GEOTLOCHCAL C>F
EFFECTS OF FLOODING UPON

WOODY VEGETATION ALONG PARTS OF

THE POTOMAC RIVER FLOOD PLAIN
Potomac River at Great Falls Park, Va., June 22, 1972.
Effects of Flooding Upon 

Woody Vegetation along Parts 

of the Potomac River Flood Plain

By THOMAS M. YANOSKY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1206

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Yanosky, Thomas M

Effects of flooding upon woody vegetation along parts of the Potomac River flood plain.

(Geological Survey professional paper ; 1206)

Bibliography: p.

1. Floodplain flora Potomac Valley. 2. Plants Effect of floods on Potomac Valley. 3. Forest ecology Potomac

Valley. 4. Potomac River Floods. 5. Flood damage Potomac Valley. I. Title. II. Series: United States. Geological

Survey.

Professional paper ; 1206.

QK191.Y36 581.5'26325 80-607999

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

Conversion factors................................................................... vi

Abstract............................................................................ 1

Introduction ........................................................................ 1

Acknowledgments ................................................................... 1

Literature review.................................................................... 1

Flooding on the Potomac River near Washington, B.C................................... 2

The monumented plot study .......................................................... 3

Methods ........................................................................ 3

The study areas ................................................................. 4

Chain Bridge................................................................ 4

Spalding farm flood plain .................................................... 5

Uplands .................................................................... 5

Changes in woody vegetation ..................................................... 5

Basal area .................................................................. 5

Survival .................................................................... 7

The effects of the Hurricane Agnes flood along the Potomac River downstream

from Great Falls ............................................................ 8

Effects of local site characteristics ................................................ 8

Species survival ................................................................. 13

Summary and conclusions............................................................ 16

References cited..................................................................... 19

ILLUSTRATIONS

Page

FRONTISPIECE. Potomac River at Great Falls Park, Va., June 22,1972.

FIGURE 1. Map of Potomac River near Washington, B.C., showing study areas at Chain Bridge and Spalding farm ................ 3

2. Map of study site below Great Falls............................................................................... 9

3, 4. Photographs showing

3. Potomac River Gorge below Great Falls.................................................................... 9

4. High water near Bifficult Run............................................................................. 9

5. Line drawing showing representation of sheltering during floods along inner and outer channel bends .................. 10

6-18. Photographs showing

6. Yellow Pond flood plain in March 1967 ..................................................................... 10

7. Lower tract of Yellow Pond flood plain in 1975.............................................................. 11

8. Upper tract of Yellow Pond flood plain in 1975.............................................................. 12

9. Rock outcrop adjacent to Yellow Pond and alluvial tract just downstream .................................... 14

10. Upper part of Old Angler flood plain during flood of February 1966........................................... 16

11. Old Angler flood plain during low flow, 1975 ................................................................ 17

12. Bownstream view from protective wall at head of Old Angler flood plain, 1975 ................................ 17

13. View of the lower part of Old Angler flood plain............................................................ 18

14. Madeira flood plain....................................................................................... 18

15. Silver maple forest on Madeira flood plain.................................................................. 19

16. Sheltered sycamore below Great Falls ..................................................................... 19

17. Flood-damaged sycamores below Great Falls ............................................................... 20

18. Bownstream view from Rocky Islands in 1968 .............................................................. 21
VI CONTENTS

TABLES

Page

TABLE 1. Summary of flooding at seven monumented plots during water years 1963-1972.......................................... 4

2. Basal area of woody vegetation at seven monumented plots............................................................ 6

3. Survival of plants at seven monumented plots ........................................................................ 7

4. Survival of single-stemmed and clumped plants at seven monumented plots............................................. 8

5. Summary of condition of trees still standing in July 1973 on a 20-m by 10-m plot on the Yellow Pond flood

plain just downstream from the protective wall................................................................... 13

6. Summary of condition of trees still standing in July 1973 on alluvial plot just downstream from the rock

outcrop adjacent to Yellow Pond................................................................................. 15

7. Summary of selected core data from Old angler flood plain (upstream tract) and Madeira flood plain along

the Potomac River.............................................................................................. 17

GLOSSARY

Adventitious growth. Stems or roots that arise from parts of the 

plant where they would not arise under ordinary conditions. Adventitious growth often occurs following injury from flooding.

Alluvium. Sediment deposited by moving water.

Basal area. Cross-sectional area of a stem or trunk at breast height.

Botanical evidence. Evidence from vegetation for the past or present occurrence of geomorphic processes or land use.

Crest. The highest altitude of a flood. 

Discharge. The rate of flow in a river or stream.

Flood. Any flow that rises above the banks and spreads across the 

flood plain.

Flood damage. In this report, the killing or injury of plants by 

flooding.

Flood plain. An area adjacent to a stream or river formed of deposition and erosion during floods.

Frequency. The average number of times per year that a given discharge is equaled or exceeded.

Monumented plot. In this report, study areas marked by concrete 

monuments that act as permanent reference points.

Peak discharge. The maximum discharge attained by a flood.

Recurrence interval. The average interval in years between flows 

that equal or exceed a given discharge.

Stage. The water-surface altitude of a stream.

Water year. The 12-month period, October 1-September 30.

Woody vegetation. In this report, trees and shrubs.

CONVERSION FACTORS

Units of measure used in this report are International System of Units (SI). The following factors can be used to convert SI units to U.S. 

customary units.

To convert from, 

centimeter (cm) 

meter (m)

kilometer (km) 

meter2 (m2) 

hectometer2 (hm2) 

meter3 per second (mVs)

To

inch (in) 

foot (ft) 

yard (yd) 

mile (mi) 

foot2 (ft2) 

acre 

foot3 per second (ftVs)

Multiply by 

2.54 

3.281 

1.094 

0.6214

10.76 

2.471

35.31
EFFECTS OF FLOODING UPON WOODY VEGETATION ALONG 

PARTS OF THE POTOMAC RIVER FLOOD PLAIN

By THOMAS M. YANOSKY

ABSTRACT

A two-part study along the Potomac River flood plain near Washington, B.C., was undertaken to investigate the effects of flooding upon 

woody vegetation. Floods abrade bark, damage branches and canopies, and often uproot trees. The first study was of vegetation in five 

monumented flood-plain plots, which differed in the frequency and 

severity of flood flow over a 10-year period. Basal area and survival of 

trees appear to be related to velocity of flood flow, which in turn is 

related to flood magnitude and channel shape. However, the effects of 

flooding also depend on the nature of the flood-plain surface and size 

and growth habit of vegetation.

In the second study, a catastrophic flood following Hurricane Agnes 

in June 1972 was found to cause large-scale changes in the age, form, 

and species composition of flood-plain forests below Great Falls, Va. 

The impact of the flood depended primarily on the flow regime of the 

river; destruction was greatest in areas exposed to the maximum flood 

force, and minimal at sheltered locations. Age determinations from 

dead trunks and surviving trees suggest that most trees in severely 

damaged areas started to grow after the last great flood, which occurred in 1942. Trees along sheltered reaches survived several previous catastrophic floods. In addition, species varied in their ability to 

withstand damage from the Hurricane Agnes flood. The least likely to 

recover were species growing on infrequently flooded surfaces, which 

may explain, in part, their absence at lower flood-plain altitudes.

INTRODUCTION

Flood-plain forests are generally different from those 

on adjacent terraces and uplands. These differences in 

part are related to the local frequency, duration, and 

velocity of floodflow and are important in gaining an 

understanding of the hydrologic environment. In this 

study, woody vegetation along a part of the Potomac 

River between Washington, B.C., and Great Falls, Va., 

was investigated to determine the effects of flooding on 

plant growth and survival. In the first part of the investigation, vegetation was studied on two reaches of 

flood plain which differ in flood frequency and severity 

of flow velocities during high water. Another study area 

was in an upland forest which is never flooded. Woody 

stems were initially mapped and measured by personnel 

of the U.S. Geological Survey in a series of monumented 

plots in 1962 (flood plain) and 1965 (upland). Stems were 

remapped and remeasured after the 1972 growing season, and data from the two mappings were compared.

The second part of the study concerns specific effects 

of the catastrophic flood following Hurricane Agnes in 

June 1972. The age, form, and survival of flood-plain 

trees were studied where destruction was locally variable. Areas exposed to the maximum current velocities 

of the flood were compared to nearby reaches where 

vegetation was sheltered from severe floodflows.

ACKNOWLEDGMENTS

The author wishes to thank Richard L. Phipps and 

Jane F. Hill for reviewing the manuscript and making 

helpful suggestions and Mrs. Esther R. Flint for her 

assistance with much of the typing. Thanks are extended to S. P. Spalding and the Madeira School for 

permission to conduct field studies on their properties. 

Thanks are also extended to Kenneth J. Fassler and 

John W. Zuke but most of all to Dr. Robert S. Sigafoos, 

who kindly provided much of the data in this report and 

greatly encouraged the author's interests in riparian 

vegetation and hydrology.

LITERATURE REVIEW

McGee (1891) defined a flood plain as land "within 

reach of the river and suffering overflows by freshets, or 

at least by great floods." More modern definitions emphasize dynamic associations between the stream and 

flood-plain formation (Kilpatrick and Barnes, 1964). 

