Scientific Investigations Report 2009–5130

Prepared in cooperation with the National Park Service

Modeling Flood Plain Hydrology and Forest Productivity of 

Congaree Swamp, South Carolina

U.S. Department of the Interior

U.S. Geological Survey
Prepared in cooperation with the National Park Service

Scientific Investigations Report 2009–5130

U.S. Department of the Interior

U.S. Geological Survey

Modeling Flood Plain Hydrology and 

Forest Productivity of Congaree Swamp, 

South Carolina

By Thomas W. Doyle
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Doyle, T.W., 2009, Modeling flood plain hydrology and forest productivity of Congaree Swamp, South Carolina: U.S. 

Geological Survey Scientific Investigations Report 2009–5130, 46 p.
iii

Contents

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

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

Coastal Plain Forest Ecosystems .......................................................................................................2

Productivity Studies of Coastal Plain Forests ..................................................................................2

Environmental Setting..........................................................................................................................3

Congaree Swamp ........................................................................................................................3

Saluda Dam...................................................................................................................................4

Climate...........................................................................................................................................4

Temperature.........................................................................................................................4

Precipitation ........................................................................................................................6

Palmer Drought Severity Index ........................................................................................8

Hurricanes..........................................................................................................................10

Purpose and Scope ............................................................................................................................10

Methods.........................................................................................................................................................10

Water Level Monitoring .....................................................................................................................10

Tree Core Collections and Measurement.......................................................................................12

Growth Calculations and Analysis...................................................................................................12

Results ...........................................................................................................................................................15

Hydrological and Trail Elevation Surveys .......................................................................................15

Flood Plain Inundation Model...........................................................................................................19

Tree-Ring Analysis..............................................................................................................................27

Forest Age and History .............................................................................................................27

Loblolly Pine Chronology Development and Productivity ...................................................28

Loblolly Pine Climate-Growth Relations.................................................................................30

Baldcypress Chronology Development and Productivity ...................................................31

Baldcypress Climate-Growth Relations.................................................................................34

Saluda Dam Effects on River Stage and Tree Growth.........................................................37

Summary........................................................................................................................................................40

Acknowledgments.......................................................................................................................................42

References ...................................................................................................................................................42
iv

Figures

1. Map of the South Carolina Midlands showing public lands, rivers, lakes, 

and location of Congaree National Park...................................................................................3

2. Graphs showing temperature records for the central region (Division 6) of South 

Carolina by month and year for the period of record from 1895 to 2007..............................4

3. Graphs showing precipitation records for the central region (Division 6) of South 

Carolina by month and year for the period of record from 1895 to 2007..............................6

4. Graphs showing Palmer Drought Severity Index (PDSI) records for the

 central region (Division 6) of South Carolina by month and year for the period of 

record from 1895 to 2007..............................................................................................................8

5. Graph showing chronology of estimated windspeeds at Congaree National Park

 from North Atlantic tropical storms and hurricanes passing over South Carolina 

in the last 120 years based on HURASIM model projections..............................................11

6. Graph showing average daily stage of maximum, mean, and minimum flows of the 

Congaree River at Columbia, S.C., by Julian date for the period 1935–2000.....................11

7. Photograph of water level gage at Fort Motte railroad crossing on the 

lower Congaree River on the southeastern boundary of 

Congaree National Park ............................................................................................................12

8. Map of water level gage locations within Congaree National Park 

in reference to longitudinal compartments that correspond to river grade.....................13

9. Aerial photography mosaic of Congaree National Park showing locations 

of park boundary, hiking trails, and tree-coring sites of primary species, 

baldcypress and loblolly pine ...................................................................................................14

10. Graphs showing stage records for the Congaree River at Columbia, S.C. .......................16

11. Graphs showing the number of days that water levels equaled or exceeded

 select stage heights for the Congaree River at Columbia, S.C., over the period 

of record from 1892 to 2000 .......................................................................................................18

12. Photograph of trail at Congaree National Park and laser level equipment 

used to establish vertical elevations of ground and water surfaces.................................20

13. Typical hydrograph of Congaree River stage and the Boggy Gut gage 

in the backswamp over a 4-year period (2002–6)..................................................................20

14. Photograph of damaged trail bridge across a narrow drain in Congaree 

National Park following high floods in spring 2003 ...............................................................21

15. Graphs showing relation of stage height records and flood loop patterns for the 

upstream gage of the Congaree River at Columbia, S.C., and the 

downstream in-channel gage at the western boundary of 

Congaree National Park ............................................................................................................22

16. Graphs showing stage relation of hourly readings from the Congaree River

 gage and the Boggy Gut gage in Congaree National Park..................................................23

17. Graphic user interface of the Congaree National Park flood plain 

inundation model illustrating functional buttons for predicting 

current and 24-hour forecasts of flood conditions on park trails.......................................24

18. Graphic user interface of the Congaree National Park flood plain 

inundation model illustrating concurrent gage heights from the 

U.S. Geological Survey National Water Information System Web 

site for the Congaree River at Columbia, S.C., and downstream 

park gages on the Congaree River and Cedar Creek ...........................................................24
v

