

for which streamflow data are inadequate or nonexistent, they 

can be of help in planning flood-plain use in expanding cities. 

The age and form of flood-damaged trees may provide the date 

of the minimum stage of a flood that occurred prior to the period of record. The botanical phenomena will, at least, indicate 

the year in which a severe flood occurred, thereby providing 

valuable assistance in a further search for other kinds of 

evidence.

INTRODUCTION

The growth and form of trees along rivers permit 

the dating of flood occurrences in the last 100 years 

and the dating of deposition of sediment on flood 

plains. Floods scar the bark, prune the tops and 

branches, or knock over or destroy most of the trees on 

the flood plain. Most trees, however, are not killed; 

new wood and bark grow over the scars, and sprouts 

grow from decapitated branches and inclined trunks. 

The trees continue to grow. The number of annual 

rings that have grown since scarring of the bark or the 

age of the sprouts is equal to the number of growing 

seasons that have elapsed since the trees were damaged. 

The year when the flood occurred, therefore, can be 

ascertained.

Some trees are partly buried by deposition of sediment, but only a few are killed. If a tree survives partial burial, the new wood formed in the buried part of 

the trunk is more like root wood than like stem wood; 

thus it is possible to date the year of burial. The surface 

upon which the trees were growing prior to burial can 

be identified by digging to the roots growing from the 

flared base of the trunk; thus the approximate depth 

of alluvium that has been deposited can be measured.

Floods are characteristic of nearly all rivers; and 

although severe floods have been recorded, measured, 

and mapped in recent years, they occur unnoticed or are 

soon forgotten by most people. A flood is any flow of a 

stream or river that overtops the banks of the channel 

and spreads across the flood plain. On the average,

Al
A2 VEGETATION AND HYDROLOGIC PHENOMENA

flows of this size or larger occur about once every 2 

years (Hoyt and Langbein, 1955, p. 15). Flood-control 

problems are discussed in a recent comprehensive book 

by Leopold and Maddock (1954).

The purpose of this report is to demonstrate, by 

examples drawn principally from work in progress in 

the Potomac Kiver basin, how botanical evidence can 

be used to deduce information on floods and flood-plain 

deposition. The examples used are important to this 

report only to the degree that they facilitate the illustration of methodology. The larger study, concerned 

with the relationship of vegetation to the hydrology 

of streams, has been confined largely to the Potomac Kiver flood plain in the vicinity of Washington, 

D.C. (Fig. 1), primarily because of the long record of 

streamflow available for this reach. From a study of 

trees, the flood history of a reach can be extended in 

time prior to the period of the streamflow record, or a 

history can be reconstructed for reaches for which 

there are no records. The depth of fluvial sediment 

can be measured, and the period of time since deposition can be determined; consequently, the botanical 

evidence will aid in distinguishing between flood plains 

and terraces.

ACKNOWLEDGMENTS

During the course of this study, many of the author's 

colleagues visited the study areas and offered invaluable encouragement and suggestions. Thanks are given 

to personnel of Kegion 6, National Park Service 

78°15'

78°15'

FIGURE 1. Map of part of Potomac River near Washington, B.C., showing study area. Base from Washington and Baltimore 1:230,000 

quadrangles.

especially to W. Drew Chick, Jr., and T. Sutton Jett 

for permission to make studies in the Chesapeake and 

Ohio Canal and Palisades Parks, and to Maurice Sullivan for help and suggestions resulting from visits to 

the field areas. Joe Brown and James Watt, Fairfax 

County Park Authority, were most helpful with phases 

of the study at Great Falls Park, Va., and they recently 

provided a canoe and other facilities at Kiver View 

Park. Without the permission and enthusiastic interest 

of several persons who own land along the Potomac 

River, this study would not have been possible. These 

include S. P. Spalding, O. V. Carper, Misses Olga A. 

Larsgaard and Doris K. Stevenson, H. D. Weant, the 

staff of Madeira School, and E. B. Burling. The author 

also wishes to thank Ethel W. Coffay, W. J. Conoyer, 

and N. C. Matalas for help in making the statistical 

analysis of vessel diameters.

MECHANISM OF PLANT GROWTH

Trees grow in height as a result of the division of a 

few cells just below the tip of each twig. These cells 

retain the ability to divide, and the tissue is called the 

apical meristem. Toward the base of the tree, just 

below the expanding tip and essentially continuous 

with the apical meristem, is the vascular cambium, 

which extends as a cylinder down the trunk to near the 

tips of the roots. This tissue also is meristematic, 

having the ability to divide, whereby cells formed to 

the outside differentiate into phloem, or part of the 

bark; and cells formed inside the tissue differentiate 

into xylem, or wood.

Growth activity of the vascular cambium probably 

begins at about the time that apical growth starts in 

the spring, and the activity proceeds downward in the 

trunk and into the roots. Trees increase in diamater in 

a general way rather rapidly in early spring; their 

growth slows later in the season and ceases near the 

end of the summer. The wood formed early in the 

growing season is different structurally from that 

formed later in the summer. Because a different kind 

of wood, here called early wood, is produced outside the 

late wood formed at the end of the previous summer, a 

distinct line is formed between wood that marks the 

end of one growing season and wood that marks the 

start of another. The concentric cylinders of wood 

between successive lines are quite distinct in cross 

sections of most trees and are known as annual rings. 

Kings seen in the cross sections of small branches and 

twigs of some trees, on the other hand, are not formed 

annually (Glock and others, 1960).

Growth activity and anatomy are less well known for 

the tree roots than they are for aerial parts. A few 

studies, summarized by Kramer and Kozlowski (1960,
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A3

p. 52-53), suggest that diameter growth of roots starts 

later in the spring than does diameter growth of the 

trunk; however, growth continues for a longer period 

in the summer in roots. Some evidence also suggests 

that root growth is rapid in the spring and again in the 

autumn. Rings are visible in cross sections of roots, 

but many of these rings are incomplete and may not 

represent annual growth (Brown and others, 1949, v. 1, 

p. 252).

The wood, or xylem, of stems and roots is composed 

of different kinds of cells; and because the size, 

number, and distribution of the cells are different in 

the two organs, stem and root wood will be described 

separately. Only the wood of trees belonging to the 

class Angiospermae, flowering plants, as opposed to 

that of trees belonging to the class Gymnospermae, 

cone-bearing plants, is described here because coniferous trees occur only rarely on flood plains in the 

latitude of Washington, D.C.

Wood of trees of flowering plants, the broad-leaved 

trees, is sometimes called porous because it contains 

cells in the shape of small tubes, called vessels, that 

appear as pores when viewed in cross section. Two 

kinds of porous woods have been defined on the basis of 

the size and distribution of vessels. In ring-porous 

woods the vessels formed early in spring are larger 

than those formed later in the growing season, and the 

change between them is abrupt. Viewed in cross 

section, a series of concentric rings are seen which are 

the rings of large vessels (figs. 10, 21). Oak used for 

furniture and ash and hickory used for small tool 

handles are examples of ring-porous woods. Vessels in 

diffuse-porous woods are about the same diameter and 

more or less evenly distributed across the annual rings. 

Some examples of diffuse-porous woods used in furniture manufacturing are cherry, maple, and birch.

Other cells that constitute wood of broad-leaved 

trees are tracheids, fibers, and parenchyma (Brown 

and others, 1949, v. 1, p. 193; Esau, 1953, p. 250). The 

size and arrangement of these kinds of cells distinguish 

root wood from stem wood. In root wood, fewer fibers 

are present; diameters of the cells are larger; and a 

larger volume of parenchymatous tissue, consisting of 

rays and cells, surrounds the vessels (Esau, 1953, p. 

512-513). Brown and others (1949, v. 1, p. 251-255) 

concurred with these differences. They also stated 

that the specific gravity of root wood averages about 20 

percent lower than that of stem wood, and that root 

wood tends to be diffuse-porous in ring porous species.

Physiological processes within the apical meristems 

play a major role in controlling the form of trees from 

which evidence of floods is discovered. Vegetative 

buds and flower buds start to form early in the

732-764 O 64 2

growing season on the current year's twig about the 

time that twig elongation stops. The buds mature well 

before the end of the summer. New twigs and leaves 

grow from the vegetative buds during the following 

spring. Plant scientists have known for nearly half a 

century that a growth hormone is produced in the 

growing tips of plants and is translocated to other 

areas where it affects physiological processes within 

the plant. Much of the following discussion is based 

on Leopold's (1955) review of the existing knowledge 

of hormones and their use in agriculture. The function of growth hormones which concerns the work 

reported here is the inhibitive effect upon lateral bud 

growth and, as a corollary, the effect of light and 

gravity upon hormone distribution in a trunk (Leopold, 1955, p. 96-102).

As a result of the inhibitive effect of growth 

hormones in lateral bud growth, plants possess an 

apical dominance as long as the terminal buds remain 

alive. Removal of growing terminal buds results in 

growth of lateral buds. Subsequent application of 

hormones to the abscised tip will inhibit growth of 

lateral buds, thus restoring apical dominance. Both 

the force of the water and the debris carried by the 

water during floods decapitate trees and shrubs in their 

path, thereby destroying the growing tips of the plants 

and, with them, the source of the growth hormones. 

The result is the initiation of growth of lateral buds 

(fig. 2) and the development of one or more sprouts.

In addition to lateral buds that start to grow upon 

loss of apical dominance, adventitious buds are stimulated, and their growth gives rise to new stems. 

Growth of some adventitious buds is initiated by 

spontaneous cell division in several tissues within the 

stem (Esau, 1960, p. 229), whereas growth of others is 

believed to arise from preexisting dormant bud cells 

(Eames and MacDaniels, 1947, p. 63; Kramer and 

Kozlowski, 1960, p. 387). Growth of sprouts starts 

from adventitious buds on mature parts of trunks that 

have been severely damaged by floods and that remain 

as stumps or as decapitated water-felled trees.

Gravity and light have been demonstrated to affect 

the movement of growth hormones in stems and thus 

affect the growth behavior of plants. Hormones become concentrated in the lower side of inclined stems 

because of gravity and migrate toward the shaded side 

of unilaterally lighted stems; thus the inhibiting effect 

of hormones on growth of buds on the lighted side is 

lost. For these reasons, stems normally grow almost 

vertically upward. Migration of hormones toward the 

shaded side of stems also explains the phenomenon, 

familiar to many, of the growth of branches on a 

previously clear tree trunk after it has been exposed by
A4 VEGETATION AND HYDROLOGIC PHENOMENA

FIGCEE 2. Sprouts that started to grow from ash stem after flood of 

May 1, 1958, damaged first growth of 1958 growing season. Photograph taken in vicinity of Chain Bridge, Washington, D.C., May 22, 

1958.

the removal of surrounding trees. Even if some 

quantity of hormones is produced in water-felled trees, 

much of it will migrate to the lower, shaded side of an 

inclined tree; this results in a loss of the inhibiting 

effect upon adventitious buds on the upper surface.

HYDROLOGIC CONCEPTS 

COLUECTION OF STREAMFTL.OW RECORDS

Although the methods used in the collection .of 

streamflow records are well described in the literature 

(Corbett and others, 1945; Linsley and others, 1958; 

Johnstone and Cross, 1949), a brief summary of U.S. 

Geological Survey methods is presented here for those 

unfamiliar with hydrologic techniques. The U.S. 

Geological Survey is responsible for the collection and 

analysis of basic water facts and for research on 

hydrologic principles which will lead to a better 

understanding of water in its natural environment. 

Part of the basic data is collected at more than 7,000 

gaging stations where continuous records of streamflow 

are obtained. In addition, partial records of lowflow 

or of floodflow are collected at many other points. At 

the gaging stations the elevation of the water surface, 

which is ordinarily called the stage, is recorded.

Periodically, generally about once a month, each 

gaging station is visited, the recording instrument is

serviced, and a discharge or streamflow measurement 

is made. The discharge is computed from the mean 

velocity of the water, measured with a current meter at 

many points across the stream, and from the area of 

the cross section of the channel at the measuring 

section. Discharge measurements are made during low 

stages and flood stages as well as at times when the 

gage is regularly serviced; thus, data are accumulated 

that permit the development of a stage-discharge relation. The stage, as recorded on the gage, is plotted as 

the ordinate against the simultaneous measurement of 

discharge. The continuous record of stage, accordingly, can be transformed into a continuous record of 

discharge, or the amount of water flowing in the 

stream. The daily mean discharge, expressed in cubic 

feet per second for all gaging stations, is published 

annually in the U.S. Geological Survey Water-Supply 

Papers. A cubic foot per second (cfs) is defined as the 

discharge of water in a section 1 foot wide and 1 foot 

deep flowing at a velocity of 1 foot per second. More 

detailed information on streamflow, including peak 

flood discharges, is available at the U.S. Geological 

Survey district offices.