During high flows, sediment may be deposited on flood 

plains, or previously deposited materials may be eroded 

away. The establishment of woody vegetation on flood 

plains is related to these processes in ways that are not 

completely understood. Seeds of some species germinate 

on thick alluvial deposits only after leaf litter and other 

organic debris have been removed by spring floods. Others are able to grow on thinly scattered alluvium overlying bedrock. Seedlings are often uprooted or buried by 

sediments due to flooding in late spring or early summer, suggesting that successful growth may coincide 

with periods of low flow (Sigafoos, 1964).

Once established, flood-plain forests may be inundated periodically for extended periods. Trees are often 

damaged by supernormal velocity and by debris transported at or near the water surface. Vegetation is most 

likely to be damaged or removed when velocity and 

quantity of debris are extreme, particularly when channels are choked with ice. Floods abrade bark, break 

branches and canopies, and often partially uproot trees. 

Although aerial parts may be lost, the roots are often 

not killed; stumps subsequently send out a profusion of 

adventitious sprouts and the tree survives.

Although literature on the ecology of flood-plain vegetation is abundant, most studies are primarily descriptions of bottomland communities and successional 

stages in their development. The most significant works
EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

are the vegetation studies along the Lower Illinois River 

(Turner, 1936), on flood plains of the Wabash and Tippecanoe Rivers, Ind. (Lindsey and others, 1961), and 

along the Raritan River, N.J. (Buell and Wistendahl, 

1955; Wistendahl, 1958). Succession on an island in the 

Mississippi River was investigated by Schull (1944) and 

on levees along the Illinois River by Turner (1931). 

These and other studies confirm differences in climate, 

soils, and species composition between upland and 

flood-plain forests. Most workers believed that vegetational zones along flood plains represent normal successional stages that in time will be replaced by adjoining 

upland species.

Sigafoos (1961) offered the alternative hypothesis 

that flood-plain vegetation is related to the frequency 

and magnitude of looding. In a study along the White 

River, Ark., Bedinger (1971) found four distinct vegetation zones related to the frequency and duration of 

yearly flooding. Although he did not conclude that this 

was necessarily cause and effect, he found strong evidence for it. It seems unlikely, however, that flood frequency and duration alone control the distribution of 

vegetation along many eastern rivers and streams. Unlike the White River flood plain, eastern flood plains are 

often highly variable in width and topography, and the 

generally steeper stream gradients result in greater velocities. Trees in some areas are continually damaged or 

killed by floods. Factors such as flood magnitude and 

site characteristics explain the more variable nature of 

vegetation.

Other investigations have concentrated on the effects 

of partial or complete inundation on flood-plain vegetation. Hosner (1958) found that first-year seedlings of six 

flood-plain species died within 32 days after complete 

submergence and that different species died at different 

rates. In more extensive studies, Hosner (1960) found 

that seedlings survived for longer periods when only the 

roots were flooded because the crowns retained the capacity for gas exchange. Turner (1930) found that few 

trees along the Illinois River were killed when flooded 

for long durations during the fall and early winter of 

1926 but suffered high mortality when inundated from 

April to mid-June in 1927. In a bottomland timber area 

permanently flooded by impoundment near the junction 

of the Illinois and Mississippi Rivers, Yeager (1949) 

found that standing water to a depth of 50 cm resulted 

in almost complete mortality of all trees within 8 years. 

Furthermore, mortality by species was highly variable, 

with white ash (Fraxinus americana L.) most resistant 

(8 years) and pin oak (Quercus pcdiistris Muenchh.) most 

susceptible (3 years).

Broadfoot and Williston (1973) state that chemical 

properties of soil determine many of the effects of inundation upon roots. Toxic reactions may occur in soils

with high sodium content, or in those which are highly 

acid. Harper (1938) attributed root mortality along the 

Canadian River, Okla., to oxygen deprivation after deposition of clay. Species vary in their abilities to develop 

adventitious roots and withstand leaf moisture deficits 

caused by the oxygen deficiency of saturated soils (Hosner and Boyce, 1962).

Other flood-plain studies have used botanical evidence to reconstruct the recent history of floods and 

flood-related phenomena. Sigafoos (1964) found that 

trees growing along the Potomac River near Washington, B.C., showed damage from numerous floods. By dating sprouts from inclined or broken trunks, flood dates 

could be determined. Sigafoos was also able to determine the year of burial of aerial shoots from changes in 

wood anatomy during subsequent growth. Similarly, 

Everitt (1968) used cottonwood (Populus sargentii 

Dode) as an indicator of channel movement and 

flood-plain development along the Little Missouri River, 

N. Dak.

Botanical evidence maybe valuable where stream-flow 

records are lacking or short (Sigafoos, 1964). Harrison 

and Reid (1967) tested this hypothesis by constructing a 

flood-frequency graph based on scarred trees along the 

Turtle River, N. Dak. Their graph approximated frequencies recorded in 25 years of hydrologic data. Helley 

and LaMarche (1973) determined radiocarbon dates for 

trees buried in ancient flood deposits to extend the 

record of known catastrophic floods of northern California streams.

FLOODING ON THE POTOMAC RIVER 

NEAR WASHINGTON, D.C.

A stream floods when the volume of flow is so great 

that stream stages overtop the banks and spread across 

the flood plain. Flow discharge is measured at streamgaging stations and is expressed in cubic meters per 

second (mVs). Once recorded, the annual peak discharges at a site can be used to define a flood-frequency 

curve, which relates discharge to recurrence interval 

(Patterson, 1964). For example, a flood with a recurrence interval of 2 years is equaled or exceeded on an 

average of once every 2 years or has a 50 percent probability of being equaled or exceeded in any 1 year. Discharges for the Potomac River near Washington, D.C., 

were continuously measured from 1930 to 1964 by a 

water-stage recorder 1.6 km upstream from Brookmont, 

Md., and from January 1965 to the present by a waterstage recorder with concrete control located at Brookmont. Datum, or zero of the gage, is 11.57 m, National 

Geodetic Vertical Datum of 1929.

Between 1930 and 1971, the flood of March 19, 1936 

(13,707 mVs), was the maximum recorded on the Potomac River near Washington, D.C. Large floods also
THE MONUMENTED PLOT STUDY

occurred in October 1942 (12,649 mVs) and April 1937 

(9,827 mVs). Prior to 1930, a flood in May 1924 was 

probably about 8,500 mVs, and a flood on June 2,1889, 

was approximately as great as that of 1936. These and 

all other streamflow data cited in this report were obtained from surface-water records of the U.S. Geological Survey.

The flood in the wake of Hurricane Agnes crested 

near Washington, B.C., at 10,167 mVs on June 24,1972 

(Bailey and others, 1975). This is the third highest discharge of the century, exceeded only in 1936 and 1942. 

The river rose to flood stage early on the morning of 

June 22 and remained above that level through June 25. 

Floods of this magnitude can be expected approximately once every 30 years (Darling, 1959).

The impact of floods in the Potomac River basin may 

be determined by factors other than magnitude of flow. 

Although floods at the same site are considered identical if they have the same peak discharge, no two floods 

have the same flow characteristics and hence the same 

potential for damaging vegetation. The duration of floodflow and the amount and type of debris may differ 

greatly. Most physical damage to trees appears to result 

from floating debris, particularly large logs or blocks of 

ice. Small winter floods preceded by unusually cold 

weather may culminate in destructive ice jams, such as 

those which occurred near Chain Bridge in 1948 and 

1968. Winter floods are generally of greater duration 

than summer freshets, exposing trees to longer periods 

of damaging flows.

Similarly, destruction along the river is often highly 

variable due to differences in gradient, channel sinuosity, width and depth, and obstructions in the bed. 

Maximum damage is most probable along steam sections where flood velocities are high, especially during 

debris-choked events of long duration. Damage is less 

likely along the higher altitudes of flood channels because roughness near shore reduces flow velocities. 

Even here, however, smaller plants may be buried under 

sediment or blocks of ice and larger trees may be 

damaged.

Most flooding along the Potomac River near Washington, D.C., takes place during winter or early spring, 

when vegetation is dormant. As was mentioned earlier, 

even prolonged inundation does not usually kill floodplain vegetation at this time, although continued 

submergence during the growing season may cause 

mortality to some species. However, flood durations seldom exceed a few days; even the greatest flows on record 

remained above flood stage for less than 5 days, and 

thus inundation alone probably kills only a small number of trees and shrubs.

THE MONUMENTED PLOT STUDY

METHODS

A series of seven monumented study plots was established in 1962 by R. S. Sigafoos along the Potomac River 

flood plain near Chain Bridge and on the upland near 

Washington, D.C. Three are on the flood plain in Washington, D.C., two are on the flood plain of the S. P. 

Spalding "Potowmack farm" in northern Fairfax 

County, and two are in an oak-hickory upland on the 

Spalding farm (fig. 1).