19. Illustrations of park trail condition predicted by the Congaree National 

Park flood plain inundation model across the trail network at select 

stages of the Congaree River at Columbia, S.C. ....................................................................25

20. Features of the Congaree National Park flood plain inundation model 

highlighting specific park trails of interest with an enlarged map 

view of trail information and photo stops in winter and summer in 

an online virtual hike format .....................................................................................................26

21. Graphs showing mean growth chronologies of annual basal area increment

 from loblolly pine populations at seven coring sites within Congaree 

National Park graphed by hydrogeomorphic association...................................................29

22. Graph showing a 7-year running mean of basal area increment chronologies

 among the different loblolly pine populations and sampling sites .....................................30

23. Graphs showing relation between mean annual basal area increment of

 loblolly pine growth with various climate and hydrology parameters...............................32

24. Graphs showing mean basal area increment growth chronologies of

 baldcypress populations within Congaree National Park by hydrogeomorphic 

association...................................................................................................................................35

25. Graph showing mean basal area increment growth chronologies of

 baldcypress by hydrogeomorphic groupings contrasting productivity of

 riverbank sites versus backswamp sites................................................................................36

26. Graph showing a 7-year running mean of riverbank and backswamp

 baldcypress chronologies ........................................................................................................36

27. Graphs showing relation between mean annual basal area increment of

 riverbank baldcypress growth with various climate and hydrology parameters ............38

28. Graph showing the number of flood days above select stages at 35.3 m, 

 36.0 m, and 36.6 m mean sea level for the Congaree River at Columbia, S.C.,

 for predam and postdam periods.............................................................................................40

29. Graphs showing mean annual basal area increment of loblolly pine and

 baldcypress populations for different predam and postdam periods on more

 or less flooded sites in Congaree National Park ...................................................................41

Tables

1. Age and diameter ranges based on tree core analysis of 

select hardwood tree species at Congaree National Park .................................................28

2. Site descriptions, site name abbreviations, and age and 

diameter ranges of loblolly pine tree core collections at 

Congaree National Park ............................................................................................................28

3. Site descriptions, site name abbreviations, and age and 

diameter ranges of baldcypress tree core collections at 

Congaree National Park ............................................................................................................34

4. Contrasts of mean annual basal area increment over a 

15-year period for five successive periods before and after 

construction and operation of the Saluda Dam near Columbia, S.C..................................41
vi

Conversion Factors and Datums

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Area

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

square centimeter (cm2) 0.001076 square foot (ft2)

square centimeter (cm2) 0.1550 square inch (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Flow rate

meter per second (m/s) 3.281 foot per second (ft/s) 

Temperature in degrees Celsius (oC) can be converted to degrees Fahrenheit (oF) as follows: 

 oF = (1.8 x oC) + 32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
Introduction 1

Modeling Flood Plain Hydrology and Forest Productivity of 

Congaree Swamp, South Carolina

By Thomas W. Doyle

Abstract

An ecological field and modeling study was conducted 

to examine the flood relations of backswamp forests and park 

trails of the flood plain portion of Congaree National Park, 

S.C. Continuous water level gages were distributed across 

the length and width of the flood plain portion—referred to 

as “Congaree Swamp”—to facilitate understanding of the lag 

and peak flood coupling with stage of the Congaree River. A 

severe and prolonged drought at study start in 2001 extended 

into late 2002 before backswamp zones circulated floodwaters. 

Water levels were monitored at 10 gaging stations over a 

4-year period from 2002 to 2006. Historical water level stage 

and discharge data from the Congaree River were digitized 

from published sources and U.S. Geological Survey (USGS) 

archives to obtain long-term daily averages for an upstream 

gage at Columbia, S.C., dating back to 1892. Elevation of 

ground surface was surveyed for all park trails, water level 

gages, and additional circuits of roads and boundaries. 

Rectified elevation data were interpolated into a digital 

elevation model of the park trail system. Regression models 

were applied to establish time lags and stage relations between 

gages at Columbia, S.C., and gages in the upper, middle, and 

lower reaches of the river and backswamp within the park. 

Flood relations among backswamp gages exhibited different 

retention and recession behavior between flood plain reaches 

with greater hydroperiod in the lower reach than those in the 

upper and middle reaches of the Congaree Swamp. A flood 

plain inundation model was developed from gage relations to 

predict critical river stages and potential inundation of hiking 

trails on a real-time basis and to forecast the 24-hour flood 

condition on the trail network within the park.

In addition, tree-ring analysis was used to evaluate the 

effects of flood events and flooding history on forest resources 

at Congaree National Park. Tree cores were collected from 

populations of loblolly pine (Pinus taeda), baldcypress 

(Taxodium distichum), water tupelo (Nyssa aquatica), green 

ash (Fraxinus pennslyvanica), laurel oak (Quercus laurifolia), 

swamp chestnut oak (Quercus michauxii), and sycamore 

(Plantanus occidentalis) within Congaree Swamp in highand low-elevation sites characteristic of shorter and longer 