COMPARISON OF FLOODS

A record of the crest stage or of the instantaneous 

peak discharge of a flood is of limited value if the flood 

cannot be related to other floods. One method of 

comparing floods is based on their frequency of occurrence in time. The concept of frequency of events in 

time is familiar to all who have an interest in natural 

history, even if that interest is merely avocational. 

The vacation camper knows he should not camp 

overnight in an arroyo because of the danger of 

frequent floods. Picnickers often state, when a thunderstorm breaks just after the food is spread, that it 

always rains when they have a picnic, implying that 

such events are frequent. The analysis of floods that 

compares one with another is called flood-frequency 

analysis and tells the average interval between floods 

that exceed a given magnitude.

The frequency of floods and their relative magnitude, as one would expect, are inversely related. The 

smaller floods, which most people never recognize, are 

frequent; and such floods may recur once every year or 

two. The severe floods, the ones described by news 

media, are infrequent; and these may happen on the 

average only once in 50 or 100 years. If many classes 

of mean daily discharge, ranging from the minimum to 

the maximum, are chosen and the number of days on 

which each of these flows occurred is tabulated, then a 

plot of their frequency distribution would tend to be 

normal. Minimum and maximum flows would occur
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A5

on the least number of days, and median and average 

flows would occur on a larger number of days.

Several methods of analyzing flood events have been 

used (Benson, 1962); the method of flood-frequency 

analysis based on peak discharge as used by the U.S. 

Geological Survey (Dalrymple, 1960, p. 5-21) will be 

outlined briefly. The annual flood, which is the 

highest peak discharge between October 1 of one year 

and September 30 of the following year (the water 

year), is listed, and the array is called the annual flood 

series. Another series, the partial-duration series, 

requires listing of all floods above a base discharge 

that is, all independent peak discharges greater than a 

selected discharge. From either of these listings, the 

average recurrence interval can be calculated. A flood 

having a recurrence interval of 2 years, for example, 

has one chance out of two, or a 50 percent chance, of 

occurring in any one year. A flood having a recurrence 

interval of 50 years, the 50-year flood, has a 2 percent 

chance of occurring in any one year.

Neither method was used in its entirety in this study, 

but floods having peak discharges above a base of 

33,100 cfs were listed. This discharge is the minimum 

flow for which evidence of damage to trees has been 

found along the Potomac River. Some difficulty was 

encountered in determining the independence of flood 

peaks in the lower range of discharges, and some of the 

floods listed might not have been independent of 

preceding floods. However, as suggested by Dalrymple (1960, p. 11-12), only those peaks were recorded 

that were separated by a trough that was 25 percent 

lower than the lower flood peak. The recurrence 

interval of only those floods above 82,100 cfs was 

calculated, using graphical methods outlined by Darling (1959). This base flow was chosen because it is the 

discharge of the flood that just cover the buried tree 

(discussed on p. 18-23) that was analyzed to develop a 

method of dating deposits on flood plains.

FL.OODSTAGE-DISCHARGE RELATIONSHIP

At a gaging station the crests of floods, as well as the 

stages of all other flows, are recorded. Because in 

many studies, as in this one, the part of the flood plain 

involved is remote from the station, other techniques 

are used to record the elevation of flood crests. These 

employ visual reading of various types of gages, 

recording the crest on a crest-stage gage, and mapping 

of flood marks. In the present study, crests were 

determined by using crest-stage gages and by mapping 

flood marks.

A crest-stage gage (Ferguson, 1942) consists of a 

length of 2-inch pipe fastened to a tree near the bank 

of a river. The pipe, which has a vented cap at the 

top and a perforated cap at the bottom encloses a

stick equal to the length of the pipe. A small amount 

of ground cork is placed in the bottom cap, so that 

when the river rises around the pipe, the holes in the 

bottom cap permit the water to rise in the pipe, 

thereby floating the cork. When the water recedes, 

the cork adheres to the stick at the highest level 

reached by the water, and the elevation of the ring 

of cork is readily determined.

Flood marks can be found on the flood plain after 

recession of the water, and these are variously mapped. 

In this study, bench marks were installed at many 

points along the river-study reach in connection with 

installation of the crest-stage gages and other instruments, so elevations of flood marks at any point in the 

reach can be determined readily by leveling. Also, 

flood marks on banks delimiting inundated areas of 

flood plain were mapped on aerial photographs.

The peak discharge of the various floods in question 

can be obtained from the gaging-station record and a 

correlation defined between stage at the various sites 

under study and the rate of flow at the reference 

gaging station. (In this study the station record is 

that of Potomac Eiver near Washington, D.C., located 

near the Washington, D.C.-Maryland boundary [Fig. 

1].) The complete gaging-station record is then examined, and the dates and quantities of all discharges that have equaled or exceeded the floods of interest 

are listed. The peak discharge of the flood, rather 

than stage, is used in listing flood events and in 

subsequent analyses. Separate floods on a given river 

having the same peak discharge do not attain exactly 

the same relative elevation throughout the reach of the 

river owing to variations in distribution of tributary 

flow or changes in physical character of channel and 

flood-plain sections. Conversely, separate floods that 

attain the same elevation at a given section do not 

always have the same discharge.

EFFECTS OF FLOODS UPON TREES

The high velocity of water and the large quantity of 

transported debris are the two characteristics of 

floodflow most damaging to flood-plain trees. With an 

increase in discharge, or streamflow, during a flood, the 

velocity of the water usually increases. Discharge 

measurements at the reference gaging station are made 

most of the time from Chain Bridge, which spans the 

Potomac Eiver in Washington, D.C., where velocities 

are so high during floods that current meters, if used, 

would be damaged or destroyed (L. W. Lenfest, oral 

commun., 1956); in fact, velocities higher than 20 feet 

per second have been recorded at Chain Bridge during 

minor floods (Leopold, 1953, p. 608). Discharge 

measurements of the peak flow during floods that occur
A6 VEGETATION AND HYDROLOGIC PHENOMENA

about once every 2 years and during larger ones that 

occur less frequently are made from a highway bridge 

over the tidal estuary about 7 miles downstream where 

velocities are lower. The velocities of water along the 

banks, over low brush-covered islands, and at the 

upstream ends of larger islands, therefore, are exceedingly high, and the force of the water is sufficient to 

injure trees in the flood path. The water alone will 

bend small trees, break off tops, and remove leaves if 

the flood occurs in the summer.

Anyone who has walked along a river after a flood is 

keenly aware that all manner of debris is carried by 

flood water. This debris consists of logs, branches, 

leaves, and other fragments from flood-plain vegetation, and of all kinds of human accouterment. Floating ice during late winter and spring floods are particularly damaging to trees and other flood-plain 

vegetation.

PHYSICAL EFFECTS OF HIGH VELOCITY AND DEBBIS

The physical damage caused by floods and floating 

ice has been described by many observers. Lindsey 

and others (1961, p. 125) stated that icefloes during the 

February 1959 flood along the Wabash River in 

Indiana abraded the bark and broke branches from 

trees along the river bank. They noted a line of 

scarring in vegetation for a considerable distance along 

the bank, and they believed it represented the crest of 

the flood. I observed a similar crest line of scarring in 

plants along the banks of the Potomac River after 

floods and along banks of the Ohio River near St. 

Marys, W. Va., in June 1959. I attribute this line on 

the Ohio River to damage to trees by floating ice in 

February 1959. Damage to trees by floating ice has 

been reported in other areas (Cribbs, 1917, p. 148), and 

on rare occasions in the Southeastern United States 

(Putnam and others, 1960, p. 20).

The results of uprooting, breaking, shearing, and 

scarring of trees by high water during floods are 

commonly seen in flood-plain forests and have been 

reported by several workers. Lindsey and others 

(1961, p. 124) observed these types of damage to trees 

along the Wabash River following the February 1959 

flood. They also noted that small willow trees (Salix 

interior] showed deformation similar to that of windtrained trees and suggested calling this a "floodtrained" form. Shrubs in the arroyos in the Rio 

Grande valley in New Mexico are knocked over and 

partly buried by high water (Gardner, 1951, p. 398). 

Shull (1944, p. 774-775) observed that willows along 

the banks of the Mississippi River suffer damage during high water and that although the tops of many trees 

may be killed, the roots of most will survive. Dore

and Gillett (1955, p. 15) stated that ice on the St. 

Lawrence River kills tree seedlings and perennial 

plants along the shore in the International Rapids section. Ware and Penfound (1949, p. 478) observed that 

most plants on sand bars in the channel of the South 

Canadian River in central Oklahoma are killed by annual floods.

Severe floods of extremely rare occurrence at any 

one place will destroy all trees on parts of the flood 

plain (Hack and Goodlett, 1960, p. 49). This type 

of destruction of flood-plain vegetation along small 

streams in the Appalachian Mountains may occur more 

commonly than is generally believed; in fact, Hack 

and Goodlett (1960, p. 55) suggested that such damage 

may be more frequent in the southern Appalachians 

than in the northern Appalachians. Landslides that 

occurred in the southern Appalachian Mountains in 

August 1940 destroyed large swaths of forests on 

hillsides and in stream valleys below (Water Resources 

Branch, 1949, p. 14). Similar slides occurred in 

central Pennsylvania on July 18, 1942, and destroyed 

stands of trees on slopes and in stream valleys (Eisenlohr, 1952, p. 78). Such catastrophic hydrologic events 

occur in Western United States as well. A most 

striking debris flow occurred during the night and 

early morning of October 2-3,1947, on Kautz Creek in 

Mt. Rainier National Park when an estimated 50 

million cubic yards of rock, gravel, sand, and logs 

moved down the valley (Grater, 1947, 1948), destroying and inundating an extensive area in a mature 

coniferous forest. Accounts of damage to vegetation 

are included in most flood reports published by State 

agencies, the U.S. Geological Survey, and other Federal agencies.

The larger study, concerned with the relationship of 

vegetation to the hydrology of streams, of which the 

one reported here is a part, is being made in areas 

along the Potomac River where stands of trees have 

survived several major floods. The magnitude and 

dates of occurrence of these floods are discussed on p. 

29. Individual trees have been found that were 

growing prior to floods in 1861, 1889, and 1924. 

Several stands that contain trees that were growing 

prior to the 1936, 1937, and 1942 floods have been 

studied. Most of the forest along one reach of the 

river in the vicinity of Chain Bridge near the northwestern boundary of Washington, D.C., consists of 

trees growing from stumps of trees that were cut off or 

felled by a severe ice jam on February 16,1948 (fig. 5). 

The relationship between flood frequency and sedimentation and the form, distribution, and age of the trees 

in this part of the river is under investigation and 

study.
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A7

All flood-plain trees that were growing prior to 

floods of record have, of course, experienced subsequent 

floods. The majority of flood-plain trees, as a result of 

this experience, show varying degrees of damage; so 

that most are small and have poor form or are, in 

forestry terms, of low quality (fig. 3). The trees are 

bent and twisted and consist of several sprouts growing from different sizes and forms of stumps. Trunks 

of trees along the banks of a channel and on upstream 

ends of islands commonly exhibit scars that have 

resulted from repeated abrasion by debris and ice 

carried by high water (fig. 4). The only concentrations of large and straight, single-stemmed trees grow 

on the higher ridges of the flood plain and at considerable distances from the river channel.

Severe floods such as the ones that crested on March 

19, 1936, and October 17, 1942, and the ice jam that 

occurred on February 16, 1948, (figs. 5, 6) inflict great 

damage to a large number of trees. Most of the trees 

in the paths of these floods or of the ice jam were bent 

over or partly uprooted, and the crowns were heavily 

pruned. As long as 25 years after such an event, the 

trunks of trees are still bent and inclined in a 

downstream direction and have scars on the upstream 

side.

High wind also fells flood-plain trees, but windthrown trees on flood plains, as well as on uplands, are 

oriented at random and not in a general downstream

FIGURE 3. View of flood-plain forest showing poor form and sprout 

origin of trees.

FIGURE 4. Floating ice overriding flood plain scars trees. Ice fragment lodged in small ash tree that shows light-colored wood exposed 

by ice abrasion on January 15, 1958.

direction; furthermore, many wind-thrown trees are 

broken into a decapitated trunk and the top, which 

eventually dies. In contrast, most trees felled by high 

water are partly uprooted. Adventitious roots start to 

grow from the part of trunk in contact with the soil, 

and the fallen tree continues to live.

The percentage of stems showing evidence of flood 

damage was determined in December 1962 in seven 

plots located at random along the Potomac River. 

The plots were approximately 50 by 50 feet square. 