Each monumented flood-plain plot is a 0.04 hm2 circle 

with a radius of 11.3 m. A concrete post marks the 

center and acts as a reference point. All trees and 

shrubs 0.8 cm or greater in diameter were identified and 

measured by Sigafoos following the 1962 growing season. Diameter measurements were at breast height 

(dbh), except for saplings less than 2.5 cm in diameter, 

which were measured near the base. The location of 

each plant was mapped by use of a plane table. No 

herbaceous vegetation was recorded. After 10 growing 

seasons, plants were remeasured and remapped by Sigafoos and the author.

Many flood-damaged species bear multiple stems, 

often arising from a buried trunk. The normal growth 

habit of some flood-plain species, such as spicebush 

(Lindera Benzoin (L.) Blume) and red osier dogwood 

(Cormis Amomum Mill.), is almost invariably a shrublike clump of stems. Each clump was considered an individual plant, although the number of aggregated stems 

was recorded. The stem size assigned to each clump, 

thus to an individual plant, was that of the largest stem.

39'

VIRGINIA Brookmont 9aging station \/ WASHINGTON

Chain Bridge Vv D. C.

0 5 MILES

5 KILOMETERS

77°

FIGURE 1. Potomac River near Washington, D.C., showing study 

areas at Chain Bridge and Spalding farm. Base is fromWashington 

and Baltimore quadrangle, 1:250,000.
EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

From the 1962 and 1972 maps and data, the following 

computations were made for each plot: (1) total number 

of woody plants, (2) basal area (cross-sectional area at 

the point of measurement) of each species, (3) total 

basal area, which gives a measurement of standing crop 

per unit area, and (4) number of plants less than 2.5 cm 

in diameter and number greater than 2.5 cm. In addition, data were analyzed by species and size classes to 

determine which plants mapped in 1962 were still living 

in 1972. Surviving plants usually increased in basal area, although in some instances, as when large trunks 

were broken off and replaced by a mass of young 

sprouts, basal area was less.

The minimum discharge required to inundate each 

study plot was estimated from floodmarks and observations during high flows. Because the plots are nearly 

level, the discharge that inundates the fringes of the 

plot differs little from that which covers the entire area. 

The number of days that each plot was inundated is 

summarized in table 1.

The two upland study plots were originally mapped 

by Sigafoos and Phipps in 1965 (oral communication) 

and are contiguous 0.04-hm2 rectangles.

THE STUDY AREAS

CHAIN BRIDGE

The Potomac River near Chain Bridge flows in a 

narrow channel bordered on the Virginia shore by steep 

palisades and on the Maryland-Washington, B.C. side 

by a broad, bedrock flood plain extending to the Chesapeake and Ohio (C & 0) Canal. Channel width at this 

point increases dramatically from approximately 

35 m at low flow to more than 335 m at flood stage 

(2,945 mVs). Bedrock increases gradually in altitude to 

the east and is covered at higher altitudes by alluvial 

deposits of variable thickness.

The straight channel, heavy flow volumes, and steep 

river bed descent of 11.6 m in 2.4 km between Brookmont, Md., and Chain Bridge result in high current 

velocities during floods. Leopold (1953) reports a maximum point velocity of 6.7 m/s at Chain Bridge during 

a minor flood on May 14, 1932. I estimated a surface 

velocity in the mainstream of at least 9 m/s during the 

flood of June 24, 1972, which had a peak discharge 

10,167 mVs.1

Sigafoos (1961) lists 24 tree species common on the 

Chain Bridge flood plain. Predominant among these are 

sycamore (Platanus ocddentcdis L.), ash (Fraxinus L.), 

boxelder (Acer negundo L.), cottonwood (Populus deltoides Bartr.), river birch (Betula nigra L.), silver maple 

(Acer saccharinum L.), elm (Ulmus L.), and swamp 

white oak (Quercus tricolor Willd.). Most vegetation on 

the low bedrock flood plain is small and shrubby and 

shows damage from numerous floods and ice jams. 

Larger trees grow near the C & 0 canal, where floods are 

less frequent and flow velocities are lower. Alluvium 

overlies the bedrock. In spring, much of this area is a 

series of shallow swamps amid luxuriant masses of entangled vegetation.

The first permanent plot is about 60 m downstream 

from Chain Bridge and 35 m from the canal. This area 

is inundated by about 2,820 mVs, although it is not 

unusual for several years to elapse between floods of 

this magnitude. Leaf litter and other organic debris 

accumulate over the alluvium, and growth conditions 

for established trees are good. The largest trees in 1962 

included an elm 43 cm and sycamores 69 and 104 cm dbh.

Most trees mapped in 1972 showed some flood damage. Few vertical trunks were observed, and many were 

deeply abraded on the upstream side. Most of this damage resulted from the June 1972 flood, which inundated 

the plot for about 4 days to a depth of 4 m at the crest. 

Despite this, the three largest trees survived.

Permanent plot 2 is about 600 m upstream from 

Chain Bridge and 150 m from the canal. Just upstream

'Estimations of surface velocities in this report were made by the surface-float method.

TABLE 1. Summary offloading at seven monumented plots during 1963-72 water years

Location

Chain Bridge ___

flood plain ____

____

Spaldingfarm __

flood plain __ _

Spalding farm _

upland _ _____

Plot

1

2

3

4

5

6

7

Discharge inundating 

plot 1 (cubic meters 

per second)

2,830

2,270

1,420

2,550

2,830

Never flooded

Never flooded

Number of 

floods during 

1930-61 water years

29

42

105

37

29

Number of floods 

expected in period 

1963-72 2

9

13

33

12

9

Number of floods 

in period 

1963-72

5

16

34

10

5

Total 

days of 

inundation

10

27

3 83

17

10

1 Based on observation of stage at plot and simultaneous gage height for the Potomac River near Washington, D.C. Discharge at plot is believed to approximate discharge at gaging station.

2 Calculated to compare flood frequencies during the study interval with the earlier period of 32 years.

3 Includes 4 days representing peaks dependent on prior flows above 1,416 mVs (see Dalrymple, I960, p. 12, para. 4.)
THE MONUMENTED PLOT STUDY

from the plot is a man-made tailrace running diagonally from the canal to the river. Parallel to the downstream edge of the tailrace is a ridge, about 3 m high, 

that acts as a levee for much of the area immediately 

below. Hence the plot is initially inundated at about 

2,270 mVs by slow-moving backwaters that deposit alluvium over the bedrock. High velocities are reached only 

during floods exceeding about 2,900 mVs, when the tailrace levee is overtopped.

Alluvial deposits and lower frequencies of damaging 

floods in the plot have favored growth of larger trees 

than on bedrock areas of comparable elevation. Boxelder and ash exceeding 20 cm dbh were measured in 

1962. The Hurricane Agnes flood (June 1972) damaged 

or destroyed all woody vegetation, completely removing 

most large trees and leaving others horizontal.

Permanent plot 3 is just upstream from the tailrace 

and at about the same altitutde as plot 2. Flooding at 

plot 3 occurs at about 1,420 mVs, and swift currents 

scour the bedrock. Alluvial deposits are confined to 

shallow depressions and other surface irregularities, 

and it is there that plants are growing. Most trees are 

shrubby and seldom exceed 8 cm dbh. This is probably 

due in part to the open environment and to the continual breakage of aerial parts and subsequent resprouting.

SPALDING FARM FLOOD PLAIN

Two flood-plain plots are located about 27 km upstream from Chain Bridge on the S. P. Spalding "Patowmack farm", in northern Fairfax County, Va. The 

channel here is about 730 m wide and the river flows 

slowly over a gradient averaging only about 1 m/km 

between Seneca, Md., and Aqueduct Dam (just above 

Great Falls). The river is shallow and dotted with islands and occasional riffles. Much of the flood plain in 

the Spalding farm study area is bordered along the 

channel by steep banks and hence not flooded until 

bankfull stage is exceeded. Alluvial soils are at least 4 m 

deep in some locations (Sigafoos, oral communication, 

1975), and bedrock is exposed only along the edges of the 

main channel.

I estimated crest velocity of the June 1972 flood to be 

about 2-3 m/s in the main channel and at lower rates at 

increasing distance from the low-water channel. The 

main-channel velocity of smaller floods, such as those 

cresting March 22, 1963, and March 9, 1967, was estimated at approximately half that of the June 1972 flood.

Most woody species noted at Chain Bridge are also on 

the Spalding farm flood plain. Ash, boxelder, hackberry 

(Celtis occidentalis L.), sycamore, and elm are predominant, some exceeding 1 m in diamter. Less common are 

black walnut (Juglans nigra L.), basswood (Tilia americana L.), swamp white oak, swamp chestnut oak 

(Quercus michauxii Nutt.), and Shumard oak (Quercus

shumardii Buckl.). Shrubs and small trees such as 

spicebush and pawpaw (Asimina triloba (1.) Dunal) 

abound in the understory. The flood-plain forest borders 

an oak-hickory upland or ends abruptly in cultivated 

fields.

Permanent plot 4 is on Patowmack Island, a small 

alluvial island near the Virginia shore about 2.3 km downstream from the mouth of Seneca Creek. The plot and 

most of the island are inundated by about 2,550 mVs. 