flood duration and related to upriver flood controls and dam 

operation. Ring counts and dating indicated that all loblolly 

pine trees and nearly all baldcypress collections in this study 

are postsettlement recruits and old-growth cohorts, dating 

from 100 to 300 years in age. Most hardwood species and 

trees cored for age analysis were less than 100 years old, 

demonstrating robust growth and high site quality. Growth 

chronologies of loblolly pine and baldcypress exhibited 

positive and negative inflections over the last century that 

corresponded with climate history and residual effects of 

Hurricane Hugo in 1989. Stemwood production on average 

was less for trees and species on sites with longer flood 

retention and hydroperiod affected more by groundwater 

seepage and site elevation than river floods. Water level data 

provided evidence that stream regulation and operations 

of the Saluda Dam (post-1934) have actually increased 

the average daily water stage in the Congaree River. There 

was no difference in tree growth response by species or 

hydrogeomorphic setting to predam and postdam flood 

conditions and river stage. Climate-growth analysis showed 

that long-term growth variation is controlled more by spring/

summer temperatures in loblolly pine and by spring/summer 

precipitation in baldcypress than flooding history.

Introduction

Congaree National Park was established in 1976 by 

congressional mandate “... to preserve and protect for 

the education, inspiration, and enjoyment of present and 

future generations an outstanding example of a near-virgin 

southern hardwood forest” (Public Law 94–545, 1976). The 

park holdings include an 8,903-ha old-growth bottomland 

hardwood flood plain— referred to as “Congaree Swamp”—

along the Congaree River southeast of Columbia, S.C. The 

flood plain has an elevation slope and fall of 6 m from the 

upstream to downstream boundary. Hydrology is considered 

the most influential force driving landscape geomorphology 

and the forest dynamics of bottomland hardwood swamps 

indicative of a diverse and complex pattern of vegetative 

communities (Wharton and others, 1982; Thompson, 1998). 
2 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Forests of baldcypress (Taxodium distichum) and tupelo 

(Nyssa spp.) occupy low-elevation and flooded backswamp 

areas of old meander bends, while the higher flood plain 

zones are intermittently flooded and dominated by a diverse 

bottomland hardwood ecosystem interspersed with loblolly 

pine (Pinus taeda) populations. The operation of the Saluda 

Dam, 78 km upstream of the park, has reduced the magnitude, 

frequency, and duration of high flood events since its 

construction in 1930 (Whetstone, 1982; Patterson and others, 

1985). Dam operation and streamside developments that 

could potentially change river stage and water quality, thereby 

posing a threat to forest diversity and productivity of Congaree 

Swamp, are major concerns of park management. Dewatering 

measures and flow regulation of river systems have led to 

water declines elsewhere that have propagated legal disputes 

among States and municipalities and put downstream 

ecosystems at risk. 

Coastal Plain Forest Ecosystems

Congaree National Park represents a remnant old-growth 

forest under wilderness protection that is typical of Coastal 

Plain alluvial flood plain forest. Forests of the Coastal Plain 

region of the Southeastern United States are among the 

most productive in North America and form the basis of a 

large timber and wood products industry (Quarterman and 

Keever, 1962). Unmanaged forests in parks and preserves 

are an important complement to managed forests for critical 

wildlife and conservation needs (for example, Marks and 

Harcombe, 1981; Bridges and Orzell, 1989; Ware and others, 

1993). Unfortunately, little quantitative data are available 

to facilitate the understanding of interannual variability and 

seasonality of climate and riverflow on Coastal Plain forest 

habitats. Few studies or tools are available to predict how 

Coastal Plain forest ecosystems will respond to either natural 

or anthropogenic changes in regional climate and watershed 

dynamics.

Old-growth forests are rare entities worldwide in need 

of special management and conservation measures. The 

Congaree National Park is a complex mosaic of wetland and 

upland forest associations that has been shaped largely by 

alluvial processes and natural climatic phenomena apart from 

any substantial influence by humans. The Congaree Swamp 

forests are among the tallest temperate deciduous forests, 

ranging from 30 to 50 m in height. Many State and national 

champion trees have been discovered at the park (Gaddy, 

1977; Jones, 1997). Timber company records of parcels within 

the park indicated that baldcypress was selectively logged 

around the turn of the 20th century. Otherwise, half of the park 

area (4,450 ha; R. Sharitz, University of Georgia, Savannah 

River Ecology Laboratory, oral commun.) is regarded as oldgrowth forest that has escaped intensive or extensive land-use 

activities by humans except primitive access roads for hunting 

purposes. 

Productivity Studies of Coastal Plain Forests

Long-term forest observations are valued commodities 

for determining the sensitivity and response of biological 

systems to environmental change. The ability to distinguish 

long-term changes in forests from short-term fluctuations 

caused by riverflow alterations or climatic anomalies may 

be impossible without long-term monitoring and tree-ring 

studies. For example, results of a long-term study at Big 

Thicket National Preserve demonstrated that only now, after 

20 years of continuous observations, can long-term trends 

related to human activities be distinguished from short-term 

responses to natural climatic perturbations (Harcombe and 

others, 1998). Generally, abrupt changes in the flood regime 

of a swamp environment, drier or wetter, can lead to abrupt 

change in growth response of flood-tolerant bottomland 

hardwood species (Conner and Day, 1992; Keeland, 1994; 