Six of them are in areas underlain by bedrock or 

fine-grained alluvium and are flooded at different 

frequencies. The surface of the seventh plot, plot 2, 

consists of alternate ridges of alluvium and flood 

channels in which bedrock is exposed in the bottom. 

Plots 1-4 are upstream from Washington, D.C., near 

the Maryland State line; plots 5-7 are on an island 

approximately 16 miles upstream from plot 4, near 

Seneca Falls (fig. 1). All woody stems in two plots, 

numbers 3 and 4, grow as sprouts from stumps or root 

crowns with the exception of silver maple 1 seedlings 

less than 1 year old. In these two plots, bedrock crops 

out and the surface is flooded on the average about 5 

percent of the time. In the other plots, the percentage 

of stems showing damage ranges from 52 to 85; the

1 Nomenclature of trees follows that used by Little (1953).
A8 VEGETATION AND HYDROLOGIC PHENOMENA

FIGURE 5. Ice jam on Potomac River at Chain Bridge, Washington, D.C., February 16, 1948. The Washington Post photograph.

FIGURE 6. Low flow on Potomac River at Chain Bridge, Washington, D.C., July 1956.
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A9

data are summarized as follows :

Plot

!_.. 

2... ...

3.- 

4 

6 -

6_._ 

7...... ...

Area

Chain Bridge..... 

 do 

.. do.............

.....do.... ... ...

 ..do....... ... ...

 do.... 

Surface l

AlluvAll. Br. 

Br... ._

Br-. 

Alluv.. 

Alluv.. 

AlluvNumber 

of 

floods 2

17 

>17 

155 

155 '15 

>15 '15

Total 

stems

53 

52 

227 

260 

33 

16 

29

Number 

of stems 

damaged

41 

44 

227 

260 

28 

14 

15

Percent 

of stems 

showing 

damage

77.4 

84.7 

100 

100 

84.8 

87.5 

51.7

1 Alluv., fine-grained alluvium; All. Br., ridges of fine-grained alluvium and 

channels having rock bottoms; Br., bedrock. 

" Period of record, 3/1/30-4/1/62, 32 years, 1 month. 

3 Approximate.

General observation at many places throughout the 

area of study leaves the impression that the proportion 

of damaged trees is of the magnitude measured in the 

plots.

EFFECTS OF INUNDATION UPON TBEES

The presence of living trees that have been flooded 

at least 20 times in more than 32 years and still 

survived is sufficient proof that high water alone does 

not kill them. In fact, detailed quantitative study 

along one part of the Potomac River shows no significant difference between vegetation that was covered 

by high water from February to May 1961 and similar 

vegetation studied during the preceding year, prior to 

high water. The quantitative studies have been briefly 

summarized, and the results of the measurements made 

prior to the high water are discussed (Sigafoos, 1961, 

p. C 248-250). This phase of the larger study is the 

subject of a second report that is in preparation.

Other workers have reported that high water alone 

during floods does not kill flood-plain trees but that 

extended periods of flooding and heavy sedimentation 

will kill them. Willow thickets along streams in 

Wyoming are believed to withstand repeated flooding 

for many years (Reed, 1952, p. 708). Taller trees, 

whose crowns extend above the crests of floods, were 

found to survive along the Mississippi River (Shull, 

1944, p. 773); however, Shull (1922, p. 205-206) 

believed that young trees would be killed if submerged 

for a long time.

Several workers have noted that submergence during 

the dormant season will not kill flood-plain tree species (Putnam and others, 1960, p. 11, 13, 20, fig. 15; 

Harper, 1938; Turner, 1936, p. 693; Wistendahl, 1958, 

p. 150; Silker, T. H., 1948, p. 432-433, p. 436), and 

Lindsey and others (1961, p. 122-123) found that high 

water during two floods on the Wabash River in 

Indiana in June 1958 killed only the inundated leaves. 

Cribbs (1917, p. 148) stated that the period of submergence by floods on streams in western Pennsylvania is 

usually too brief and the amount of sediment deposited 

is generally too small to injure trees. Putnam and

others (1960, p. 13, 17) stated that flooding even in 

spring will not kill most bottom-land tree species and 

that many will remain dormant until the water recedes; however, total submergence for long periods 

after breaking of dormancy will kill seedlings and 

small saplings. High water on the Illinois River from 

September to December 1926, in January 1927, and 

from April to mid-June 1927 is believed to have killed 

an estimated 90 percent of the mature timber on the 

flood plain (Turner, 1930). Turner believed that the 

high water from April to mid-June was most effective 

in killing the trees. In a study of the effects of 

flooding by the Kentucky Reservoir on the Tennessee 

River, Hall and Smith (1955) found that all woody 

species were killed if flooded about 60 percent of the 

time during the growing season. Bottomland species 

survived, they found, if flooded less than 42 percent of 

the time.

Submergence even for short periods during the 

growing season, however, will kill some trees and, for 

extended periods of several years, will kill all species. 

Flooding as a result of a beaver dam built across Carp 

Creek in Michigan killed all upland species in 1 year; 

however, black ash and American elm, species characteristic of flood plains, were not killed during this 

interval (Gates and Woollett, 1926).

Inundation for a period of 5 to 6 years by backwater 

of a dam on the Mississippi River killed all flood-plain 

tree species; and a study made during this period 

showed that young, rapidly growing trees evinced the 

greatest resistance to death, whereas old trees and 

young saplings were the first to die (Yeager, 1949, p. 

37).

GROWTH OF TREES AFTER FLOOD DAMAGE

Trees continue to grow after they are damaged 

during floods; and subsequent growth, in a sense, 

preserves the injury. The identification and interpretation of this growth and of the flood-inflicted damage 

are the subjects of this section. It will be shown that 

parts of the bark and underlying tissues are killed and 

that scar tissue grows over the damaged area. Initiation of growth of the scar tissue can be dated. It will 

be shown also that sprouts grow from the residual 

stumps of flood-felled and decapitated trees and that 

the start of growth of these sprouts can be dated. The 

dates obtained from all specimens are those of the first 

growing season following the flood event.

CIBCTTMFEBENTIAL GBOWTH ABOUND TBITNK SCABS 

A sharp blow to a tree trunk, whether made by a 

lawn mower, by a baseball bat in the hands of a small 

boy, or by debris carried by high water, will crush, and 

thus kill, the living tissues between the bark and the 

wood. One of these living tissues beneath the bark is
A10 VEGETATION AND HYDROLOGIC PHENOMENA

the vascular cambium, composed of thin-walled meristematic cells that divide, causing the tree to increase in 

diameter and circumference by the growth of annual 

rings. If the external force is sufficiently strong, or if 

prolonged, as during a flood, the bark is abraded and 

the wood becomes exposed.

Upon death of the cambium, a homogeneous tissue, 

called callus, forms around the margin of the scar and 

across it if the scar is small. New wood is not formed

over the damaged area upon resumption of growth. 

Vascular cambium later differentiates within the callus and ultimately bridges the scar. As the cambium 

is forming, new wood also starts to form around the 

margin of the scar; and, in time, a new layer of wood 

will cross the scar, and a complete annual ring will 

encircle the trunk (Esau, 1953, p. 392, pi. 56, 57). 

This process of wound healing is illustrated diagrammatically in figure 7.

Scar

Vascular cambium

Bark

A Before damage B After damage

, Callus Vascular 

cambium'

Bark

C First growth 

after damage

D After first growing 

season

Bark

3 annual rings

E After three growing seasons 

following flood damage

FIGURE 7. Diagrams representing circumferential growth of trunk after destruction of bark and cambium. Width of the cambium is

greatly exaggerated.
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION All

The extent of healing and the length of time 

required for the scar to be completely covered with 

new wood are affected by the size of the initial scar, by 

the frequency of the event that causes the damage, and 

apparently by the time of year that the damage is 

inflicted. Scars that are a few inches long on twigs 

and stems smaller than about 1 inch in diameter will 

heal by the end of the growing season following the 

damage. Thus, small scars made during the spring or 

early summer generally are covered by new wood and 

bark by the end of the same summer. Larger scars, 

which encircle half the circumference of stems as small 

as half an inch in diameter and extend along most of 

the length, generally do not heal for several years. 

Large scars formed by ice abrasion of silver maple 

trees along the Tippecanoe River in Indiana in 1956 

were seen in 1959 to be healing (Lindsey and others, 

1961, p. 126). Because floods that damage trees on the 

lower surfaces of flood plains occur more often, on the 

average, than once a year, the repeated injury prevents 

many scars from ever healing.

A scar can be dated by taking a section through the 

trunk, tracing the last ring formed prior to injury 

along its circumference to the sound part of the stem, 

and counting the number of rings that lie outside. 

This number is equal to the number of growing seasons 

that have elapsed since injury. To count the annual 

rings in wood directly outside the scar is rarely 

possible, because the wood consists of callus, which 

does not contain annual rings, and generally shows an 

incomplete number of rings. Note in figure IE that 

three rings are represented outside the last ring formed 

before injury, yet only one complete ring overlies the 

center of the scar. If a large area of the stem is 

damaged and if damage, furthermore, is severe, part of 

the wood formed prior to damage may be eroded, so 

that the outer ring in the scarred area is not the last 

ring formed. The wood exposed by abrasion of the 

bark also may rot and subsequently erode before the 

scar is healed.

A description of the scars shown in one dissected 

small tree (fig. 8) and the record of floods will 

illustrate the relationship between flood damage and 

subsequent circumferential growth of stems, The tree 

was collected along the Potomac River near Washington, D.C., in a low area adjoining the channel which 

had experienced 147 floods in the period of streamflow 

record of 26 years and 10 months prior to the time of 

collection. j

From the spring of 1952, when the oldest part of the 

stem shown in figure 8 started to grow, to January 20, 

1958, when the specimen was collected, the stem was 

subjected to 28 floods. Since 1954, when the youngest

732-764 OFIGUEB 8. Parts of a stem of an ash tree growing in an area along the 

Potomac River where a minor ice jam occurred on January 15, 1958. 

The letters A-F correspond to sections shown in figure 10. The stem 

is 1% inches in diameter at A.

part of the main stem included in figure 8 started to 

grow, the stem was subjected to 12 floods. Four of 

these floods are recorded in cross sections of the stem, 

and a fifth is evident in the stem itself. The stem is 

a sprout that grew from a stump that is inclined in a 

downstream direction. Six floods that could have 

felled the parent tree occurred after the growing 

season of 1951 and before the growing season of 1952. 

The use of ages of sprouts as evidence of floods is 

discussed on pages 12-18. The bark (fig. 8) was abraded 

by floating ice that covered the area on January 15,

FIGURE 9. Fragments of ice deposited on flood plain of Potomac River 

on January 15, 1958, in area where tree in figure 8 was collected. 

The lightly shaded parts of the stems are bare wood and are on the 

upstream side of the stems.
A12 VEGETATION AND HYDKOLOGIC PHENOMENA

1958 (fig. 9). Subsequent observations in this area 

suggest that some stems of trees similarly damaged 

in 1958 have died, whereas many others have survived. 

Indeed, sections of a number of trees collected in late 

August 1960 show two rings outside a scar that is 

attributed to the 1958 ice jam.

Cross sections through parts of the stem (fig. 10A-F) 

show healed scars on the upstream sides of the sections which are at the top in figure 10. Three annual 

rings have grown outside the scar hi sections A, Z?, 

and F; thus the scar was formed before the growing 

season of 1955. The scar is on the outer part of the 

ring that grew in 1954; therefore, damage occurred 

between the fall of 1954 and the spring of 1955 during 

any one or all of four floods that covered this area 

during that time. Another scar is present between 

the 1955 and 1956 rings in section E ; the fourth highest 

flood of record on the Potomac River occurred on 

August 20, 1955 (fig. 11, 12). The scar present within 

the 1956 ring in sections C and D may have resulted 

from a sharp rise in the discharge of the Potomac 

River at Chain Bridge to a peak of 72,500 cfs on July 

21, 1956, following an intense local storm which occurred in the lower part of the Potomac River basin on 

July 20-21, 1956 (Water Resources Division, 1964). 

Summarized below are the types of botanical evidence 

of flood-induced damage seen in the sections in figure 

10, the years when they were created, the dates of the 

possible floods involved, and the peak discharge.

Botanical evidence of 

flood damage

Sprout from decapitated stump.

Scar_ _ _ _ _

Scar _ _ _

Scar _ _ _._

Abraded barkYear of growing season 

following damage

Started to grow 

1952.