Six trees greater than 25 cm dbh were mapped in 1962, 

the largest a basswood 55 cm dbh. Spicebush was abundant in the understory.

Permanent plot 5 is on the Virginia shore, opposite 

plot 4 and about 100 m from the channel. It is inundated 

at about 2,830 mVs. Eleven trees exceeded 25 cm dbh in 

1962, with a swamp white oak and a Shumard oak measuring 66 cm and 67 cm dbh, respectively. The understory 

was primarily spicebush and boxelder.

Vegetation on the Spalding farm flood plain shows 

damage from the Hurricane Agnes flood but not to the 

extent noted at Chain Bridge and at many other downstream areas. Some canopy and subcanopy sized trees 

were scarred or partly uprooted, although nothing comparable to the destruction of entire flood-plain forests 

was observed. Some small members of the understory 

were damaged or killed following burial by flood debris.

UPLANDS

Plots 6 and 7 are in an upland forest on the Spalding 

farm tract and have never been flooded. Most of this 

forest was selectively cut-over about the time of World 

War I (Sigafoos, oral communication, 1975) and is presently a privately owned wildlife refuge. Dominant canopy species include white oak (Quercus alba L.), scarlet 

oak (Quercus coccinea Muenchh.), red oak (Quercus rubra 

L.), black oak (Quercus velutina Lam.), mockernut hickory (Carya tomentosa Nutt.), pignut hickory (Carya glabra (Mill.) Sweet), and sour gum (Nyssa sylvatica 

Marsh.). Beech (Fagus grandifolia Ehrh.), tulip tree 

(Liriodendron tulipifera L.), cherry (Prunus L.), sassafras (Sassafras attndum (Nutt.) Nees), and flowering 

dogwood (Cornus florida L.) abound in the understory.

CHANGES IN WOODY VEGETATION

BASAL AREA

Numerous factors affect the growth and mortality of 

trees, and it is difficult to correlate changes in basal area 

solely with flooding. This study indicates that changes 

in basal area are most striking on frequently flooded 

surfaces at sites where flow velocities are high. Changes 

become less apparent on ground that is seldom flooded, 

and at sites where slower currents during high water 

are less likely to damage vegetation.
6 EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

Basal area measurements at upland and flood-plain 

locations are shown in table 2. If flood plain plots are 

ordered by the minimum discharge inundating their 

surfaces, basal area cannot be related solely to flood 

frequency because of differences in the channels at the 

two locations. As was mentioned previously, flow velocities are higher at Chain Bridge than at the Spalding 

farm. However, basal areas at Chain Bridge increased 

as inundation discharge increased. For example, the 

1962 basal area of plot 3 was 5.4 percent that of plot 2 

and 2.8 percent that of plot 1. In 1972, the percentages 

were 38.6 and 3.3, respectively. This agrees with most 

field observations that trees at Chain Bridge are largest 

on infrequently flooded surfaces and progressively 

smaller nearer the river. Sigafoos (1961) related flood 

frequency to three general vegetation zones based on 

tree size and form near Chain Bridge.

Near the Spalding plots, woody vegetation is scarce 

along the steep banks that separate the low-water channel from the flood plain. Trees along the low-water 

channel are generally smaller than trees on the flood 

plain. However, unlike at Chain Bridge, the average size 

of trees of the same species seemingly does not increase 

on less frequently flooded portions of the flood plain. 

The basal area of plot 5 was nearly twice that of plot 4 

after both mappings, even though both plots are inundated during great floods for approximately the same 

time interval (table 1). Plot 4, located more toward 

mainstream, may be exposed to greater flood velocities 

than is plot 5. Furthermore, near plot 5 are frequently 

flooded stretches with less sloping banks, commonly 

with large trees growing near the edge of the low-water 

channel. In some places, large trees grow on surfaces 

flooded by less than 550 mVs. These frequently flooded 

surfaces are exposed during floods to low velocities that 

deposit alluvium and rarely inflict serious damage to 

vegetation.

Differences between 1962 and 1972 basal area measurements were greatest at Chain Bridge, especially in 

the two most frequently flooded plots. Plot 2 decreased 

in basal area by 80 percent, while plots 1 and 3 increased

by 23 percent and 44 percent, respectively. At the Spalding farm, where flood velocities are not as great, plot 4 

decreased by 4 percent and plot 5 increased by 14 percent. The basal area of the two upland plots decreased by 

7 percent and 1 percent in a somewhat shorter period.

Because large trees make up a greater percentage of 

total basal area in most of the plots, their fate has the 

greater influence on changes in basal area. In plot 1, for 

example, three large trees accounted for nearly 85 percent of the total 1962 basal area. Had one or more been 

killed between mappings, the 1972 figure would have 

been far less. However, each survived, and the additional 10 years of radial growth accounted for an increase in basal area despite high mortality among many 

smaller trees from the Hurricane Agnes flood. Basal 

area will continue to depend on these three trees, and 

since they survived a major flood equaled or exceeded on 

the average once in 30 years, they are unlikely to be 

killed or uprooted by more common floods. Mortality is 

more likely from other agents, such as windthrow, disease, insects, lightning, or senescence. Large fluctuations 

in basal area within this plot may thus be independent 

of flooding.

On frequently flooded surfaces, however, the range in 

size is generally less than on infrequently flooded surfaces, and basal area does not depend on a few large 

trees. Changes in stem number result in high percentage changes in total basal area. Basal area declines 

when flooding destroys many stems or increases when 

there is a profusion of new growth. For example, all 

stems measured in plot 3 in 1962 had diameters less 

than 8 cm; thus basal areas of largest and smallest 

stems differed only slightly. Frequent floods and extreme velocities during high water have prevented the 

growth of large trees. Germination of seedlings and 

continued production of sprouts from established roots 

cause large percent increases in plot basal area. On infrequently flooded surfaces, such growth has a negligible percent effect on total basal area. On the other 

hand, because stems in plot 3 are repeatedly broken off 

during high water, there are also large percent

TABLE 2. Basal area of woody vegetation at seven monumented plots

Location

Chain Bridge

flood plain

Spalding farm

flood plain

Spalding farm

upland 1

Plot

1

2

3

4

5

6

7

Inundation (cubic 

meters per second)

2,830 

2,270 

1,420

2,550 

2,830

Never flooded 

Never flooded

Basal area 

(square meters)

1962

1.618 

0.841 

0.045

1.087 

2.123

1.234 

0.942

1972

1.992 

0.169 

0.065

1.040 

2.414

1.149 

0.931

Basal area 

change (percent)

+23.1 

-79.9

+44.4

-4.3 

+ 13.7

-6.9 

-1.2

' Upland plots were mapped in 1965.
THE MONUMENTED PLOT STUDY

decreases. Basal area probably fluctuates significantly, 

being greatest after the growing season and smallest 

after winter or early spring floods.

It thus seems probable that fluctuations in basal area 

at Chain Bridge are greatest on frequently flooded surfaces and become less pronounced at higher altitutdes. 

Further supporting evidence is that all other study 

plots, including those on the Spalding farm flood plain 

and on the uplands, derived more than 99 percent of 

1972 basal areas from continued growth of stems 

mapped in 1962; less than 1 percent resulted from stems 

which became measurable after that date. In plot 3, 

however, only 78.5 percent of the 1972 basal area resulted from originally mapped stems.

The 80 percent decline in basal area at plot 2 resulted 

primarily from the Hurricane Agnes flood. Perturbation on this scale had not occurred since 1948, when 

most vegetation was destroyed by an ice jam (Sigafoos, 

1964). The presence of trees up to 20 cm dbh and a total 

basal area of 0.84 m2 in 1962 suggest that even the August 1955 flood, 6,117 mVs, did not damage vegetation as 

severely as did the Hurricane Agnes flood. Minor floods 

during 1963-1971 water years, the largest being 4,163 

mVs in March 1967, probably removed only a small 

number of trees; hence, just prior to the Hurricane 

Agnes flood, basal area was probably at least as great as 

at the 1962 mapping.

SURVIVAL

The survival and continued growth of originally 

mapped plants are primarily responsible for changes in 

basal area. It has already been noted that survival depends to a great extent on the channel characteristics at 

a particular location and not on flood frequency and 

duration alone. In addition to these factors, the size and 

growth habits of vegetation are important. In none of 

the flood-plain plots, for example, did survival of plants 

less than 2.5 cm exceed that of larger plants; on the 

uplands, however, survival for both size classes was

about the same. At all sites, plants with multiple stems 

(clumps) survived at higher rates than did singlestemmed plants.

Numbers and survival percentages of plants at seven 

monumented plots are shown in table 3. On the uplands, 

90 percent of the originally mapped plants survived in 

comparison with 40 percent on the five flood-plain plots. 

Differences in age and species composition only partly 

explain the lower survival at flood-plain sites. Survival 

of flood-plain plants ranged from 7 percent at plot 2 to 

66 percent at plot 3; at plots 1, 4, and 5, which are 

inundated by flows exceeding 2,550 mVs, survival was 

approximately half that of the uplands.