Young and others, 1995). Studies of baldcypress growth 

have shown that flowing and fluctuating water regimes 

stimulate more growth and healthier trees than do stagnant 

and continuously flooded swamps (Brown, 1981; Lugo and 

Brown, 1984; Conner and Flynn, 1989; Dicke and Toliver, 

1990; Megonigal and Day, 1992). Changes in water quality 

such as increased nutrient concentrations can spur increased 

forest productivity in receiving wetlands (Hesse and others, 

1998; Day and others, 2006). Dewatering effects leading to 

streamflow reduction in the Apalachicola River flood plain 

ecosystem over the last few decades were shown to shift forest 

composition toward drier forest types of fewer flood-tolerant 

tree species (Darst and Light, 2008). In Congaree Swamp, 

forest plots were established to inventory the damage and 

recovery of bottomland hardwood forests following Hurricane 

Hugo in 1989 (Putz and Sharitz, 1991; Sharitz and others, 

1993; Allen and others, 1997; Allen and Sharitz, 1999); these 

plots have been remeasured every 4 years since and may offer 

some much needed insight into the rate and process of forest 

regrowth and regeneration as related to disturbance and flood 

history. 

Tree-ring studies of temperate forest species offer 

the means to quantify forest productivity, to examine 

climate-growth relations, and to relate growth patterns to 

environmental effects and disturbance events (Fritts, 1976; 

Fritts and Swetnam, 1989). Tree rings from old-growth 

sites provide centuries of climate information on the basis 

of ring size and density as opposed to relatively short-term 

observations from the existing network of meteorological 

stations within the Coastal Plain region, most of which 

were placed in service post-1948. Streamflow and drought 

reconstructions from tree rings of baldcypress have been 

achieved to determine the probabilities of water surplus and 

deficit cycles in select watersheds in Arkansas and North 

Carolina over millennial time scales (Stahle and others, 1985, 

1988; Cleaveland, 2000). Baldcypress forests in coastal river 

outlets of South Carolina demonstrated negative growth effects 

from salinity exposure elevated by extended droughts, high sea 

level, and storm surge overwash from tropical storms (Doyle 
Introduction 3

and others, 2007). Tree growth variations in flood plain sites 

dominated by baldcypress along the Savannah River below 

Thurmond Lake Dam, Ga., demonstrated growth alterations 

synchronously affected by streamflow regulation with dam 

operation (Palta, 2005). Cook and others (2001) conducted 

a regional assessment of climate history for the West Gulf 

Coastal Plain including parts of Louisiana and east Texas by 

using tree-ring chronologies from 38 site locations within and 

surrounding Big Thicket National Preserve. Their analysis 

chronicled the effects of logging and settlement periods, 

prolonged drought episodes, and hurricane disturbance in the 

region. Differences in the sensitivity of species to seasonal 

rainfall, drought, and temperature extremes were identified 

between pine and prominent hardwood species. The density of 

record-size trees in Congaree Swamp provided a comparative 

opportunity to further our understanding of how climate and 

flooding interact to control productivity and species response 

in South Atlantic Coastal Plain ecosystems. 

 Environmental Setting

Congaree Swamp

Congaree National Park is a unit of the National Park 

Service located in the South Carolina Midlands, approximately 

35 km southeast of the State capital, Columbia (fig. 1). The 

park was established as a national monument in 1976 when 

Congress set aside 6,126 ha to protect an outstanding example 

of an old growth bottomland hardwood forest, known as 

Congaree Swamp. Considered the largest remaining intact 

tract of its kind in the United States, this unique wetland 

ecosystem was preserved for scientific, educational, and 

recreational purposes. In 1988, Congress funded the expansion 

of the park boundaries by designating 6,075 ha as wilderness 

and 2,768 ha as potential wilderness, bringing the total acreage 

Lake Murray

Saluda River

Broad River

Congaree River

Columbia

Richland County, South Carolina

0 4.5 9 18 27 36

Kilometers

Congaree National Park

Saluda Dam

City area

Interstate highway

Figure 1. Map of the South Carolina Midlands showing the public lands, rivers, lakes, and location of Congaree National Park, 

approximately 35 km southeast of the State capital, Columbia.
4 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

of the park to approximately 8,900 ha. In 2003, the South 

Carolina congressional delegation introduced legislation 

to elevate the status of the park from its early beginnings 

as a monument to that of a national park. In addition, the 

legislation expanded the park’s eastern boundary by 1,862 ha, 

stretching southeast to the confluence of the Congaree and 

Wateree Rivers. 

Congaree Swamp is situated in the southeast corner 

of Richland County along the north bank of the Congaree 

River. The lands surrounding the swamp are mostly cultivated 

for agriculture and timber production, but commercial and 

industrial interests are encroaching from upstream. The 

Congaree Swamp floods on average 10 times a year from 

flashy, short-duration pulses (Patterson and others, 1985). The 

Congaree River originates with the confluence of the Saluda 

and Broad Rivers in the City of Columbia. 

Saluda Dam

Saluda Dam was completed in the early 1930s to create 

Lake Murray, a large hydroelectric reservoir, from the Saluda 

River near its confluence of the Broad River and headwaters of 

the Congaree River. Construction was initiated on September 

21, 1927, and was completed on June 30, 1930. The drainage 

area of the Congaree River basin is approximately 21,471 km2. 