1955--. _________

1956.___ _. ____

During 1956

1958__-_-_-- ___

Flood dates

( 1/3/52 

1/29/52 

2/6/52 

3/13/52 

3/25/52 

( 4/29/52 

10/17/54 

1/1/55

0/7/55 

3/24/55 

8/20/55 

2/8/56

d/lo/ob 

4/9/56 

7/21/56

1/15/58

Discharge 

(1,000 cfs)

59. 2 

66.0 

59. 2 

108.0 

46.2 

148.0 

131. 0 

70.9

78.6 

99. 5 

216.0 

50. 1

vZ. 0 

68.6 

72.5

The types of scars described in the preceding discussion are inferred to have been caused by floods: 

they are on the upstream side of the stem and are 

dated. Moreover, floods were recorded during the 

interval when the scars were formed. Many trees 

from this area have been sectioned, and all show evidence of damage that can be related to a flood. Abrasion of bark by floating ice and debris which had occurred during the period of this study was observed

during the particular flood, and the effects were subsequently seen within a few days after the water had 

receded. These observations exclude, of course, trees 

cut by boys, by fishermen, or in lumbering operations. 

Although many floods occurred for which evidence was 

not found in the sections of the specific tree discussed 

here, scars resulting from some of these floods may 

be embedded in parts of the stem that were not sectioned. Scars formed during the time of some of these 

other floods, however, have been found in other trees 

growing in the same area.

GROWTH OF SPROUTS FROM STUMPS AND 

WATER-FELLED TREES

The physical effects upon trees of floods of high 

magnitude that occur less often than once every 2 

years, on the average, and evidence of flood damage 

have been inferred only from the form of trees growing on surfaces known to have experienced these 

severe floods (fig. 11,' 12). Floods of high magnitude 

have not occurred since this study began.

Trees larger in trunk diameter and height than those 

described in the preceding section show evidence of 

severe flood damage of a different kind. These trees 

consist of sprouts growing from a stump or inclined 

trunk, show a sharp angle near the base, or look, at 

the base, like a pole implanted in the ground. They 

have a form that is more treelike, consisting of one 

or more straight trunks that are unbranched for a 

third or more of their length. They are not multistemmed, much-branched shrubby plants. Wistendahl 

(1958, p. 140-141, 150) found that willow, river birch, 

sycamore, and box elder trees produce sprouts following damage by floods on the Raritan River in New 

Jersey. Shull (1944, p. 774-775) noted that sprouts 

grew from root stocks of willow trees whose tops had 

been killed during floods along the Mississippi River.

Heavy flood debris materially damages flood-plain 

trees and locally breaks the trunks near the ground, 

leaving a ragged stump. Severe ice jams have cut 

off most trees in large areas of the Potomac River 

flood plain. Sprouts start to grow from these stumps 

during the first growing season following the damage 

and, if not subsequently damaged by another flood, 

continue to grow as straight-stemmed trees (fig. 13). 

The sprouts pictured in figure 13 started to grow in 

1948; the parent tree was probably cut off by an ice 

jam on February 16, 1948, that was about 15 feet deep 

over the area (fig. 5). Apparently all heavy ice flows 

do not cut off most trees. Lindsey and others (1961, 

p. 125-126) reported that floating ice also twisted and 

bent flood-plain trees during the flood of February 

1959 on the Wabash River in Indiana. They observed
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A13

FIGURE 10. Cross sections through stem shown in figure 8. The dark dots are large vessels formed during early growth in the spring. 

One annual ring consists of a ring of vessels and the more or less even-textured wood extending outward to the next ring of vessels. In all sections the outer ring was formed in 1957. The outermost tissue, seen only at the bottom of the sections, and 

especially in F, is bark; it was abraded from the other side of the stems by the ice. The diameter of the section in A is 1% 

inches.
A14 VEGETATION AND HYDROLOGIC PHENOMENA

that fast-growing sprouts started to grow from the 

bent trees and stumps during the following summer. 

They observed before 1959 that trees on the lower surfaces of the Wabash River flood plain showed the 

effects of what they believed to be periodic ice 

damage.

Many years after a flood, sprout trees can be recognized as one or more trunks growing from a common 

root stock. The old stump can be seen at the base of 

the sprouts; or with a little digging in the center of 

the area surrounded by the circle of sprouts, rotten 

wood can be turned up; or the sprouts may appear as 

closely grouped trunks, seemingly growing out of the 

ground, until an excavation around them reveals that 

they are all connected to the same root system. Such 

stumps have been buried by alluvium deposited during 

subsequent floods. This growth form cannot be distinguished with certainty from sprouts growing from

FIGURE 11. Potomac River on August 20, 1955. View is in upstream 

direction from Chain Bridge. Flood peak discharge was 216,000 cfs 

at 2:30 p.m. Maximum depth of the water here above low flow is 

about 35 feet. Photograph by Abbie Rowe, National Park Service.

FIGURE 12. Potomac River, viewed in upstream direction from Chain 

Bridge, at low flow, June 1956. Two fishermen, can be seen standing 

on a rock to the left of center. This point is just upstream from the 

large waves in the center of figure 11.

FIGURE 13.- -Sprouts from stump cut off by an ice jam on February 16, 

1948.

a sawed stump; but sprouts growing from rotten 

stumps in a flood-plain area, in which many trees of 

the same age are sprouts growing from inclined 

trunks, probably date from a severe flood.

More commonly, flood-plain trees that have been 

damaged by high water are partly uprooted or bent 

over, and the subsequent form consists of vertical 

sprouts from an inclined trunk (fig. 18). Just as a 

scar on the side of a trunk can be dated by counting 

the rings overlying it, so can the year that a tree was 

felled be determined.

The simple form consists of a vertical sprout growing from an older inclined trunk. The roots are upstream and the trunk is inclined in a downstream direction. In line with the axis of the inclined trunk and 

at the extreme downstream side of the tree, the stub 

of the parent trunk consists of a protrusion of rotten 

wood that may be only partly covered with wound 

tissue (fig. 14). The vertical sprouts start to grow 

from the inclined trunk during the first growing season 

after the tree was felled; thus, a count of the annual 

rings in a core from the base of the sprout that includes the outer- and inner-most rings tells the number of growing seasons since the tree was knocked 

down.

The bark of the older inclined trunk is rougher, the 

scales or plates are larger, and ridges and grooves are 

deeper than are those of the sprout. The inclined 

trunk looks more mature than the sprout, and the two 

invariably have a visible line of demarcation between 

them. The L-shaped form and differences in texture 

of the bark persist for a long period of time (fig. 15).
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A15

FIGURE 14. Vertical sprout growing from Inclined trunk, showing stub 

of rotten wood at end of parent trunk that was felled by 1948 ice jam. 

Difference In texture of bark of sprout and of trunk and line between 

them are apparent. Downstream Is to the left.

FIGURE 15. Trunk of tree that has sharp angle near Its base suggesting 

origin as a sprout from the part inclined In a downstream direction to 

the right. Trunk above angle started to grow In 1861.

Damage from several floods can be seen in the 

sequential composition of occasional trees: sprouts 

arising from older, inclined sprouts which in turn 

arise from the inclined parent trunk (fig. 16). The 

history of the ash tree illustrated in figure 16 is shown

diagrammatically in figure 17 and indicates by way of 

a case study how botanical evidence of floods can be 

interpreted from sprouted, flood-felled trees. The two 

oldest sprouts started to grow in 1930, the first growing season after a flood in October 1929. In the summer of 1929, the tree was probably fairly small and 

probably consisted of a single vertical trunk. Floods 

occurred on April 18 and October 23,1929, prior to the 

construction of the stream gage near Washington, D.C. 

From the record of the Point of Rocks gage, 41 miles 

upstream, I estimated that the discharge at Chain 

Bridge during these floods was 190,000 cfs and 125,000 

cfs. Both, though not large, would have covered the 

tree and could have damaged it.

Evidence that the 1929 floods damaged this tree is 

somewhat questionable; however, the evidence of subsequent floods is more reasonable. The sprouts that 

started to grow in 1930 were, by the autumn of 1942, 

small trees having trunks about 5 inches in diameter. 

On October 17,1942, the second highest flood of record 

crested and bent the tree in a downstream direction.

FIGURE 16. Ash sprouts of four ages growing from a stump, to the left 

out of the photograph, on the Potomac River flood plain near Chain 

Bridge. The smallest sprouts, in upper half of photograph, are nearly 

vertical and started to grow in 1(956. Second^age sprouts, shown 

forming a gentle arc and Inclined to the right at about 70° from the 

horizontal, started to grow in 1948. The third-age sprout, the short 

segment in lower third of photograph, is inclined to the right at about 

25° from the horizontal and Is growing from a convex split trunk ; 

this sprout started to grow in 1943. The oldest sprouts are a pair 

that started to grow in 1930.
A16 VEGETATION AND HYDROLOGIC PHENOMENA

A 1928 D End of summer, 1942

B Apr. 18, Oct. 23, 1929 E Oct. 17, 1942, second highest 

flood of record; 447,000 cfs

C Summer, 1930 F Summer, 1943

FIGURE 17. Diagrams showing events that produced the form of the ash tree illustrated in figure 16.
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A17

1943 sprout

G End of summer, 1947 J Early summer, 1955

H Feb. 16, 1948, ice jam 

to 34 ft above mean sea level

1948 sprout - 

1943 sprout 

K Aug. 20, 1955, fourth highest 

flood of record; 216,000 cfs

/ Summer, 1948 

Two sprouts that started to grow in 1930 persist today; but, for simplicity, only one is shown.

L Present time as illustrated 

in fig. 16
A18 VEGETATION AND HYDROLOGIC PHENOMENA

A sprout started to grow in 1943. By the end of the 

summer of 1947, this sprout was about 1.5 inches in 

diameter and was Lent over or decapitated by the ice 

jam on February 16,1948. From this inclined sprout a 

new sprout started to grow in 1948 and by August 20, 

1955, the new sprout was about 1.5 inches in diameter 

when it was bent over by the fourth highest flood of 

record.

The fact that evidence of the maximum flood of 

record, on March 19, 1936, and of the third highest, on 

April 28, 1937, is missing in this tree points out a 

major problem in using botanical evidence for recreating a flood history. This tree shows certain evidence 

of three floods; but like any other one specimen; it 

cannot be expected to yield information on all floods 

that inundated it during its lifetime. Chance alone 

can explain why a single tree escaped injury, or the 

1942 flood and the 1948 ice jam could have destroyed 

evidence of earlier floods. The most abundant evidence that has been found in this study stems from the 

most recent floods. Either other floods which are 

known to have occurred did not damage trees that 

show evidence of earlier and later inundation, or the 

later floods destroyed the earlier evidence.

Evidence of nearly every flood on the Potomac Kiver 

in the vicinity of Washington, D.C., since the flood of 

1936 has been found along a reach extending about a 

mile upstream and a mile downstream from Chain 

Bridge. Single sprouts of several ages from inclined 

trunks have been found that date from floods that 

occurred prior to the period of record. These single 

sprouts only suggest earlier floods and, in the absence 

of other evidence, do not date them. If many sprouts 

of the same age were found, however, they would be 

offered as proof. The upright trunk of the larger tree 

shown in figure 15 started to grow in 1861. Sanderlin 

(1946, p. 220) stated that a "freshet" in April 1861 was 

reported to have damaged the Chesapeake and Ohio 

Canal that borders the river. Sprouts of two trees 

upstream from Great Falls started to grow in 1889, 

and the maximum flood prior to 1936 occurred in May 

and June 1889 (Grover, 1937, p. 334). Grover (1937, 

p. 334) listed another flood in February and March 

1902 for which botanical evidence has also been found. 

In the vicinity of Chain Bridge, a sprout was found 

that started to grow in 1924. A severe flood occurred 

on May 13, 1924, that was reported to have overflowed 

the Chesapeake and Ohio Canal levee for a distance of 

1 mile in the vicinity of Chain Bridge (The Washington Post, May 14,1924). This flood caused such heavy 

damage throughout the length of the canal that the 

Canal Company failed to recover, and traffic on the 

canal finally ended (Sanderlin, 1946, p. 277-278).

GROWTH OP TREES FOLLOWING BURIAL BY 

ALLUVIUM DURING FLOODS

The sediment carried by a flood accumulates in 

places on the flood plain where it may surround the 

bases of some upright tree trunks or partly cover 

water-felled trees, giving both a characteristic appearance which can be easily recognized. If a tree remains 

buried through at least one growing season, ring counts 

and study of wood structure made in the laboratory 

can determine with accuracy the year in which the 

burial took place.

Trees that grow on fairly stable surfaces have a 

characteristically flared base, which is buried when the 

upright trees are subject to aggradation during a flood. 

Such partly buried trees look like poles or posts stuck 

in the ground. Hadley (1960, p. 14) noticed this form 

in trees on flood plains in the Cheyenne Eiver basin in 

Wyoming and interpreted it as an indication of active 

aggradation. Jahns (1947, p. 98) learned that the 

minimum thickness of recent flood-plain sediments 

could be determined by the depth of buried tree trunks.