Flood-plain plants with stem diameters exceeding 

2.5 cm consistently survived at rates higher than did 

smaller plants. In plots 4 and 5, survival of plants 

greater than 2.5 cm compares favorably with that on the 

uplands; at plot 1, however, survival was about 25 percent lower, probably as a result of greater flow velocities 

at Chain Bridge. A similar trend was observed for 

plants less than 2.5 cm in diameter; survival on the 

Spalding farm flood plain exceeded that at plot 1 by 

about 50 percent, and on the uplands nearly 91 percent 

of the plants survived. Better established root systems 

may account for the increased survival of larger plants, 

while smaller plants may be more easily uprooted or 

buried by debris.

Plots 2 and 3 represent the extremes of flood-plain 

survival. Vegetation at plot 2, normally sheltered by the 

tailrace levee just upstream, was nearly all destroyed. 

At plot 3 there is no sheltering effect, and flooding is 

even more frequent; yet this plot had the highest survival percentage of any flood-plain tract. Part of the 

explanation may be that because frequent flooding has 

kept the vegetation small, trees were not battered by 

surface debris during the higher stages of the Hurricane Agnes flood. The larger trees in plot 2 were destroyed during higher flood stages when velocities and 

amounts of surface debris appeared greater. Debris

TABLE 3. Survival of plants at seven monumented plots

Total number of plants

Location

Chain Bridge _ 

flood plain __

Spalding farm _ 

flood plain ___

Spalding farm _ 

upland 1

Plot

1 

2 

3

4 

5

6

7

Inundation 

(cubic meters 

per second)

2,830 

2,270 

1,420

2,550 

2,830

Never flooded 

Never flooded

1962

142 

226

47

204 

765

183 

161

1972

66 

16 

31

106 

336

164 

144

Proportion 

surviving 

(percent)

46.5 

7.1 

66.0

52.0 

43.9

89.6 

89.4

1962

66 

90 

23

160 

690

129 

120

Total plants 

<2.5 cm

1972

18 

5 

13

68 

275

121 

105

Proportion 

surviving 

(percent)

27.3 

5.6 

56.5

42.5 

39.9

93.8

87.5

1962

76 

136 

24

44

75

54 

41

Total plants 

>2.5 cm

1972

48 

11 

18

38 

61

43 

37

Proportion 

surviving 

(percent)

63.8 

8.1 

75.0

86.4 

81.3

79.6 

90.2

1 Upland plots were mapped in 1965.
8 EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

carried by flows of about 1-2 m above the surface of plot 

3 possibly causes the greatest amount of stem breakage. 

However, velocities at such discharges were estimated 

at about 1.5 m/s and are unlikely to uproot plants.

There were small trees at plot 2, and nearly all were 

destroyed. A second factor may be the bedrock character of the flood plain above the tailrace. Because bedrock 

is resistant to erosion over short intervals of time, the 

surface probably remained essentially unchanged by 

flooding over the 10-year period. Due to frequent flooding, only those plants most firmly anchored in the bedrock have survived. Great floods may have no greater 

impact on this area than more frequent floods. In plot 2, 

however, alluvium was severely eroded by the Hurricane Agnes flood, in places stripping it to the bedrock 

and washing away much of the vegetation. It seems 

likely, therefore, that the flood-plain surfaces also 

influence survival of vegetation.

Many plants tallied in both 1962 and 1972 consisted of 

multiple stems. For example, of the 101 stems mapped 

in plot 3 in 1962, only 31 were single-stemmed plants; 

the remaining 70 stems comprised 16 plants. It is unknown how and to what extent this growth habit 

affected survival of 1962 plants. In all seven plots, however, plants with multiple stems survived at higher 

rates than those with, single stems (table 4).

THE EFFECTS OF THE HURRICANE AGNES

FLOOD ALONG THE POTOMAC RIVER

DOWNSTREAM FROM GREAT FALLS

The effects of the Hurricane Agnes flood upon woody 

vegetation were observed in 1973 along a 4-km reach of 

the Potomac River downstream from Great Falls(fig.2). 

Below the steep cataracts of the Falls, the river enters 

a deep and narrow gorge along a fault (U.S. Department 

of the Interior, 1970) where extremely high flow levels 

and velocities are reached during floods (figs. 3 and 4). 

Vegetation growing along the exposed rock walls of the 

gorge is flooded several times each year and is generally

shrubby and of poor form. Sycamore, willow (Salix L.), 

elm, and ash are abundant. On higher, infrequently 

flooded terraces, red oak, chestnut oak (Quercus prinus 

L.), post oak (Quercus stellata Wangenh.), mockernut 

hickory, pignut hickory, and Virginia pine (Pinus virginiana Mill.) predominate.

In the lower part of the 4-km reach below Great Falls, 

the channel widens near Difficult Run and turns sharply 

to the northeast, followed closely by a sharp southeastern turn. Alluvium overlies the bedrock in numerous 

localities, supporting mixed flood-plain forests of sycamore, willow, boxelder, silver maple, river birch, and 

cottonwood. The four latter species are rare in the upper 

gorge.

Specific sites in the gorge and near Difficult Run were 

arbitrarily chosen where damage was greatest, and in 

other tracts where damage was minimal. At each site, 

the condition of flood-plain forests was related to the 

flow regime during floods. At some places, ages of survivors were determined. Also investigated was the ability of various flood-plain and upland-terrace species to 

recover from damage incurred during Hurricane Agnes. 

Tallies of species were taken in transects of arbitrary 

length and in several 10-m by 10-m plots. Species that 

are rare on the steep bedrock in the gorge were studied 

on broad alluvial flood plains near Difficult Run.

EFFECTS OF LOCAL SITE CHARACTERISTICS

This study indicates that maximum damage to vegetation occurred on flood plains adjoining outside bends 

of the main channel, where floodflow velocities appeared 

greatest. In these areas, large tracts of flood-plain 

forest were destroyed. Along the inside of channel 

bends, where trees were sheltered from high floodflow 

velocities, damage was minor (fig. 5). This "sheltering 

effect" was also observed on a local scale where destruction otherwise was greatest; some trees immediately 

downstream from rock outcrops suffered little damage 

even though other trees in the general vicinity were

TABLE 4. Survival of single-stemmed and clumped plants at seven monumented plots

Single-stemmed plants

Location

Chain Bridge

flood plain

Spalding farm

flood plain

Spalding farm

upland 1

Plot

_ _ 1

2

3

4

5

6

. _ _ 7

Number 

1962

139 

223 

31

153 

664

175 

153

Number 

surviving, 

1972

64 

15 

20

69 

270

156 

136

Proportion 

surviving 

(percent)

46.0 

6.7 

64.5

45.1 

40.7

89.1 

88.9

Number 

1962

3 

3 

16

51 

100

8 

8

Clumped plants

Number 

surviving, 

1972

2 

1 

11

36 

66

8 

8

Proportion 

surviving 

(percent)

66.7 

33.3 

68.8

70.6 

66.0

100 

100

1 Upland plots were mapped in 1965.
THE EFFECTS OF THE HURRICANE AGNES FLOOD DOWNSTREAM FROM GREAT FALLS

ii A1 tr\ )/Yellow Pond Island \\ '{ y/r\t^^ ~io:~

500 1000 METERSContour lnterval=100 Feet

APPROXIMATE SCALE

FIGURE 2. Potomac River downstream from Great Falls showing study area. Map is enlarged from Falls Church, Va., quadrangle, 1:24,000.

FIGURE 3. Downstream view of the Potomac River along the upper 

gorge below Great Falls in November 1968. Discharge is approximately 85 mVs. Floods such as those occurring in 1936,1937,1942, 

and 1972 covered the tops of the gorge.

destroyed. These findings suggest that characteristics 

of the main channel and flood plain, in addition to flood 

frequency and duration, determine many differences in 

composition, age, and form of flood-plain forests.

Many trees were killed on a 100-m by 30-m alluvial 

tract ("Yellow Pond flood plain") about 600 m east of

FIGURE 4. High water (about 4,250 mVs) in lower study area just 

upstream from Difficult Run. The water is about 6 m above summer 

levels.

Difficult Run. The location of this area and others is 

shown in figure 2. Prior to Hurricane Agnes, a silver 

maple-ash-cottonwood forest grew on this tract just 

downstream from a series of high rock walls. Even during the 4,163 mVs flood peak of March 9,1967, inundated 

trees were sheltered from main-channel velocities, and
10 EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

little damage was observed (fig. 6). The Hurricane 

Agnes flood, however, roared through a large gap in the 

rocks and swept across the downstream half of the 

tract, destroying all vegetation (fig. 7). In July 1973 a 

20-m by 10-m plot on this lower part supported no trees, 

and in some areas the alluvium had been stripped to 

bedrock. A plot of identical size on the upstream half of 

the flood plain supported 20 trees. Tops of the rock walls 

sheltering this plot remained above the Hurricane 

Agnes flood, and 8 of 11 trees directly below were still 

living (fig. 8). Four showed little or no damage. Trees in 

the remainder of the upstream plot, however, grew just 

below a smaller wall inundated by about 5,000 mVs, and 

only 3 of 9 trees survived (table 5).