A real-time gage is maintained by the U.S. Geological Survey 

(USGS) on the left bank at the southwest boundary of the park 

at river mile 150.7 (lat 33°48’38’’, long 80°52’02’’, Richland 

County, S.C.).

37.8

26.7

15.5

4.44

-6.77

0

Month

Temperature, in degrees Celcius

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Figure 2. Temperature records for the central region (Division 6) of South Carolina by month and year for the period of 

record from 1895 to 2007 (data source: National Oceanic and Atmospheric Administration, National Climatic Data Center, 

Asheville, N.C.). A, Mean monthly temperature by year for the period of record. B, Mean spring/summer (March–August) 

temperature by year. C, A 7-year running mean of spring/summer temperature. D, Number of months with above average 

monthly temperature by year.

A

Climate

The entire park boundary and all sampling sites are 

located within Richland County, S.C., and Division 6 of the 

National Oceanic and Atmospheric Administration (NOAA) 

State Climate Divisions for South Carolina. Monthly climate 

data were obtained for air temperature, precipitation, and 

Palmer Drought Severity Index for the period of record from 

1895 to present from the NOAA National Climatic Data 

Center. 

Temperature

Mean monthly temperature ranges from a winter low in 

January of 7.7°C to a summer high in July of 26.9°C (fig. 2A). 

The mean spring/summer temperature pattern from March to 

August during the growing season for trees varies from year to 

year from lows near 20.5°C to highs near 23.1°C (fig. 2B). A 

7-year running mean of spring/summer temperature exhibits 

abrupt swings in high versus low periods (fig. 2C). A general 

trend can be detected showing monotonically increasing 

temperature over the first 55 years from 1900 to 1955 followed 

by a rather sharp decline in temperature to a centennial low 

beginning in the early 1960s followed by an incremental rise 

to the end of the 20th century (fig. 2C). This spring/summer 

trend in the long-term temperature pattern closely follows the 

number of months of above average monthly temperature for 

the entire year, indicating dramatically hotter and cooler years 

for single and successive years (fig. 2D). 
Introduction 5

B

C

D

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

20.5

21.1

21.7

22.2

22.8

23.3

Year

Temperature, in degrees Celcius

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

20.8

21.1

21.4

21.7

21.9

22.2

22.5

Year

Temperature, in degrees Celcius

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

2

4

6

8

10

12

Year

Number of months
6 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Precipitation

Mean monthly precipitation ranges from a winter low 

in November of 6.0 cm to a summer high in July of 13.8 cm 

(fig. 3A). The spring/summer precipitation pattern from March 

to August during the growing season for trees varies from 

year to year from lows less than 6.4 cm per month to a high 

near 17.8 cm per month (fig. 3B). A 7-year running mean of 

spring/summer precipitation exhibits a zig-zag pattern of high 

and low periods (fig. 3C). The number of months with above 

average precipitation shows a fairly evenly distributed pattern 

of wet and dry years (fig. 3D).

A

B

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

5

10

15

20

Year

Mean precipitation, in centimeters

0

5

10

15

Month

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Precipitation, in centimeters

Figure 3. Precipitation records for the central region (Division 6) of South Carolina by month and year for the period of record 

from 1895 to 2007 (data source: National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, N.C.). 

A, Mean precipitation by month for the period of record. B, Mean spring/summer (March–August) precipitation by year. C, A 

7-year running mean of spring/summer precipitation. D, Number of months with above average monthly precipitation by year.
Introduction 7

C

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

8.9

10.1

11.4

12.7

14.0

Year

7-year

mean precipitation, in centimeters

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

2

4

6

8

10

Year

Number of months

D
8 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Palmer Drought Severity Index

The Palmer Drought Severity Index (PDSI) is an 

integrated function of precipitation and temperature that 

approximates soil moisture recharge predicated on current 

month conditions and previous month index values on a 

relative scale from +6 to -6 (positive values indicate wetness, 

and negative values indicate dryness and prevailing drought) 

(Palmer, 1965). Mean monthly PDSI ranges from a spring 

low (May) of -0.35 to a summer (June) high of -0.08 (fig. 

4A). The spring/summer PDSI pattern fluctuates from +4 to 

-4 in the March–August growing season (fig. 4B). A 7-year 

running mean of spring/summer PDSI exhibits an oscillating 

downward trend to the mid-1950s before rising sharply 

to a high of +1.5 by the early 1960s followed by another 

downtrend to the mid-1980s followed by another upturn and 

downturn by the end of the century (fig. 4C). The number of 

months with above average PDSI by year exhibits a distinct 

multiyear drought period in the 1950s (fig. 4D). The seasonal 

and annual trends for PDSI generally follow the pattern of 

precipitation and to a lesser degree have an inverse relation 

with temperature.

Figure 4. Palmer Drought Severity Index (PDSI) records for the central region (Division 6) of South Carolina by month and year 

for the period of record from 1895 to 2007 (data source: National Oceanic and Atmospheric Administration, National Climatic Data 

Center, Asheville, N.C.). A, Mean PDSI by month for the period of record. B, Mean spring/summer (March–August) PDSI by year. C, 

A 7-year running mean of spring/summer PDSI. D, Number of months with above average monthly PDSI by year.