Digging around the base of a buried tree will expose 

the roots and may show the flared base that was once 

above the ground surface. The buried, flared base 

suggests that the tree grew during much of its life 

while little or no sediment was deposited around it, 

and the sediment now above the flared base suggests 

that aggradation was recent and heavy (fig. ISA). A 

rounded or inverted cone-shaped base on a buried tree, 

on the other hand, may indicate that aggradation 

occurred early in the life of the tree, before a flared base 

could form, and that aggradation has continued at 

repeated intervals (fig. 18B). The relationship of the 

shape of the base to the rate of deposition has not been 

demonstrated in the field; but a number of individual 

trees of the same species having the two forms have 

been found, suggesting the postulated formation.

Felled trees, buried or partly buried by alluvium, 

develop one or more vertical sprouts on the upper side 

of the part of the inclined parent trunk that protrudes 

from the ground. This trunk, like those of all 

water-felled trees, is alined nearly parallel to the 

direction of streamflow (fig. 19). Sometimes only a 

row of sprouts or young trunks is visible because the 

parent trunk is completely buried.

Since many living trees of all flood-plain species 

have been found partly buried in the Potomac Kiver 

flood plain, it is apparent that severe damage and 

burial are not necessarily lethal. Adventitious roots 

commonly start to grow from the buried part of the 

trunk from tissues, which Eames and MacDaniels 

(1947, p. 289) called root germs, located close to the 

vascular cambium. Kramer and Kozlowski (1960, p. 

391) pointed out that roots grow from branches still
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A19

Deposited 

alluvium

Surface upon which 

tree started to grow

Adventitious 

roots

FIGURE 18. Diagram showing forms of tree bases postulated as result of different rates of deposition. A, Flared base suggests recent 

and heavy deposition; B, inverted cone-shaped base suggests early and repeated deposition.

FIGURE 19. Ash tree partly uprooted prior to growing season of 1936, probably during flood of March 19, 1936, the maximum flood of record. 

A, Tree, showing parent trunk protruding from flood plain in downstream direction and three vertical sprouts; B, exposed buried trunk, showing flared base. iShovel is 4% feet long; point rests about 1 foot below the approximate level of the flood plain prior to 1936.

attached to trees when the branches come into contact 

with moist soil.

In the area studied along the Potomac Eiver, I have 

not discovered any trees that were killed by deposition 

of sediment as great as 3 feet deep; but Harper (1938, 

p. 47) found that thick deposits of alluvium resulted in 

poor drainage which killed trees along Deep Fork of 

North Canadian Eiver in Oklahoma; he attributed the

heavy sedimentation to straightening and dredging of 

an upstream reach of the channel. Featherly (1941) 

noted that deposition of alluvium to depths of as much 

as 10 feet in Oklahoma killed mature forest trees; 

however, he found a small ash tree that started to grow 

in 1928 and that, by 1940, it was still alive in spite of 

the 52 inches of sediment deposited around it. Cottonwood, willow, baldcypress, and tupelo trees survive
A20 VEGETATION AND HYDROLOGIC PHENOMENA

heavy siltation on flood plains in the lower Mississippi 

River valley (Putnam and others, 1960, p. 20).

The approximate level of the surface upon which 

partly buried water-felled trees were growing before 

being uprooted is determined by interpolating the level 

of the flared base by figuratively rotating the parent 

trunk to a nearly vertical position. Application of 

this system to the tree represented in figure 195 shows 

that about 2,y2 feet of alluvium lies above the original 

base level. The fact that the vertical sprouts started to 

grow in 1936 indicates that the tree was uprooted 

during the flood of March 19, 1936. The alluvium has 

been deposited since then.

ANALYSIS OF STRUCTURE OF WOOD FROM BURIED 

TRUNK

Because the wood formed in the buried part of the 

trunk after burial has a structure different from that 

formed prior to burial, the year of deposition can be 

determined. Other investigators have found by both 

experimentation and observation that wood formed in 

buried or submerged stems structurally resembles the 

wood of roots. Bannan (1962, p. 15), citing Wieler, 

stated that wood of the kind found in lateral roots can 

be produced in a stem by placing it under water. In 

anatomical studies of stemgrafts in fruit trees, Beakbane (1941, p. 361) showed that some characteristics of 

the wood in a stem grown below the ground closely 

resembled those of root wood; other characteristics 

were intermediate between those of stems and roots. 

Wood in the swelled butts of ash trees growing in wet 

swamps is less dense than wood higher in the trunks, 

and the cells of the wood in the butts are larger 

(Koehler, 1933, p. 14-15). Pillow (1939, p. 135-136) 

found that wood of low density extends farther above 

ground in deeply flooded trees than in shallowly 

flooded ones.

The method of study and analysis of data used to 

determine the sedimentation history of buried trees 

was developed from data collected from an ash tree 

close to the one shown in figure 19. The site on which 

these trees grow has been covered by high water at least 

26 times between March 19, 1936, and May 10, 1960, 

and the tree in figure 19 has probably experienced a 

complex history of alternate burial and exhuming in 

these 24 years. The tree used to develop the method, 

however, was merely felled during one flood and 

buried during another. Its history has been simple. 

The tree is on the east side of Conn Island approximately 1,000 feet upstream from Washington Aqueduct Dam in Montgomery County, Md. The surface 

around it has an elevation of 156 feet and is approximately 3.5 feet above low water, which is controlled by 

the dam.

The surface around the tree was just covered by the 

peak flow of 82,100 cf s on May 7,1958. This discharge 

has a recurrence interval of 1.25 years, based on the 

annual flood series. The water was about 1 foot deep 

during the crest on May 10, 1960, which had a peak 

discharge of 124,000 cf s and a recurrence interval of 

1.9 years.

The tree (figs. 20, 21) consisted of a parent trunk 

that was almost completely embedded in alluvium and 

a nearly vertical sprout growing from its extreme 

downstream end. As the sprout started to grow in ' 

1936, the tree must have been felled during the March 

19, 1936, flood. Another sprout, already dead and 

buried when dug, had grown from near the parent ' 

base; therefore, the tree was not buried when it was , 

felled. The level of the base shows that about 2i£ feet 

of sediment was deposited on the flood plain during 

one or more of 25 floods after March 19, 1936, and 

before November 25, 1957, when the tree was studied. I 

In the paragraphs that follow, it will be shown that , 

the tree was buried in 1937, 1 year after it was felled, 

and that no significant amount of sediment has eroded 

subsequently from around the tree. [

To determine when the tree was buried, a laboratory 

study was made of cores taken from the parent trunk 

at 1-foot intervals ("C" in fig. 21) from the base 

of the sprout, at ground level, to the previously buried 

base. Microscopic examination of each core showed 

that the wood lying outside one particular annual ring 

is softer and more even textured than the wood toward 

the center from the ring. The ring that marked the 

change could not be dated directly in each core because 

some rings, especially those near the outer part of the 

trunk, are indistinct; so the ring was cross-dated with 

those in the core taken at the ground surface. The 

distinctive wood was first formed in 1937 throughout 

the length of the buried trunk. The third highest 

flood of record occurred on April 28 of that year.

A large wood sample was taken from the buried 

trunk 2% feet from the vertical sprout and approximately li£ feet below the surface of the ground. 

Diameters of 1,100 vessels in 11 selected rings, dating 

from 1930 through 1946, were measured in a prepared 

cross section that included rings formed in 1936, 1937, 

and 1938.

The data show that the relative diameters of the 

vessels in the ring that grew in 1937 and in rings that 

grew in later years were different from the relative 

diameters of the vessels in rings that grew before 1937. 

As illustrated in figure 22, the early wood vessels are 

smaller and late wood vessels are larger in the 1937-40 

rings than the corresponding vessels in the 1934-36 

rings. When the difference in the mean diameters of
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A21

early wood vessels and late wood vessels of 11 selected 

rings dating from 1930 through 1946 was calculated, 

and when the ratio of this difference to the mean of the 

early wood vessels was determined and plotted for each

FIGURE 20. Ash tree, showing Inclined buried trunk and nearly vertical sprout. Buried dead basal sprout is barely visible to the left of 

the shovel. White line is horizontal, and shovel is 4% feet long.

ring (fig. 23), it could be seen that the major change in 

the difference in mean diameters occurred in 1937.

Statistical analysis of these data shows that the 

vessels formed after 1936 are significantly different in 

diameter from those formed before 1936. The mean 

diameters of the late wood vessels of rings formed 

from 1930 to 1936 were compared with those of late 

wood vessels formed from 1938 to 1946. Early wood 

vessel diameters were similarly compared. The means 

of the two types of vessels formed in 1937 also were 

compared with the means of the earlier rings and of 

the later rings. The assumption was made that the 

vessel diameters are samples of a normal population. 

The results are summarized on page A22.

The £-test, which is a measure of the deviation of the 

mean of a sample from that of the population, indicates that the vessels in rings formed from 1938 

through 1946 have less than a 0.25 percent chance of 

being similar to vessels in rings formed from 1930 

through 1936; in other words, there is 1 chance out of 

400 that the vessels of the two groups of years are similar. The analysis of the early wood vessels in the 1937 

ring further shows that there is about a 90 percent probability that these vessels are similar to those in rings 

formed later, whereas there is only about a 5 percent 

probability that they are similar to those formed 

earlier. These analyses support the hypothesis that 

the tree was buried in 1937.

Approximate level pre-1936 surface

30 -

40 50 

INCHES

90

FIGURE 21. Drawing of ash tree, showing basal sprout, location of cores (C) and wood sections, and approximate level of flood plain prior

to 1936.
A22 VEGETATION AND HYDROLOGIC PHENOMENA

1934 1935 1936 1937 1938 1939 1S40

FIGURE 22. Photomicrograph of wood from buried trunk of ash tree, showing that the early wood vessels are smaller and the late wood 

vessels are larger In 1937-40 rings than in 1934-36 rings. Early wood vessels are the larger white ovals on left side of each ring.

Comparison of vessel diameters summary

Late Wood Vessels

Mean _ _ _ _

Variance ____ _ _ _ _

Group 1 (zi) 

1930-36

0. 0489

. 0000477

Group 2 (as) 

1938-46

0. 0873

. 0000489

1937

0. 0653

Probability (zi=z2) (J-test; <=8.727, 8 degress of freedom) 

<0.25 percent.

Probability 1937 belongs to Group 1<2 percent. 

Probability 1937 belongs to Group 2<2 percent.

Early Wood Vessels

Mean ___

Variance. _ _ _

Group 1 fa) 

1930-36

0. 2243 

. 000651

Group 2 (xt) 

1938-46

0. 1720 

. 000258

1937

0. 1742

Probability (xi=x2)(t-test; t 3.874, 8 degrees of freedom) 

<0.25 percent.

Probability 1937 belongs to Group 1^5 percent. 

Probability 1937 belongs to Group 2^90 percent.

Study of the buried sprout that was dead when the 

tree was dug (figs. 20, 21) supports the thesis that the 

tree was buried after the growing season of 1936 and 

not when it was knocked over, prior to this growing 

season. The sprout, which started to grow when the 

tree was felled, must have begun life as a stem, rather 

than as a root, for it has pith at the center roots do not 

contain pith; therefore, the part of the tree from which 

the sprout arose was not buried at the time the sprout 

began to grow. The inner ring of the sprout is 

composed of wood similar to that found in a stem, bul 

wood toward the outside is more like that in roots. 

This sprout must have started to grow after the tree 

was knocked over, while the base was still above the 

ground surface, and must have continued to grow for a 

time after the tree was buried.

By way of summary, then, the tree was knocked over 

in 1936, and two sprouts grew from the inclined trunk: 

one from the base and one from the distal part. In 

April 1937 the tree was buried, and the wood that
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A23

formed in the buried trunk contains vessels that are 

different from those formed in 1936 and earlier. The 

vessels formed in 1937 are like those that formed later, 

and their size distribution is similar to that of vessels 

formed in root wood.

EVIDENCE OF FLOODS AND DEPOSITION IN TREES 

ALONG SMALL STREAMS

The trees described in the preceding section are 

growing along the Potomac River, where large floods 

innundate the flood plain for several days and to 

depths exceeding 10 feet at the peak. Floods of this 

duration and magnitude commonly fell large trees. 

On small streams, on the other hand, evidence of floods 

in the form of sprouted inclined trunks indeed is rare. 

A measure of the effective force necessary to abrade 

bark or to uproot trees has not been devised but seems 

to be related to the depth and velocity of the water and 

to the duration of innundation in relation to the size of 

the tree. Floods on small streams are generally of 

shorter duration, and depths over the flood plain at the 

peak are lower than on large rivers. As a flood 

progresses downstream, in other words, the flood wave 

spreads out (Hoyt and Langbein, 1955, p. 38, 40), and 

the lag in time between the occurrence of the storm 

and the passage of the flood crest becomes longer. All 

other factors being equal, the peak discharge varies 

almost directly with the size of the drainage area 

or the size of the stream (Benson, 1962b, p. B53). 