At the age of the main channel and adjacent to Yellow 

Pond flood plain is a large rock outcrop. The upstream 

face forms a vertical cliff about 10 m above the river at 

low flow, with the downstream wall sloping steeply to an 

alluvial flat at its base. In 1973, this flat measured 

35 m by 20 m and was inundated by approximately 

1,130 mVs. Prior to Hurricane Agnes, willow and silver

FIGURE 5. Representation of sheltering during floods along inner 

and outer channel bends.

EXPLANATION

Low water channel

 Flood plain limits

Sheltering

FIGURE 6. The lower tract of the Yellow Pond flood plain along the Potomac River during the flood of March 9,1967, peak flow 4,163 mVs. Trees 

are sheltered from flood currents by a rock wall just upstream. View is from the Virginia shore toward the main channel, which flows from 

left to right.
THE EFFECTS OF THE HURRICANE AGNES FLOOD DOWNSTREAM FROM GREAT FALLS 11

FIGURE 7. The lower tract of the Yellow Pond flood plain in 1975. View is from the main channel looking toward the Virginia shore.

maple grew on the lower alluvium, with cottonwood, 

ash, and bitternut hickory (Carya cordifarmis (Wangenh.) Koc) predominating on the higher alluvial surfaces. Red oak, chestunut oak, and black locust (Robinia 

pseudoacacia L.) grew along the downstream rock wall. 

Vegetation is sheltered from high velocities, presumably until the entire outcrop is inundated at about 

8,500 mVs. Even at about 5,700 mVs, when water is 

within about 4 m of the top of the rock, a large eddy 

extends immediately downstream; minor abrasion and 

breakage of stems may be caused by large pieces of debris floating in the backwash, but large trees are rarely 

destroyed. The Hurricane Agnes flood completely inundated the outcrop and eroded away part of the alluvium below (fig. 9), battering vegetation with currents 

estimated to exceed 9 m/s. Although the number of 

uprooted trees is unknown, all willows and most small

trees were removed. Of those remaining, most were 

damaged, some so severely that they could be identified 

only by wood characteristics. Of 14 trees still standing 

in July 1973 on the downstream half of the tract, 10 were 

dead; 16 of 24 trees on the upstream half of the island 

were also dead (table 6).

Of four trees directly below the highest part of the 

rock wall, however, a red oak and two bitternut hickories showed no signs of damage. A third bitternut had 

a broken canopy. These four trees were cored with an 

increment borer to determine the years of germination. 

Unfortunately, all trees had rotten centers and exact 

ages could not be obtained. The partial core showed that 

the largest hickory started to grow before 1890. The 

crown-damaged hickory could be less precisely dated 

because a larger part of heartwood had rotted. The 

hickory was at least 38 years old at the time of the
12 EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

FIGURE 8. View of the upper tract of the Yellow Pond flood plain in 1975 and part of the rock walls just upstream. A small pond may be seen 

in the background at left. Note the striking difference between the upper and lower tracts of the flood plain. (See fig. 7.)

hurricane Agnes flood. Three cores from different sides 

of the red oak trunk averaged 24 years in 70 mm. Because this is only about one fourth of the total radius, 

the tree is probably considerably older.

At least three of these sheltered trees survived great 

floods in 1924,1936,1937, and 1942 that approximated or 

exceeded the discharge of the Hurricane Agnes flood. 

The oldest hickory (and perhaps the red oak) was alive 

during the flood of June 2, 1889, which was approximately the same magnitude as the March 1936 flood.

Downstream from the Yellow Pond flood plain, the 

river turns sharply to the southeast. Damage was minimal to trees on the Virginia shore ('Madeira flood 

plain') downstream from the inner bend. The gently 

sloping, alluvial flood plain supports a silver maple-ashcottonwood forest; some trees reach 1 m dbh, and many

grow on surfaces flooded by less than 700 mVs. Few 

trees were uprooted during the Hurricane Agnes flood, 

and most damage was confined to minor bark abrasions 

and shearing of low limbs.

On a 600-m reach of Maryland shore ('Old Angler 

flood plain') along and downstream from the outer bend 

most trees received the major force of the flood and 

were partly uprooted or destroyed (figs. 10,11, and 12). 

This was most striking on a 100-m reach along the upstream part of the Old Angler flood plain. During floods 

of less than about 4,250 mVs, this area is sheltered 

by a rock outcrop just upstream, and a silver maplesycamore-cottonwood forest developed. Downstream 

from this forest the sheltering effect is gradually lost, 

resulting in thin alluvial deposits or exposed bedrock at 

most locations. In contrast to the denser forest of larger
THE EFFECTS OF THE HURRICANE AGNES FLOOD DOWNSTREAM FROM GREAT FALLS 13

TABLE 5. Summary of condition of trees stitt standing in July 1973 on 

a 20-m by 10-m plot on the Yellow Pond flood plain just downstream 

from the protective wall

Species

Diameter at 

breast height 

(centimeters)

Status 1 Condition 2

Behind rock shelter covered by the Hurricane Agnes flood

Ash

Boxelder __ 

Boxelder __ 

Birch, river 

Blue beech _

Hickory, bitternut. 

Maple, silver.

15/13

8

10 

51/56

31

S/S

D

D

S

D

D 

D/D

D

S

4/4

Diameter unknown.

Possibly dead prior to 

Hurricane Agnes.

Behind rock shelter not covered by the Hurricane Agnes flood

Ash

Ash

Birch, river

Black locust

Blue beech

Blue beech

Blue beech

Tulip tree 3

Basswood

Hickory, bitternut __

Oak, red

31

20

10

10

13

8

5

51/10

13

38

10

S

S

D

S

S

D

D

S/S

S

S

S

2

1

1

2

1/1

3

1

Cut by beavers in

1973.

1 Status: D=Dead; S~ survived.

2 Condition: l=no or little flood damage; 2=partially uprooted; 3=horizontal; and 

4=broken or heavily damaged crown.

3 Diameter, status, and condition for two-trunked tree.

trees along the upstream reach, most vegetation on bedrock was small and shrubby prior to Hurricane Agnes, 

consisting primarily of sycamore, elm, ash, and willow. 

Several sycamore and cottonwood reaching 60 cm dbh 

grew on localized stretches of thick alluvium, particularly near the low-water channel. The Hurricane Agnes 

flood heavily damaged most vegetation on bedrock and 

uprooted many of the larger trees; some parts overlain 

with thin alluvium were eroded to bedrock (fig. 13).

All flood-plain trees along the upstream tract were 

destroyed or heavily damaged. Survival was greatest 

just downstream from the protective wall, but even 

there, trees were partly uprooted or broken along the 

trunk. Age determinations indicated that none of the 

trees were growing prior to 1944 (table 7), which strongly 

suggests that the forest had grown up since the last 

major flood (1942). On the Madeira flood plain (figs. 14 

and 15), however, trees were found that had germinated 

prior to floods, in 1924,1936,1937, and 1942. Large-scale 

destruction apparently did not occur. As a result of the 

sheltering effect below the inner bend, trees have a 

greater range in size and age. In addition, silver maple 

is dominant on the alluvium laid down by reduced current flow and rare on thinner alluvium and bedrock 

along lower parts of Old Angler flood plain.

In addition to the sheltering of large stretches of 

flood-plain forest, similar sheltering of smaller tracts 

was observed along the walls and rims of the gorge. Due 

to the velocity and flow levels attained by the Hurricane 

Agnes flood, destruction along this section was severe. 

In the midst of widespread damage, however, individual 

trees or small groups of trees immediately downstream 

from rock outcrops were often undamaged, particularly 

when the entire tree (or group) was sheltered (fig. 16). 

However, trees with canopies above the level of the shelter often were broken at some point along the trunk, 

commonly at altitudes approximating the top of the 

shelter (fig. 17).

While many examples of small-tract sheltering were 

observed, numerous other trees growing in apparently 

sheltered areas were destroyed. This suggests that sheltering decreases the probability of damage rather than 

entirely precludes it, although it is sometimes difficult 

to predict which areas will indeed provide shelter during high flows. Some flood plains just downstream from 

outcrops are inundated by severe backwash flows, resulting in destroyed or partly uprooted trees leaning 

heavily upstream. In some cases, deposition over roots 

kills trees that are undamaged by floating debris.

SPECIES SURVIVAL

A final consideration of the study below Great Falls 

concerns the survival of different species following the 

Hurricane Agnes flood. The poor form and sprout origin 

of many flood-plain trees are evidence of continued 

growth after numerous floods. However, observations in 

the summer of 1973 revealed that many damaged trees 

still standing after this flood died soon after the flood or 

within the following year. Furthermore, as the greater 

mortality of certain species indicates, differential survival is a factor in the distribution of flood-plain species.

Tallies of surviving trees do not completely describe 

the impact of the Hurricane Agnes flood, because the 

numbers and kinds of trees uprooted are unknown. 

Even frequently flooded trees, many of which previously 

had recovered from severe damage, were destroyed in 

great numbers as sandbars and gravel bars were completely eroded by the flood.

Willow and sycamore are the dominant trees on frequently flooded bedrock near Rocky Islands (fig. 2). 