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

-6

-4

-2

0

2

4

6

Year

Mean monthly PDSI

A

B

-0.4

-0.3

-0.2

-0.1

0

0.1

Month

PDSI

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.
Introduction 9

C

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

-3

-2

-1

0

1

2

Year

Mean PDSI

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

2

4

6

8

10

12

Year

Number of months

D
10 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Hurricanes

Hurricanes are large-scale climatic phenomena 

that generate high winds, tidal surges, and inland floods 

responsible for wide-area damage and destruction of 

developed property and forest resources. Direct effects of 

windthrow, branch breaking, and leaf stripping are commonly 

observed within storm swaths that can be tens and even 

hundreds of miles wide and long as storms move inland. The 

disturbance impacts of major hurricanes on stand structure, 

composition, and successional processes on bottomland 

hardwood forests were investigated after Hurricane Hugo in 

1989 (Gardner and others, 1991; Gresham and others, 1991; 

Hook and others, 1991; Putz and Sharitz, 1991) and Hurricane 

Andrew in 1992 (Doyle and others, 1995). Even though 

Congaree National Park is inland from the coast, the strength 

of Hurricane Hugo was still producing low Category 3 winds 

(51.4 m/s) sufficient to cause moderate to severe treefall 

damage within the park (Putz and Sharitz, 1991; Sharitz and 

others, 1993) and lingering impacts on forest recovery and 

woody vine density (Allen and Sharitz, 1999).

Hurricanes can modify the growth potential of trees 

either by creating canopy gaps or by direct injury of crown 

branches and leaf loss that can add a confounding influence 

to interpreting growth trends (Doyle and Gorham, 1996). 

Generally, hurricanes lose strength as they make landfall and 

proceed inland. A hurricane simulation model, HURASIM 

(Doyle and others, 1995; Doyle and Girod, 1997), was applied 

to reconstruct hurricane history and windfield distribution over 

Congaree National Park to gain a perspective of the dates and 

magnitude of storms capable of inflicting forest damage and 

influencing tree growth in Congaree Swamp. Model results 

showed that Hurricane Hugo was the strongest storm (in terms 

of windspeed) to impact the park in the last 120 years (fig. 5). 

A total of 15 storms have been recorded from 1886 to present 

with at least tropical storm wind strength (>17.4 m/s) passing 

over the park. Of these, six were strong enough to circulate 

hurricane force winds above 33.0 m/s. Only two storms have 

exceeded 44.7 m/s winds over the last century, including 

Hurricane Hugo, indicating that some forest thinning and 

disturbance from hurricanes may occur once every 50–60 

years. Some tree species show greater resistance to wind 

damage, most notably baldcypress, which is prominent in the 

Congaree Swamp (Putz and Sharitz, 1991; Doyle and others, 

1995).

Tropical depressions and storms are also capable of 

causing major flood events in Coastal Plain and Piedmont 

rivers including the Congaree River. Because of the relative 

infrequency of storms, there is too little effect on overall mean 

river stage during summer and fall months of hurricane season 

to be an important factor on tree growth. Maximum river 

stage records for 1935–2000 show the expected rise in spring 

streamflow and corresponding flood potential with periodic 

tropical storms and weather systems in late summer and fall 

(fig. 6). 

Purpose and Scope

The purpose of this ecological study was to assess, 

characterize, and predict the effects of hydrological 

fluctuations of the Congaree River on park resources. Study 

tasks focused on tree-ring analysis and the development of a 

flood inundation model to predict the effects of flood events 

and flood history on forest resources and visitor safety. Park 

management lacked the scientific basis and understanding 

of how riverflow and stage of the Congaree River influenced 

backswamp flood relations, forest productivity, and inundation 

of hiking trails. While hydrological studies have been 

conducted to predict floodwave behavior between river 

reaches, there has been nominal monitoring of the actual flood 

plain to relate flood patterns and flows in backswamp areas. 

The upstream Saluda Dam was constructed in the 1930s, 

creating Lake Murray and effectively reducing historical flood 

frequency and magnitude downstream (Whetstone, 1982; 

Patterson and others, 1985). Park officials were interested 

in knowing whether dam operations and river flow over the 

past century had influenced the productivity of tree species 

within Congaree Swamp. Specific project objectives included 

(1) monitoring backswamp water levels and surveying the 

elevation of park trails; (2) simulating flood plain inundation 

in relation to Congaree River stage; and (3) developing 

tree-ring chronologies of select species from different 

hydrogeomorphic settings to determine effects of flooding 

related to historical streamflow regulation.