Although the relative quantities of sediment transported during floods of different frequencies can be used as 

a measure of the effectiveness of floods (Wolman and

Floods of:

Mar. 19, 1936 

Apr. 28, 1937

FIGURE 23. Ratio of difference between mean of early wood and late 

wood vessel diameters to the-mean early wood vessel diameters for 

selected years. The points represent the end of the growing season for the respective years, and the line between them the period 

from the end of one growing season to the end of the next. Largest difference between ratios of successive years is between 1936 

and 1937.

FIGURE 24. Scar on channel side of river birch on Difficult Run, Va. 

Scar was formed before 1959 growing season and abraded again before 

the 1961 growing season.

FIGURE 25. <Alder sprout growing from parent stem knocked over in 

1956 along Scott Run, Va. Intense cloudburst flood occurred in the 

area on July 21. Parent stem, was subsequently buried.

Miller, 1960, p. 55), they do not qualify as a measure of 

the relative effect of floods upon trees.

The same type of evidence of floods and flood-plain 

deposition described in this report from data collected 

from trees along the Potomac River has also been 

found in small trees and shrubs growing on the flood 

plains of four small streams in Fairfax County, Va.,
A24 VEGETATION AND HYDROLOGIC PHENOMENA

FIGURE 26. Ash sprout growing from inclined trunk that was knocked 

over by flood prior to 1928 growing season along Scott Run, Va.

and Montgomery County, Md. (figs. 24, 25). In 43 

specimens collected along these streams, 35 scars or 

sprouts were found that are related to recorded floods. 

Growth after 11 floods started prior to the period of 

record on the stream where the evidence was found. 

One tree consists of a sprout that started to grow from 

an inclined trunk knocked over prior to the 1928 

growing season (fig. 26).

One small tree (fig. 27), collected by D. G. Anderson 

on Tripps Run, Fairfax County, Va., was studied 

intensively and shows evidence in the wood structure 

and growth form of partial burial on two occasions 

and of felling on two more. This damage occurred 

prior to the period of record on Tripps Run. The 

discharge of two floods was determined on nearby 

streams. These floods occurred during years when the 

tree was partly buried and partly felled. The data 

from the trees on small streams are summarized as 

follows:

Damage to trees on flood plains of small streams as related to floods

Stream

Difficult Run, Va ..

Scott Run, Va ___

Little Falls Branch,

Md. 

Total.... .

Drainage area 

(sqmi)

58

4.69

4.55

4.1

Period 

of record

12/34-

9/57 

8/60-

3/12/63 

9/59-

3/12/63 

6/44-

5/63

Number 

of specimens

11

5

8

19

43

Number 

of scars 

related 

to flood 

record

14

1

6

14

35

Number 

of scars 

formed 

before 

record

4

7

11

Number 

of scars 

of questionable 

origin

2

2

EVIDENCE OF SEQUENTIAL DEPOSITION AND 

EROSION ON THE FLOOD PLAIN

The discussion in the preceding sections has been 

concerned with the growth behavior of trees after 

different kinds of flood damage and after deposition of 

alluvium around or over the trunk. The form and 

wood structure of some trees show evidence of a 

sequence of floods, deposition, and erosion. Study of 

an ash tree (figs. 19, 28) reveals a complex pattern of 

wood structure suggesting such a history of alternate 

deposition and erosion of alluvium around it since the 

parent trunk was felled during the March 19, 1936, 

flood. This tree is about 15 feet downstream from the 

ash tree described on pages 18-23.

The large vertical sprouts (fig. 19) started to grow 

in 1936, and since that time about 2y2 feet of sediment 

has been deposited on the surface around the tree.

FIGURE 27. Ash tree collected from flood plain of Tripps Run, Fairfax 

County, Va., showing the form of trees damaged by floods. Scale is 

1 foot long and at 1962 ground level.
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A25

10 11 12 13

FEET

FIGDKB 28. Drawing of buried part of the ash tree in figure 19.

Because sprouts A, B, and C (fig. 28) have pith in 

centers of cross sections through their bases, they must 

have started to grow as stems. The lower part of 

sprout A grew above ground for 2 years, became 

buried, and grew for an indeterminate interval beneath 

the soil surface; later, it grew above ground for 3 years 

and then was again buried and continued to grow 

below the surface until collected. Basal sections 

through sprouts B and C show similar histories but are 

not correlative in time. Sections through higher levels 

in the buried sprouts also show varied wood structure, 

but their sequences of stem wood and rootlike wood 

cannot be related to sequences in any other sections. 

Although these sprouts were alive when collected, the 

rings in the rootlike wood are indistinct and" cannot be 

dated with certainty; furthermore, it is not known 

when sprouts B and C started to grow. Their records 

are, therefore, truncated at the beginning and the end 

and are obscured in the middle.

Swollen growths at various levels on the buried 

sprouts from which other sprouts and roots are growing appear to be analagous to root crowns of trees that 

have been damaged repeatedly (Lutz, 1934, p. 60, fig. 

15). There appear to be three levels of these growths, 

suggesting that three floods cut off the stems at the 

respective levels and that later floods deposited alluvium over them. Again, it is not possible to date their 

initial growth.

Although it has been impossible to relate the evidence of deposition and erosion to specific floods, it is 

evident that this tree has experienced a varied history.

Study of a core from the parent trunk between sprouts 

B and C indicates that this part of the trunk was 

buried in 1937 and remained buried until sampled. 

Another core, taken 1 foot below the surface and to the 

left of the large vertical sprout in the center of figure 

19, suggests a varied history of sedimentation similar 

to that of the sections of the small buried sprouts.

In summary, after the tree was felled in 1936, it 

could have been partly buried in 1937. Later, some 

floods could have added additional layers of alluvium 

on top of the 1937 surface, and other floods could have 

eroded all or part of these layers. Apparently no flood 

subsequent to the one in 1937 eroded to the pre-1936 

level.

The analysis of the form and the wood structure in 

the trunk of another tree (fig. 29) is presented as 

another example of the reconstruction of at least a 

general history of floods and sedimentation. This tree 

also grows on the Potomac River flood plain, about 2 

miles downstream from Great Falls. The inclined 

trunk and vertical sprouts show that the tree was 

knocked over. The roots growing from the fallen 

trunk show also that the trunk was buried; a small 

basal sprout, however, attests that the tree was buried 

during a flood later than the one that felled the tree. 

Subsequent erosion is confirmed by the exposed roots. 

The tree was knocked over after the 1942 growing 

season, probably during the flood of October 17, which 

was the second highest flood of record. The year of 

burial cannot be determined from the cores that were 

taken because some rings cannot be identified; however,
A26 VEGETATION AND HYDROLOGIC PHENOMENA

FIGURE 29. Ash tree, on flood plain, that was knocked over by one 

flood, partly buried by another, and exposed by a third.

the fact that alluvium was eroded from around the tree 

between the growing seasons of 1950 and 1951 can be 

deduced from a change in wood structure in the fallen 

trunk between rings of these years. The wood formed 

in the exposed roots also changed in 1951; wood which 

has grown since then is more like stem wood. Three 

floods, on November 26, December 6, and December 10, 

1950, covered the site and could have exposed the roots.

The foregoing detailed description of the tree shown 

in figure 29, as well as even superficial observations of 

flood-plain trees, reveals that alluvium is eroded from 

around roots on flood plains and in channel banks. 

Wood of ash roots is diffuse-porous, whereas the outer 

layers of wood in exposed ash roots are ring-porous. 

The change from diffuse-porous to ring-porous is 

abrupt and occurs between adjacent rings. This 

change is attributed to exposure by erosion, and the 

year of exposure can be determined by counting the 

rings lying outside the change in wood. Annual rings 

in the ring-porous wood of exposed roots are distinct.

Detailed study by other investigators has shown 

anatomical differences between wood of buried roots 

and wood of exposed roots. Morrison (1953) studied 

the wood anatomy of roots of six species of trees and 

found that the length, diameter, and concentration of 

vessels and the width of rays in exposed roots differed 

from those in buried roots. Others have found that 

exposure of lateral roots by erosion results in growth 

of new wood that is more like that in stems (Bannan, 

1962, p. 15; Esau, 1953, p. 512-513).

In review, the botanical evidence of floods, deposition, 

and erosion consists of the growth of trees after the 

flood has subsided. This growth manifests itself in the 

healing of scars on trunks of trees, in the growth of 

sprouts from inclined trunks, and in the change in wood 

structure in buried stems and in exposed roots. The 

growth and change occur during the first growing 

season after the hydrologic event and can be dated by 

counting the annual rings that have formed since the 

change.

BOTANICAL PHENOMENA RELATING TO THE 

FORMATION OF THE FLOOD PLAIN

The relatively level land bordering a stream, called a 

valley flat (Twenhofel, 1932, p. 806) or flood plain, is 

composed of sediments reworked by the river and is 

subject to flooding (Hoyt and Langbein, 1955, p. 12). 

The types of deposits that underlie the flood plain have 

been described by Happ and others (1940, p. 22-25) 

and by Lattman (1960). Wolman and Leopold (1957 

p. 95, 96) have shown that flood plains are formed 

primarily by the lateral migration of the channel 

across the valley flat and are thus composed mostly of 

deposits of lateral accretion. Different definitions of 

the flood plain exist, but the currently accepted ones 

seem to agree that a flood plain is the lowland 

adjoining the channel, subject to flooding, and composed of sediments carried and reworked by the river. 

Similar differences of opinion in the definition of the 

river bank exist in the literature, and some authors 

have expressed difficulty in measuring the height of a 

river bank (Hack, 1957, p. 48; Schumm, 1961, p. 33; 

and Wiitala and others, 1961, p. 53). It has been 

suggested, furthermore, that bankfull floods, which are 

of intermediate frequency that is, floods that occur on 

the average of about once a year or once every 2 

years are more effective in causing the lateral migration of the channel, thus the formation and modification 

of flood plains, than the larger, less frequent floods 

(Wolman and Miller, 1960, p. 66).

The sections that follow show that, not only along 

part of the Potomac River but probably along similar 

rivers in the East, the channelward limit of flood-plain 

trees represents the bank. It is a line that can be 

mapped, and it owes its initial location to the low flow 

regime of the river. Trees can become established on a 

flood plain only during extended periods of low 

flow; yet, once established, they mature and stabilize 

the banks. The only hydrologic events that remove the 

trees are the slow lateral undercutting of banks, which 

causes the trees to topple into the river, and infrequent 

catastrophic floods. The presence of mature trees 

along the channel, then, marks the bank; their age
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A27

allows the determination of the frequency, albeit 

crude, of the reworking of the flood plain deposits by 

the river.

RELATIONSHIP BETWEEN FLOOD-PLAIN TREES AND 

LOW PLOW

A demarcation line readily seen along a stream 

channel and closely related to the flow regime, as well, 

is the margin of the perennial vegetation. This line is 

used to map rivers on topographic maps and, in the 

humid parts of the United States, coincides with the 

channelward limit of tree species. The relationship 

between the flow of rivers and the establishment and 

growth of trees on the channel banks is only partly 

understood, because the sequence of events believed 

necessary for the establishment of trees on banks has 

not occurred along the main stem of the Potomac River 

during the period of study; therefore, the conclusions 

presented here are only tentative.

For trees, or any plants, to become established, three 

basic elements are necessary: (1) a supply of viable 

seed, (2) a favorable seed bed, and (3) a period of 

time during which the environment is favorable for 

the growth of young plants. Although the yield of 

tree seed and the time of dissemination are being 

studied in connection with the larger project, only a 

few data will be presented here. The time of dissemination, the yield, and a few observations of the fate of 

silver maple and box elder seed are cited as examples 

of the relationship of these botanical processes to the 

flow regime of the river. Behavior of other species 

river birch, cottonwood, American elm, and sycamore could be discussed; however, the pattern is 

similar.

Silver maple trees along the Potomac River near 

Washington, D.C., drop their seed in late spring, 

mostly during a 2-week period ending about May 1. 

The seed are capable of germinating immediately, and 

hundreds of newly sprouted ones are seen each May. 

Seed have been caught and counted in traps and in 

marked plots on the ground. Over a 4-year period, the 

number of seed per square yard ranged from 55 to 243; 

and on one day they fell at a rate of 5 per square yard 

per hour. These data show that an adequate supply of 

silver maple seed is broadcast on the flood plain.