Willow is most abundant along the lowest altitudes of 

the flood plain, often forming dense clumps, and is rare 

on surfaces flooded by more than about 570 mVs. Sycamore is also common, but also grows on less frequently 

flooded surfaces. Transects were established at elevations flooded by less than 850 mVs along bedrock channels at Rocky Islands and along the lower walls of the
14 EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

FIGURE 9. The large rock outcrop adjacent to Yellow Pond as seen from near the Maryland shore in 1975. Note living trees just downstream 

(to the left) from the rock wall. Several large cottonwood trees damaged by the Hurricane Agnes flood can be seen near the middle of the 

alluvial island; these trees were living in 1973, but most were dead at the time of the photograph. The alluvial island stretched at least 

20 m farther downstream prior to the flood.

gorge just downstream. A tally of 148 willow and 236 

sycamore was recorded. Of these, only 9 willow and 32 

sycamore were dead, some of which may have died prior 

to Hurricane Agnes or succumbed to agents unrelated 

to flooding. Similarly, Schull (1944) found that black 

willow (Salix nigra Marsh) along the Mississippi River 

withstood great floods that often damaged aerial parts 

but seldom killed roots.

Other common species included in these transects 

were elm, ash, red osier dogwood, and black haw (Viburnum prunifclium L.). Most trees were small and 

shrubby, and nearly all showed damage from the Hurricane Agnes flood. In addition, mortality was greater 

among ash and elm than among willow and sycamore; 

13 of 30 ash and 11 of 20 elm were dead.

Two 100-m transects at Rocky Islands along surfaces 

flooded by less than 2,800 mVs showed that despite 

severe damage most trees survived. An exception was 

one 10-m by 10-m tract where extreme deposition of 

unknown thickness had occurred. Seven trees still 

standing were dead. These included elm measuring 13, 

18, and 18 cm and ash measuring 10,20,23, and 23 cm at 

dbh. Trees were partially uprooted and girdled at points 

along the trunk, but no sprouting was observed.

Because river birch and cottonwood are rare on bedrock, these species were investigated downstream near 

the Yellow Pond flood plain and on transects along 

nearby reaches where both species are common. It was 

found that trees growing in areas sheltered from high 

velocities during the Hurricane Agnes flood generally
THE EFFECTS OF THE HURRICANE AGNES FLOOD DOWNSTREAM FROM GREAT FALLS 15

TABLE 6. Summary of condition of trees still standing in July 1973 on 

the alluvial flat just downstream from the rock outcrop adjacent to 

Yellow Pond

Species

Boxelder _. 

Boxelder 3 __ 

Persimmon.

Diameter at 

breast height 

(centimeters)

Status 1 Condition2

Downstream half of flat

Maple, silver

Maple, silver

Maple, silver

Maple, silver

Maple, silver

Maple, silver 3

Cottonwood

Cottonwood

25

51

25

10

25

8

31/10

41

61

S

D

D

D

D

D

S/S

D

S

2,4

2,4/2,4

Dead at start of 1974

10 

5/8 

15 

15/15

D 

D/D

D 

S/D

D

growing season

2,4

Diameter unknown

Upstream half of flat

Maple, silver 3

Maple, silver

Maple, silver

Maple, silver

Maple, silver 3

Cottonwood 3

Cottonwood

Cottonwood

Boxelder

Elm

Elm3 _

Ash

Ash

Ash__ _____

Ash

Ash

Ash

Ash

Ash_ _ _

Ash

Birch, river

Oak, red 4

Hickory, bitternut 4 _

Hickory, bitternut4 _

Hickory, bitternut 4 _

15/25

20

51

13

15/20

61/20

61

51

5

10

15/15

18

10

51

13

10

5

5

5

5

18

61

18

31

33

D/D

S

S

S

S/S

S/S

S

S

S

D

D/D

D

S

S

S

S

D

D

D

D

D

S

S

S

S

2

2

2

2/4

1/4

2

2

3

2

3

4

Cut by beavers

in 1973

1

4

1

1

1 Status: D=dead; S=survived.

2 Condition: l=no or little flood damage; 2=partially uprooted; 3=horizontal; and 4= 

broken or heavily damaged crown.

3 Diameter, status, and condition for two-trunked tree.

4 Grows directly below the highest part of the rock wall.

survived. For example, transects along the Madeira 

flood plain and parts of the Virginia shore downstream 

tallied 58 cottonwoods and 69 river birch. Of these, only 

6 Cottonwood and 12 river birch were dead. However, 

along reaches where many other flood-plain species survived severe damage, 20 of 34 cottonwood and 18 of 23

river birch were killed. In addition, several large cottonwood trees surviving the flood died during the 1973 or 

1974 growing season. Turner (1930) similarly observed 

high mortality of cottonwood along the Illinois River 

after the 1927 flood.

Trees were also tallied on surfaces that are inundated 

only by great floods. These high terraces were sampled 

on Rocky Islands at altitudes above the 6,117 mVs flood 

of August 1955 and have been flooded in this century in 

1924, 1936,1937,1942, and 1972 (fig. 18).

Species greater than 2.5 cm dbh were counted on 

three main terraces. In addition, a 25-m transect and 

two plots measuring 10-m by 10-m were made at similar 

altitudes along parts of the Maryland shore just downstream from Rockey Islands. Dominant species are Virginia pine, post oak, and red oak; mockernut hickory, 

pignut hickory, black locust, and shadbush (Amelanchier Med.) are less common.

Virginia pine grows mainly on shallow alluvium 

flooded by more than 5,750 mVs and was the most abundant species tallied. Of 218 individuals, 129 were dead 

and many were partly uprooted (often nearly horizontal) or broken off along the trunk or canopy. Most survivors had minor damage, such as shearing of small 

stems or bark abrasions along upstream-facing trunks, 

and none showed adventitious sprouting. Most adjacent 

hardwoods, however, survived despite often severe damage to aerial parts and, in addition, rarely were partly 

uprooted. The greater proportion of uprooted pines may 

be due to their establishment in marginal sites where 

roots do not firmly anchor to underlying bedrock or to 

the generally shallow root systems. It thus appears that 

the survival of Virginia pine is precluded by damage 

often well tolerated by hardwood species.

Post oak is rare on surfaces flooded by less than about 

4,250 mVs and, like Virginia pine, is confined to alluvium on terraces underlain by bedrock. It is rare on 

surfaces that are never flooded. Red oak grows at all 

elevations along the Potomac River but is most abundant on infrequently flooded surfaces. Both species near 

Rocky Islands were tallied less often than was Virginia 

pine, and most individuals were heavily damaged. 

Sprouting along basal parts was common, however, and 

28 of 46 post oak and 24 of 29 red oak were still living. 

Other species in these infrequently flooded areas included mockernut and pignut hickory, although only 18 

trees were tallied. Ten were living. Black locust, shadbush, chestnut oak, red cedar (Juniperus virginiana L.), 

and persimmon (Diospyros virginiana Mill.) were less 

common.

These results are preliminary and are not an attempt 

to rank species by differential mortality due to flood 

damage. The high survival rates of bedrock floodplain species, particularly those at frequently flooded
16 EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

FIGURE 10. View of the flood plain forest on the upper part of Old Angler flood plain along the Potomac River during the flood of February 

15, 1966, peak flow 2,512 ms/s. Trees are sheltered from main-channel velocities by a rock wall just upstream.

elevations, suggest that even a great flood does not 

significantly alter species composition. Form of trees, 

however, may be highly altered by growth of new 

sprouts from broken trunks, resulting in aerial shoots 

much younger than roots. Vegetative reproduction appears to be the primary manner by which new aerial 

stems are produced. Seedlings are locally rare on frequently flooded bedrock, probably due to the lack of 

seeds or available sites for their establishment or to 

mortality from flooding.

On infrequently flooded surfaces, great floods may 

decrease numbers of individuals and alter species composition due to site factors or different genetic tolerances to inundation and damage. The time of year at 

which flooding occurs may also be important, with tolerance probably being lowest during the growing season. 

The absence at lower altitudes of Virginia pine, post 

oak, and mockernut and pignut hickory is likely due in 

part to these species' intolerance of continued flooding. 

The recovery of severely damaged trees in the subcanopy may also depend on the fate of larger trees within

the stand, for not only are genetic considerations important in recovery but also the amount of light is often 

critical in sprouting of adventitious stems.

SUMMARY AND CONCLUSIONS

Flooding is the dominant environmental influence 

upon flood-plain forests of the Potomac River near 

Washington, B.C. Because catastrophic floods are particularly destructive to vegetation, such floods determine the age, form, and composition of many flood-plain 

forests. Trees are uprooted or severely damaged during 

smaller floods, but the simultaneous destruction of 

great numbers of trees is unlikely.

The variability in destruction by the Hurricane 

Agnes flood indicates that damage is mainly due to factors other than flood discharge. Characteristics of the 

flood channel determine where flood velocities are 

greatest. Destruction was most evident near Chain 

Bridge and the reach below Great Falls, where maximum velocities during the Hurricane Agnes flood were 

observed; conversely, the slower-flowing currents above
SUMMARY AND CONCLUSIONS 17

FIGURE 11 View from the middle of Old Angler flood plain looking 

toward main channel during low flow in 1975. Note sprouts from 

inclined boxelder (right) that survived the Hurricane Agnes flood. 