Methods

Water Level Monitoring

To determine flood plain inundation with river stage, 

water level recorders (gages) were installed in drainage guts 

along the river and feeder creeks and sloughs within the 

interior backswamp zones (fig. 7). Gages were deployed 

across the landscape gradient in the upper, middle, and lower 

reaches of Congaree Swamp proximal to trail locations and 

tree-coring sites (fig. 8). Gages consisted of transducer units 

(Infinity model 210) programmed to record continuous hourly 

readings. Gages were installed in the fall of 2001, but because 

of a severe and prolonged drought, there was no backswamp 

flooding until late fall of 2002. In contrast, 2003 was a high 

water year that brought several extreme floods that resulted 

in destruction of some gages and hiking trail bridges, which 

hampered backcountry access and tree core collections. Water 

level data were collected over a 4-year period from late 2002 

through the winter/spring of 2006. 
Methods 11

26.8

31.3

35.8

40.2

44.7

49.2

53.6

1886

1890

1894

1897

1903

1908

1913

1918

1924

1928

1934

1939

1944

1949

1954

1959

1964

1970

1976

1981

1987

1993

19992006

Year

Windspeed, in meters per second

Figure 5. Chronology of estimated windspeeds at Congaree National Park from North Atlantic tropical storms and hurricanes 

passing over South Carolina in the last 120 years based on HURASIM model projections. A total of six storms of hurricane strength 

have impacted the park forests since 1886; Hurricane Hugo represents the strongest storm, with estimated windspeeds at 51.4 m/s.

0

1.5

3.0

4.6

6.1

7.6

9.1

10.7

1 27 53 79 105 131 157 183 209 235 261 287 313 339 365

Julian date

Relative gage height, in meters

Mean

Minimum

Maximum

Figure 6. Average daily stage of maximum, mean, and minimum flows of the Congaree River at Columbia, S.C., by Julian date for 

the period 1935–2000, illustrating the expected rise in spring streamflow and corresponding flood heights with periodic tropical 

storms and weather systems in late summer and fall.
12 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Figure 7. Photograph of water level gage (Infinity model 210) at 

Fort Motte railroad crossing on the lower Congaree River on the 

southeastern boundary of Congaree National Park.

Tree Core Collections and Measurement

Tree cores were collected from multiple species and 

forest stands to facilitate understanding of forest history and 

effects of flooding on long-term forest productivity. Species 

selection and sampling intensity focused on two primary 

conifer species and five secondary hardwood species to 

accomplish the development of tree-ring chronologies and 

age-size relations, respectively. Primary species selection 

included common conifer species, loblolly pine and 

baldcypress, on the basis of their wide distribution, various 

hydrogeomorphic settings, and contrasting flood tolerance. 

Multiple populations of each of these primary species were 

sampled across the flood plain in settings of greater or lesser 

flood retention and hydroperiod (temporary [dry], seasonal 

[bnk], and semipermanent [wet]; fig. 9). Some common 

hardwood species (green ash [Fraxinus pennslyvanica], laurel 

oak [Quercus laurifolia], swamp chestnut oak [Quercus 

michauxii], sycamore [Plantanus occidentalis], and water 

tupelo [Nyssa aquatica]) were also cored to establish age-size 

relations and determine aspects of forest and flood history.

Three different types of hydrogeomorphic settings were 

identified within Congaree Swamp that were also associated 

with primary tree-coring sites described herein as riverbank, 

backswamp, and seepage zones. Riverbank areas are located 

directly adjacent to the main channel on the highest ground 

and on slopes defining the natural levee and full-bank stage. 

Trees in this zone usually include mixed hardwood species 

sometimes interspersed with baldcypress associates. Flooding 

is somewhat intermittent and seasonal, occurring during the 

highest flood events when reaching full-bank flood stage and 

usually of short duration in days or less. Backswamp areas are 

located beyond the natural levee in the interior flood plain at 

lower elevations that store floodwaters and remain inundated 

for longer periods. Seepage zones are old meander bends 

(oxbows) at the base of the flood plain escarpment that receive 

a fairly constant supply of groundwater seepage and surface 

water runoff from adjacent upland areas. These areas remain 

fairly saturated near soil surface year round but undergo deep 

flooding when the river stage nears full bank and overtops 

levees. Upland drainage and rainfall can also bring localized 

flooding in backswamp depressions and sloughs without river 

overflow.

Tree core and site collections were composed of 

10–15 trees per species in proximity within a common 

hydrogeomorphic setting and forest stand. In all cases, 

two cores were extracted from opposite sides of each 

tree. Diameter at breast (core) height, tree height, and 

percent live crown were recorded for each tree to calculate 

stemwood production (basal area increment) from ring-width 

measurements and to associate crown class with tree vigor. 

In the case of buttswell trees and species, such as water 

tupelo and baldcypress in frequently flooded zones, cores 

were taken above the buttswell, usually 2–3 m above ground 

level. Standard dendroecological procedures were applied 

to collect, process, date, and measure annual growth rings 

(Stokes and Smiley, 1964). The dating process involved 

identifying a synchronous growth history within trees and 

sites and assigning a calendar year to each ring, the pattern 

of which corresponds with year-to-year variation associated 

with climate and environmental changes. Ring measurement 

was accomplished with the aid of a dissecting microscope, 

graduated stage, and optical encoder device to the nearest 0.01 

mm. Ring measurements were checked for correct crossdating with cross-correlation analysis.

Growth Calculations and Analysis

Radial measurements of growth rings of opposite radii 

and cores from each tree were used to calculate tree growth 

metrics, diameter increment, and basal area increment. 

Diameter increment (Dinc) is the increase in diameter for a 

particular year on a given tree. Annual Dinc is calculated by 

using the following equation:
Methods 13

C ongaree

R iver

Upper

reach

Middle

reach

L ower

reach

State Highway 4 8

StateHighway769

S

ta

te

H

i

g

h

wa

y

5

6 U

.S

.