Box elder seed are disseminated from autumn to 

early spring; and they, like silver maple seed, are 

capable of germinating when the weather becomes 

warm in May, when hundreds of newly sprouted seed 

of this species are also seen. The number of seed that 

have fallen annually range from 100 to 250 per square 

yard.

Although attempts have been made to collect data on

the rate of germination of silver maple and box elder 

seed on different kinds of soil, all quantitative results 

have been negative. All newly germinated seed and 

seedlings of these species have been found only on 

mineral soil, either recently deposited by a flood or 

recently exposed by erosion of overlying organic matter, or in thin layers of finely comminuted organic 

flood debris. Some of the seed traps are on the flood 

plain at levels that have not been inundated during the 

period of study; therefore, the ground is covered with 

several years' accumulation of organic matter and leaf 

litter. Although silver maple and box elder trees are 

not common near these traps on the higher surfaces, 

some seed of these species are caught annually. No 

seedlings of silver maple or box elder, or of any 

flood-plain tree species, for that matter, have been 

found on surfaces covered with humus and leaf litter 

in the 6-year period of observation.

As examples of the fate of seed and seedlings, a 

series of observations will be discussed. On April 30, 

1958, 155 newly fallen silver maple seed were counted 

in a plot having an area of 25 square feet. One week 

later the plot was inundated by a flood, and the seed 

were either washed away or buried. None on the plot 

germinated. On May 16, 1962, 1,150 silver maple seed 

were counted in five 1-square-yard plots underlain by 

fine-grained alluvium. These plots were examined 

periodically, and no seed germinated. On June 21, 

1962, numerous new silver maple seedlings were observed outside the plots, but, as late as August, none 

appeared in the plots. The surface upon which the 

new seedlings grew, as well as the plots, however, was 

inundated by floods on March 7 and March 22, 1963, 

which destroyed the seedlings and deposited 2-4 inches 

of alluvium on the plots. Environmental conditions at 

the five marked plots were not favorable for germination, and, as will be seen subsequently, the 1963 floods 

probably would have destroyed any seedlings if they 

had germinated.

Seedlings of silver maple and box elder have been 

found, counted, and observed. Hundreds of silver 

maple seedlings were found in a small ridge of finely 

comminuted flood debris on June 8,1961. In a marked 

plot extending 6 feet along the ridge and 1 foot across, 

228 seedlings were counted. These were destroyed by 

burial under several inches of sediment on March 23, 

1962. Similarly, box elder seedlings were found in two 

flood-debris ridges in 1958. From late March to late 

April 1958 the river rose three times, each time to a 

lower level than before, leaving three debris lines. On 

May 2, 1958, 64 box elder seedlings were counted in 10 

linear feet along the late March line, 13 along the 

mid-April line, and none along the late April line. A
A28 VEGETATION AND HYDROLOGIC PHENOMENA

few seedlings were growing on the surface between the 

upper and lower debris lines, and none below the lower 

line. Because the count was made just 2 days after the 

late April rise, it seems reasonable to believe that the 

late April rise destroyed any seedlings that may have 

germinated on the surface inundated at that time. 

All seedlings, including those in the higher debris line, 

were destroyed by the flood of May 7, 1958, which 

crested at an elevation higher than the late March 

flood.

In the examples cited, the entire annual crop of seed 

of silver maple and box elder trees either failed to 

germinate because the local environmental conditions 

were not favorable for germination, or the seedlings 

were destroyed by later floods. For seed of these 

species to germinate and grow into seedlings and these 

to become established as a new generation of trees, a 

new crop of seed must be produced in another year. 

Because young saplings of all flood-plain tree species 

are present in the forest along the Potomac River, a 

sequence of hydrologic events associated with unique 

climatic conditions must have prevailed in the recent 

past that has permitted their establishment. This 

sequence however, did not occur during the period of 

study.

These data suggest that the orderly sequence of 

botanical, hydrologic, and as yet unknown environmental events that must prevail before seedlings will 

become established is as follows: Streamflow during 

the winter must remove humus and freshly fallen leaf 

litter from the surface; seed must fall on mineral soil; 

microclimatic and edaphic conditions must favor germination and growth of seedlings; and finally, the 

surface must not be flooded again until the seedlings 

have grown enough to withstand mechanical injury. 

The interval necessary to insure the trees a chance of 

survival between seed germination and the first flood is 

not known, because no floods have recurred during the 

period of study in areas where young seedlings grow.

Other investigators have noted the orderly sequence 

of events required for the germination and establishment of plants along rivers. Putnam and others 

(1960, p. 13) found that regeneration of all tree species 

in southern river valleys will fail if seedlings are 

repeatedly flooded during the growing season. Dietz 

(1952, p. 252) observed that, for willows to become 

established, seed dispersal must occur during the 

falling stage of a flood, at which time the floating seed 

are deposited from the receding water in a line on the 

bank. For seed to germinate, the bank must remain 

moist; and, for willows, no flood may occur for several 

months while the seedlings are becoming established. 

Horton and others (1960) made thorough laboratory,

FIGURE 30. Mature silver maple trees on low bank that was flooded at 

least 60 times between 1931 and 1960. Flow at time of photograph. 

February 18, 1963, was about 5,000 cfs. Average annual discharge 

is 11,000 cfs. The 2-year flood is about 10 feet deep at this section.

greenhouse, and field studies of seed germination and 

seedling establishment of riparian species common in 

the semiarid southwest. They reported (p. 14-16) 

that tamarisk and seepwillow seed are spread on river 

banks during receding streamflows in the autumn and 

winter. If the bank remains moist in early autumn 

and in spring, many seed will germinate and become 

established. Both species can survive submergence for 

as long as several weeks but will be eroded from the 

soil if subjected to any current. Gardner (1951, p. 

398-399) found that seed of creosote bush germinate 

on exposed gravel bars in the channel of the Rio 

Grande in New Mexico and that shrubs become established during subsequent dry years.

Mature trees lining long reaches of river banks are 

inundated as often as several times a year by high 

flows and by floods. This fact, plus the close relationship between seedling growth and stream hydrology, 

shows that the establishment and maturation of trees 

on the bank constitute a botanical process that is 

adjusted to the low-flow regime of the river. As 

Leopold and Wolman (1957, p. 63) pointed out, if a 

river does not use the entire width of the channel, as 

during periods of low flow, vegetation will quickly 

occupy the new ground; furthermore, as they stated, 

growth of vegetation contributes to the deposition of 

sediment. Shull (1944, p. 776) also noted that several 

successive years of low water are required for the
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A29

establishment of trees on banks of the Mississippi 

River. The channelward limit of flood-plain trees, 

therefore, represents the high-water mark of the maximum stages during periods of low flow (fig. 30).

The surface that is exposed during low flow, upon 

which trees become established along rivers that slowly 

migrate laterally, is comparable to a point bar (Leopold, 1962, p. 528; Wolman and Leopold, 1957, p.

91-92). It is initially formed by high flows and floods 

which also keep it bare, and trees become established 

across the surface from close to the water's edge only 

during periods of low flow. The elevation of the 

surface is raised by deposition during subsequent high 

flows in a manner similar to that described for point 

bars in pastures (Wolman and Leopold, 1957, p.

92-93). These processes are represented diagrammatically in figure 31. The rate of change in point-bar 

formation in forested flood plains is slower than in 

pastures.

The deposition of sediment around mature floodplain trees, which occurs during high flows and which 

was discussed in a previous section, does not adversely 

affect most trees. The death and destruction of 

flood-plain trees is more complex, involving internal 

physiological as well as hydrologic processes related to 

floodflow; the effects of hydrologic processes upon the 

development of the forest and formation of flood-plain 

banks are discussed in the following sections.

SIGNIFICANCE OF OLD TREES ON BANKS

Detailed mapping of tree species along most of both 

shores for an approximately 20-mile reach of the 

Potomac River (fig. 1) indicates that for much of their 

length these banks are characterized by old trees 

growing on the vertical face of the bank or, on a few 

places, on top of it. These banks are, in fact, for most 

of their length, armored with heavy, old roots, tree 

bases, and trunks, whose presence indicates that the 

bank has not changed position for a period equal to at 

least the age of the trees. The flood-plain deposits, at 

these places at least, have not been reworked for a long 

period of time. Leopold and Wolman (1957, p. 69) 

stated that heavy tree roots bind alluvium of flood 

plains and that erosion of the banks is extremely slow. 

They emphasized that the banks of some rivers are not 

eroded during periods as long as 200 years (Wolman 

and Leopold, 1957, p. 100).

Active erosion of banks of the Potomac River is 

manifest in a few places in exposed tree roots, trees 

leaning over the water, and bare cut banks (fig. 32). 

Old trees grow on top of these banks. The rate of 

erosion has not been determined at these places, and at

only a few of them would repeated measurements be 

feasible.

Frequent floods those that occur on the average of 

once in two years do erode the bank in places, 

undercutting some of the trees and toppling them into 

the river. Such trees have been observed from time to 

time in the course of fieldwork. During this period, 

1958 to the present, no floods have exceeded the bank at 

the places where the trees fell into the water. Elsewhere, newly exposed roots are seen extending out 

from eroded banks after recession of floods (fig. 32).

The presence of many old trees along the banks of 

the Potomac River suggests that even the catastrophic 

floods here are no more effective in the erosion of banks 

and the destruction of the flood-plain forest than 

are frequent floods. Leopold and Wolman (1957, p. 

64) stated that river banks and the trees that grow on 

them in the Eastern United States are severely eroded 

only by the larger floods. In mountain streams of the 

Eastern United States, Hack and Goodlett (1960, p. 

48-51) showed that a single catastrophic flood can 

determine the character of the flood plain, and they 

believe that forested banks can be moved only by floods 

of such magnitude. The tree shown in figure 15 is 

more than 100 years old and has grown on the vertical 

face of the bank through three major floods which 

were of sufficient magnitude to be considered catastrophic and which Wolman and Miller (1960, p. 57) 

suggest recur once in 50 to 100 years. In the vicinity 

of Washington, D.C., the March 1936 flood has a 

recurrence interval of about 70 years, and the November 1942 flood, of about 50 years. The third flood to 

pass over the tree occurred in May and June 1889, 

prior to the establishment of recording gages on the 

Potomac River; but it has been estimated as approaching the March 1936 flood in size (Grover, 1937, p. 334).

In addition to these catastrophic floods, the tree (fig. 

15) has been inundated by many other floods of lesser 

magnitude. The flood of May 10, 1960, reached nearly 

to the level of the base of the sprout and branch above 

the knife on the smaller tree in the center of the figure. 

Such a flood has a recurrence interval of 1.9 years; and 

18 floods, including those in 1936 and 1942, have 

equaled or exceeded it in 33 years.

Evidence has been presented which supports the 

theory that, once the trees become established, they 

will persist for a long period of time and will 

coincidentally preserve the location of the bank. The 

location of the bank, then, is established at a lower 

elevation close to mean low water and well below the 

level of floods that recur about once every 2 years. 

The old trees and the tops of the eroding banks are generally at about the 2-year-flood level.
A30 VEGETATION AND HYDROLOGIC PHENOMENA

Erosion and accretion

Approximate level 

of 2-year flood

A Normal climate

River level approximately 

75 percent of time

B Drought climate

Seedling establishment

Maximum stage for n years

No seedlings; trees here 

repeatedly damaged by floods

Erosion and accretion

C Normal climate

Approximate level 

of 2-year flood

River level approximately 

75 percent of time

Renewed accretion

FIGURE 31. Diagrams representing establishment of trees in relation to the flow regime of the river and the processes modifying the flood plain.
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A31

FIGUEE 32. Cut bank of Potomac River showing newly exposed roots 

and recently fallen trees. Two-year flood just reaches top of the 

bank. Area is almost 200 yards upstream from area in flgue 30.

BOTANICAL EVIDENCE OF CUBKENT PKOCESSES 

AFFECTING THE FLOOD PLAIN

A discussion of the lack of agreement on whether 

(1) lateral accretion due to point bar deposition, (2) 

deposition on flood plains in backwater during floods, 

or (3) erosion and deposition in a sequence of floods 

makes the major contribution to the deposited sediment is beyond the scope of this report. This controversy, however, fails to note an important aspect 

shown by the botanical evidence. On some flood 

plains, certainly on forested ones of rivers that are 

moving laterally very slowly, local erosion by overbank flows produces rapid interchange of alluvium 

only to be followed by deposition in flows of comparable magnitude either in the same year or in subsequent 

years. Flood-plain sediments appear to be constantly 

reworked to variable depths.

This process of trading of flood-plain materials 

without intervention of channel movement may affect 

large amounts of material. It is probably enhanced by 

vegetation, owing to local concentration of flow, whereas it may be nearly absent on pastured or grassy flood 

plains. This trading process is shown by botanical 

evidence to be of greater magnitude than had heretofore been realized, but it probably occurs at this magnitude not only on the Potomac River but also along 

other eastern rivers of similar character. To the 

extent that this trading is important the significance

of the origin of the first deposited material lateral deposits or overbank deposits is proportionately 

diminished.