Much of the flood debris was cleared to make way for a canoe 

launching site.

Great Falls caused relatively little damage to trees on 

the Spalding farm flood plain. Even below Great Falls, 

however, destruction was highly variable. Many forests 

normally sheltered during smaller floods were totally 

destroyed when sheltering was lost during the Hurricane Agnes flood. Other areas are sheltered even during 

the greatest recorded floods, and widespread damage 

has apparently never occurred. As a result, trees in 

close proximity may differ strikingly in size and age 

depending on the shelter afforded. Similarly, the composition of flood-plain forests may be controlled by the 

different abilities of species to become established on 

highly altered surfaces and subsequently to withstand 

severe mechanical damage.

Flood-plain forests at many sites are destroyed by 

catastrophic floods, grow again, and are uprooted or 

killed during subsequent great floods. Understanding 

the rate of recurrence of these events is important in 

applying botanical evidence to determining past hydroFIGURE 12. A downstream view in 1975 from the protective wall at 

the head of Old Angler flood plain. Survival was greatest just below 

the wall (foreground), where numerous sprouts have grown from 

buried or severely damaged trunks.

logic events. Evidence may extend records of great 

floods or indicate recent occurrence on streams where 

data are unavailable. Evidence of catastrophic floods is 

most probably in areas where streamflow velocities are 

greatest and vegetation is not sheltered by obstructions 

or the channel geometry.

TABLE 7. Summary of selected core data from Old Angler flood plain 

(upstream tract) and the Madeira flood plain along the Potomac 

River

Species Core height 

(centimeters)

Tree diameter 

(centimeters)

Age at onset of

1972 growing season

(years)

Old Angler's flood plain (upstream tract)

Cottonwood

Boxelder

Sycamore

Sycamore

Sycamore

Sycamore

Silver maple

Silver maple

Willow ___ _____

25

20

15

10

stump

stump

stump

stump

25

38

36

23

23

31

18

(partial core)

18

(partial core)

25

25

25

25±1

25±1

24

28
18 EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

FIGURE 13. The lower part of Old Angler flood plain 3 years after the Hurricane Agnes flood. Note the contrasting state of the Madeira

flood plain on the opposite shore.

TABLE 7. Summary of selected care data from, Old Angler flood plain 

(upstream tract) and the Madeira flood plain along the Potomac 

River Continued

Species Core height Tree diameter 

(centimeters) (centimeters)

Age at onset of 

1972 growing season 

(years)

Madeira flood plain

Ash

Ash

Ash

Ash

Silver maple

Silver maple

31 56

46 41

20 23

91 43

71 61

38 89

29 

92 

(partial core) 

33 

78 

(partial core) 

48 

(partial core) 

40 

(partial core) FIGURE 14. View of the Madeira (Virginia) flood plain opposite the 

lower portion of Old Angler flood plain. Photograph was taken in 

1975 from midstream.
REFERENCES CITED 19

Flooding directly or indirectly influences the distribution of vegetation on flood plains. Just as upland and 

flood-plain forests are composed of different species, 

there appear to be vegetation zones on the flood plain 

based on some of the factors considered in this report. 

These zones do not represent successional stages in 

which flood-plain species are eventually replaced by upland species. Zonation is undoubtedly further complicated by vast differences in the topography and lateral 

size of flood-plain surfaces. Studies are needed that relate flooding to the distribution of flood-plain species, 

possibly leading to a botanical delineation of flood-plain 

boundaries.

REFERENCES CITED

Bailey, J. F., Patterson, J. L., and Paulhus, J. L. H., 1975, Hurricane 

Agnes Rainfall and Flood, June-July 1972; U.S. Geological Survey 

Professional Paper 924, 403 p.

Bedinger, M. S., 1971, Forest species as indicators of flooding in the 

Lower White River Valley, Arkansas, in Geological Survey Research 1971: U.S. Geological Survey Professional Paper 750-C, p. 

C248-C253.

Broadfoot, W. M., and Williston, H. L., 1973, Flooding effects on southern forests: Journal of Forestry, v. 71, p. 584-587.

Buell, M. F., and Wistendahl, W. A., 1955, Flood plain forests of the 

Raritan River: Torrey Botanical Club Bulletin, v. 82, p. 463-472.

Dalrymple, T., 1960, Flood frequency, Manual of hydrology: Part III. 

Floodflow techniques: U.S. Geological Survey Water-Supply 

Paper 1543 A, 104 p.

Darling, J. M., 1959, Floods in Maryland, magnitude and frequency: 

U.S. Gelogical Survey Open-File Report, 9 p.

Everitt, B. L., 1968, Use of the cottonwood in an investigation of the 

recent history of a flood plain: American Journal of Science, v. 266, 

p. 417-439.

Harper, H. J., 1938, Effect of silting on tree development in the flood 

plain of Deep Fork of the North Canadian River in Creek Co.: 

Oklahoma Academy of Science Proceedings, v. 18, p. 46-49.

Harrison, S. S., and Reid, J. R., 1967, A flood frequency graph based on 

treescar data: North Dakota Academy of Science, Proceedings, 

v. 21, p. 23-33.

Helley, E. J., and LaMarche, V. C., Jr., 1973, Historic flood information 

for Northern California streams from geological and botanical 

evidence: U.S. Geological Survey Professional Paper 485-3, 16 p.

Hosner, J. F., 1958, The effects of complete inundation upon seedlings 

of six bottomland tree species: Ecology, v. 39, p. 371-373.

___ 1960, Relative tolerance to complete inundation of fourteen 

bottomland tree species: Forest Science, v. 6, p. 246-251.

Hosner, J. F., and Boyce, S. G., 1962, Tolerance to water saturated soil 

of various bottomland hardwoods: Forest Science, v. 8, p. 180-186.

Kilpatrick, F. A., and Barnes, H. H., Jr., 1964, Channel geometry of 

Piedmont streams as related to frequency of floods: U.S. Geological Survey Professional Paper 422-E, 10 p.

Leopold, L. B., 1953, Downstream change in velocity of rivers: American Journal of Science, v. 251, p. 606-624.

Lindsey, A. A., Petty, R. 0., Sterling, D. K., and Van Asdall, W., 1961, 

Vegetation and the environment along the Wabash and Tippecanoe Rivers: Ecological Monographs, v. 31, p. 105-156.

McGee, W. J., 1891, The flood plains of rivers: Forum, v. 11, p. 221-234.

Patterson, J. L., 1964, Magnitude and frequency of floods in the United 

States, Part VII. Lower Mississippi River basin: U.S. Geological 

Survey Water-Supply Paper 1681, 636 p.

FIGURE 15. Silver maple forest on Madeira flood plain in 1975. Many 

trees grow on surfaces flooded by less than 700 mVs.

.* .<iti*t TtfiffTiiiti~TTTIlTl*~~ii 1

FIGURE 16. Sycamore growing in a shelter behind a rock wall downstream from Great Falls in 1975. This tree is larger and of different 

form than trees growing in more exposed locations. Direction of 

current is from left to right. Discharge is about 255 mVs.
20 EFFECTS OF FLOODING UPON WOODY VEGETATION, POTOMAC RIVER FLOOD PLAIN

FIGURE 17. View of several sycamore broken along the trunk during the Hurricane Agnes flood at altitudes corresponding to shelter height. 

Photograph was taken in 1975 and looks directly upstream from a small cove in the lower study area.
REFERENCES CITED 21

Schull, C. A., 1944, Observations of general vegetational changes on a 

river island in the Mississippi River: American Midland Naturalist, v. 32, p. 771-776.

Sigafoos, R. S., 1961, Vegetation in relation to flood frequency and 

magnitude near Washington, D. C., in Geological Survey Research 1961: U.S. Geological Survey Professional Paper 424-C, 

p. C248-250.

___ 1964, Botanical evidence of floods and flood-plain deposition: 

U.S. Geological Survey Professional Paper 485-A, 35 p.

Turner, L. M., 1930, The 1926-27 floods and the Illinois River Valley 

vegetation: Illinois State Academy of Science, Transactions, v. 22, 

p. 95-97.

___ 1931, Plant succession on levees in the Illinois River Valley: 

Illinois State Academy of Science, Transactions, v. 24, p. 94-102.

___ 1936, Ecological studies in the lower Illinois River Valley:

Botanical Gazette, v. 97, p. 689-727.

U.S. Department of the Interior, 1970, The river and the rocks: 46 p. 

Wistendahl, W. A., 1958, The flood plain of the Raritan River, New

Jersey: Ecological Monographs, v. 28, p. 129-153. 

Yeager, L. E., 1949, Effect of permanent flooding in a river-bottom

timber area: Illinois Natural History Survey, Bulletin, v. 25,

p. 33-65.

FIGURE 18. View from Rocky Islands looking downstream along the 

Potomac River in 1968. Discharge is about 850 mVs. The main river 

is in the background. Upland species were tallied on high pinnacles 

that are rarely flooded.

 h U.S. GOVERNMENT PRINTING OFFICE: 1982-361-594/113