Sediment found on the flood plain is not part of the 

original point bar or overbank deposit; but it is the 

result of processes erosion or deposition that are 

much more recent than those associated with the initial 

flood-plain formation. The surface one sees is of 

relatively recent origin, even if the average elevation 

of the flood plain has not changed with the passage of 

time- The flood plain consists of a mosaic of erosional 

and depositional surfaces in space, and tree form 

indicates that this sequence is characteristic of any one 

place at different times. The net effect of this process 

is temporary storage of alluvium on the flood plain, 

similar to temporary storage of alluvium in point bars 

(Wolman and Leopold, 1957, p. 95). The flood plain, 

its vegetation, and the complete flow regime of the 

river are elements in dynamic equilibrium with each 

other (Hack, 1957, p. 90; 1960, p. 85-87; Hack and 

Goodlett, 1960, p. 58).

The conclusion that banks and flood plains of the 

Potomac River are eroded only slightly, even by floods 

having a recurrence interval of 70 years, agrees with 

the conclusions of Langbein and Schumm (1958). 

They showed that the annual sediment yield in the 

United States decreases with an increase in precipitation above about 14 inches per year to a minimum 

quantity in densely forested regions of the humid East 

and the Pacific Coast States. Much of the sediment 

carried by the Potomac River during high flows may 

well come from scour of its bed (Leopold, 1962, p. 

520), as well as from erosion throughout the drainage 

basin (Leopold and Maddock, 1953, p. 21), and, during 

floods, from erosion of its flood plain.

The concept that vertical erosion and deposition are 

effective in modifying flood plains in the humid East 

complements the seemingly opposed concept of predominance of lateral corrasion and deposition. Forested river banks are relatively stable because they are 

composed of cohesive materials bound by heavy roots 

(Leopold and Wolman, 1957, p- 64). In the semiarid 

West, however, river banks, they report, are composed 

of more friable materials, and flood plains support a 

less dense plant cover; therefore, these banks are 

susceptible to erosion by floods of lower magnitude and 

higher frequency. It follows that the dominant processes that modify a flood plain are determined by the 

flow regime and the vegetation on the flood plain, as 

well as by the materials composing the bank. One 

would expect, therefore, other conditions being equal, 

that different processes will be dominant in valleys 

supporting different types of vegetation. Differences
A32 VEGETATION AND HYDROLOGIC PHENOMENA

in the type of vegetation may result from the use of 

the land by man or from differences in regional 

climate.

According to this reasoning, change of the natural 

vegetation due to change in land use or change in 

climate will result in a change in dominance of 

processes that act on the flood plain. Upon destruction 

of the forest on flood plains in the humid East through 

expansion of urbanization, "improvement" of streams 

through channelization, or clearing for agriculture, 

lateral corrasion and deposition will become the dominant processes on the flood plain and will remain so as 

long as the banks are kept clear of trees. A change in 

climate to a drier one would result also in an increase 

in the dominance of lateral corrasion and deposition. 

This change with change in climate is supported by the 

conclusions of Langbein and Schumm (1958, p. 1084), 

who showed that, as conditions change from humid to 

dry, the sediment load increases, probably as the result 

of increasing lateral corrasion of the banks.

An interpretation of the botanical phenomena 

against the background of the flow regime of the river 

defines the flood plain as the frequently flooded surface 

adjoining the channel and composed of alluvium 

deposited and reworked by the river. The bank is the 

channelward limit of perennial vegetation tree species in the humid east. The surface is considered to be 

flooded whenever the level of the river exceeds the 

bank; and, along the parts of the Potomac River that 

have been studied, the frequency of these floods ranges 

from about five times per year to once in 2 years.

In summary, the establishment, growth, maturation, 

and death of flood-plain trees in humid regions are 

inseparably merged with the complete flow regime of 

the river and the erosion and deposition of sediment. 

The bank, marked by the channelward line of trees, 

represents the maximum high water line during extended periods of low flow or the margin of the 

laterally eroding channel. Once the flood-plain trees 

become established, they will persist through several 

severe floods. Locally, a few trees are torn out by 

catastrophic floods, but most die when they reach a 

maximum size or when they topple into the river as the 

bank is eroded. Growth of the trees is modified by 

hydrologic processes resulting in tree forms that record the flood history of the surface.

ECONOMIC SIGNIFICANCE OF BOTANICAL EVIDENCE 

OF FLOODS

Some knowledge or undersanding of the magnitude 

and frequency of floods and flood-plain formation is 

necessary for the design of structures along rivers and 

the use of flood plains by man. Some of these

structures are flood-control reservoirs, levees, and 

channels, whose design most obviously must be based in 

a large part upon the characteristics of floods. The 

demand for knowledge of floods, needed also to design 

storm-water drainage systems and to prepare ordinances controlling the use of flood plains, is especially 

great as a direct result of the seemingly unabated 

construction of suburban housing. The decisions made 

to answer the two basic questions underlying any 

proposal involving streams is it needed? and, how 

large should it be? are influenced, as Langbein and 

Hoyt (1959, p. 127-142) pointed out, by uncertainties 

in the hydrologic data.

Some of the uncertainties stem from a complete lack 

of data from the drainage basin where a project is 

proposed or from the fact that data have been collected for too short a period of time. About three-fourths 

of the stream gaging stations in the United States are 

on streams draining areas larger than 100 square miles, 

and the total length of small streams greatly exceeds 

that of larger streams (Langbein and Hoyt, 1959, p. 

13-16). Furthermore, about half of the stations have 

been in operation for less than 20 years.

Botanical evidence of floods and flood-plain deposition discussed in this report will aid in reducing 

uncertainties. Although the methods are crude and 

much additional research is needed, a group of fallen 

trees, a number of partly buried tree trunks, or a 

tangle of exposed tree roots on a flood plain are 

positive evidence that something has happened; and 

further study will reveal the cause of the anomalies in 

the tree growth. In the section that follows, the value 

and limitations of botanical evidence are treated as a 

source of data to extend the records of floods backward 

in time and to provide a record of events at places 

where none exists. Finally, the value of botanical 

evidence of sedimentation is discussed.

VALUE OF BOTANICAL EVIDENCE OF FLOODS AND 

FLOOD-PLAIN DEPOSITION

In the absence of flood records, engineers must use 

indirect methods to determine the frequency of floods 

of different magnitudes (Jarvis and others, 1936, p. 

28-68; Corbett and others, 1945, p. 98-109; Benson, 

1962; and Dalrymple, 1960). These methods are 

estimates of peak flood discharges from which heights 

of flood crests can be estimated. As Langbein and 

Hoyt (1959, p. 240) emphasized, " * * * raw hydroloic data are still strange to many * * *." Many 

cannot translate the concept of a flood of a given 

magnitude and frequency to a mental picture of water 

flowing down a valley in a broad river, covering the 

entire flood plain. The methods described in this
BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A33

study might provide positive evidence of the occurrence of floods along reaches where the magnitude and 

frequency are estimated from a minimum of data.

The botanical evidence of floods, consisting of 

scarred trunks and felled trees, are positive evidence of 

a catastrophic event. The simple techniques used to 

analyze this evidence show almost at once when the 

event occurred. With a little effort a brief history of 

floods can be developed. At present, however, the 

evidence is only of the occurrence of a flood. It tells us 

nothing of the magnitude of the flood except that it 

exceeded by an appreciable amount the flood that 

would have just reached the trees. Furthermore, most 

of the evidence is of the most recent floods and of those 

that have occurred since the most recent one of 

catastrophic size. These severe floods, as will be 

discussed in the following paragraphs, generally remove evidence of earlier ones of lesser magnitude.

All indirect methods of estimating the magnitude 

and frequency of floods are based on some type of 

record of hydrologic data, whether it be rainfall or 

streamflow. Thus, the value of the analysis is dependent upon the record. Dalrymple (1960, p. 13-14) 

stresses the importance of searching for historical 

evidence of extreme floods and showed that knowledge 

of only one flood that exceeded the maximum flood of 

record lengthens the record back to that historical 

occurrence.

The botanical evidence of floods discovered in the 

course of this study has not shown the occurrence of 

such a flood in the past. Two reasons, it is felt, are 

responsible for this failure. The maximum flood of 

record, from 1895 to 1963, occurred on March 19, 1936. 

All known historical evidence (Grover, 1937, p. 334; 

Sanderlin, 1946) indicates that the 1889 flood did not 

exceed the 1936 flood and the previous maximum flood 

in 18TT was appreciably smaller than these. Secondly, 

the floods of 1889 and 1936, as well as the ones in 1937 

and 1942, and the ice jam in 1948, probably destroyed 

evidence of any possible floods prior to 1877 that could 

have exceeded the 1936 flood.

The fact that one felled tree has been found that 

suggests evidence of a flood in 1861 (fig. 15) indicates 

that an intensive study might reveal botanical evidence 

of a flood that exceeded the maximum of record, 

thereby extending the record in time. The probability 

of finding this evidence in any flood-plain forest is 

extremely low; and on the Potomac River, where a 

catastrophic flood occurred 27 years ago, the probability is still lower. For some rivers and streams, however, especially those for which the record is short or for 

which several kinds of evidence strongly indicate the 

occurrence of floods many times higher than the

highest of record (Stewart and Bodhaine, 1961, p. 

25-27), the probability of finding botanical evidence is 

greater.

Erosion and deposition have been intensively studied 

by engineers, geologists, soil scientists, biologists, and 

others for more than 30 years, and even a brief 

summary of these studies is beyond the scope of this 

report. Much has been written about accelerated 

erosion, which is believed by some to be induced by 

man's use of the land, as opposed to natural erosion; 

however, doubt exists in the distinction between the 

two processes and of deposits resulting from them 

(Strahler, 1956, p. 623). The nature of the problems 

in measuring erosion quantitatively, summarized by 

Leopold (1956, p. 639-641), is such that the effect of 

man's use of the land upon erosion and deposition is 

most difficult to evaluate. The distinction between 

deposits laid down under the present climate and those 

laid down under past climates is difficult to make, and 

mechanical analysis of samples of some sediments of 

different origin fails to show significant differences 

(Hack, 1953, p. 185). Langbein and Hoyt (1959, p. 

183-184) emphasized that evidence of accelerated erosion and deposition in valley bottoms is based chiefly 

on the distinction between modern sediments and the 

buried "original" soil. They reported that physical 

and cultural characteristics of the deposits such as 

bottles, cans and charcoal layers are used for separating them. They (p. 184) stressed the need for research 

in the mechanism of the processes and for long-term 

observations of erosional and depositional phenomena.

The methods described here provide a means by 

which the rates of erosion and deposition at specific 

sites on banks and flood plains can be determined. 

Other advantages of the study of botanical evidence of 

floods and sedimentation phenomena are that, within 

limits, the trees can be studied at a time selected by the 

researcher and that the trees can contain a record that 

would otherwise require many decades of observation. 

Wolman and Eiler (1958, p. 8) found that after just a 

few months the sediment deposited by the catastrophic 

hurricane-Diane floods in August 1955 in New England 

was difficult if not impossible to distinguish from 

flood-plain deposits present prior to the floods. As 

Langbein and Hoyt (1959, p. 245) stressed, funds for 

the collection of basic water facts generally are not 

appropriated until some crisis stresses the need; thus 

there is a considerable lag between the need for data 

and theii* availability. Botanical evidence of floods 

also can reveal evidence of previously unknown historical floods, and thus provide an incentive for a study of 

other sources for evidence of these floods and of recent 

floods in places where intense application of indirect
A34 VEGETATION AND HYDROLOGIC PHENOMENA

hydrologic methods would provide an estimate of their 

magnitude and frequency.

LIST OF PLANTS

Ash _. Fraxinus pennsylvanica

Marshall 

Baldcypress . Taxodium distichum

(Linneaus) Richardson

Birch, river . Betula nigra Linneaus 

Boxelder Acernegundo Linneaus 

Cotton wood _. Populus deltoides Bartram 

Elm, American _. Ulmus americana Linneaus 

Maple, silver . Acer saccha.rinum Linneaus 

Oak, swamp white_. Quercus bicolor Willdenow 

Sycamore . Platanus occidentalis Linneaus 

Tupelo Nyssa sylvatica Marshall 

Walnut, black ___. Juglans nigra Linneaus 

Willow . Salix Linneaus

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BOTANICAL EVIDENCE OF FLOODS AND FLOOD-PLAIN DEPOSITION A35

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U. S. GOVERNMENT PRINTING OFFICE : 1964O - 732-764