H

igh

wa

y

6 0

1

0 1,90 0 3,80 0 5,70 0 7,60 0 950

Meters

1 1 2 3 4 0

Kilometers

R eal-time gageGage locationsTrailsC ongaree National ParkPark headquarters

Figure 8. Map of water level gage locations within Congaree National Park in reference to longitudinal compartments that correspond to river grade (upper, middle, and lower river reaches).
14 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Figure 9. Aerial photography mosaic of Congaree National Park showing locations of park boundary (light blue line), hiking trails (white lines), and tree-coring sites of primary species, baldcypress (yellow labels) and loblolly pine (green labels). Tree-coring sites for baldcypress included Boardwalk (BWK), Toms Creek flood plain (TCF), Lower Toms Creek (LTC), Horsepen (HRS), Lower Cedar Creek (LCC), and Virgin Cedar Creek (VCC). Tree-coring sites for loblolly pine included West Access Road (WAR), Headquarters A (HQA), Headquarters B (HQB), Elevated Boardwalk (EBW), Garrick Road (GAR), Service Road (SRV), and Pine Island Pines (PIP).
Results 15

Dinct = rwAt + rwBt, (1)

where Dinct is the diameter increment of the tree in year (t), 

rwAt is the measured ring width (rw) of core A for a given 

calendar year (t) on one radius of the tree, and rwBt is the 

ring width of the same calendar year (t) of core B from the 

opposite radii of the same tree. Basal area increment (BAI) is 

the area of stemwood production for the entire growth ring for 

a given year for a tree of given size at year (t). Annual BAI is 

calculated by first decrementing diameter growth (Dinc) from 

measured diameter (Diam) at core height for every successive 

year and deriving the inside and outside radius of each ring 

boundary for the entire growth record:

BAIt = ((Diamt / 2)2 * p ) - (((Diamt - Dinct) / 2)2 * π) (2)

BAI is the preferred and most comprehensive metric 

of tree growth given that it considers both the annual 

growth increment and size of tree. BAI is also more readily 

comparable between different chronologies and sites than 

is Dinc, which varies with tree age and size. Most trees and 

species will reach a stable BAI with canopy establishment for 

a given site quality and comparable canopy leaf area. 

Tree-ring data constitute longitudinal data such that a 

subject class of individual trees (that is, site collection) is 

measured repeatedly on an annual basis (that is, ring widths). 

Single-year comparisons primarily identify climate-induced 

growth deviations of wet and dry year cycles, while multiyear 

growth comparisons isolate growth changes or inflections that 

may correlate with long-term climate trends or disturbance 

events, such as Hurricane Hugo or the Saluda Dam effect 

on riverflow changes. Growth response differences between 

age cohorts and different forest stands within a region are 

used to interpret the source of variation and disturbance 

type and magnitude. Normalized difference algorithms were 

used to compare within-tree variance related to stemwood 

distribution pattern that also chronicles the onset of standlevel disturbances, such as logging activities and hurricane 

disturbance (Doyle and Gorham, 1996). Mean chronologies 

were developed for primary tree species, baldcypress and 

loblolly pine, to relate long-term productivity with differences 

among sites that represent greater or lesser flood potential 

or hydroperiod. Sites of common hydrogeomorphic setting 

were grouped to allow analysis of flooding potential on 

growth performance and to evaluate species response to 

changes in hydrology before and after 1934 relating the 

effect of riverflow change from Saluda Dam construction and 

operation. A two-way analysis of variance (ANOVA) was used 

to test for differences between growth periods by species or 

hydrogeomorphic setting in the immediate periods prior to 

dam construction from 1915 to 1929 and the postdam period 

from 1935 to 1949.

Results

Hydrological and Trail Elevation Surveys

Long-term hydrological records exist for the Congaree 

River at Columbia dating from 1892 to present. Missing 

records in the data record were mostly caused by the impact 

of highest floods that damaged gage structure, mechanics, 
16 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

or electronic components. Overall, the hydrological record 

for the Columbia gage is fairly robust with less than 5 

percent of missing daily records (fig. 10A). Mean annual 

stage has increased slightly since 1940 in part by more 

evenly distributed flows released from the Saluda Dam for 

hydroelectric generation and likely changes in river gradient 

(fig. 10B). Water volume of floods on the Saluda River that 

passed downriver in hours and days prior to dam installation 

in the 1930s have since been stored in Lake Murray to be 

released days and weeks later, effectively raising the mean 

daily river stage of the Congaree River (Conrads and others, 

2008). Maximum stage height shows the year-to-year extremes 

and variability of high and low flow years attributed to climate 

(fig. 10C). More years are near normal than extremely wet 

or dry by comparison. Typical high (1975) and low (1988) 

flow years are differentiated more by increased frequency of 

singular flood events than by sustained water flow of higher or 

lower stage on a daily basis (fig. 10D).

These same data are useful for examining stage 

differences pre- and post-Saluda Dam operation to determine 

effects of discharge schedules and volume on downstream 

flow conditions and patterns. Counting the number of days 

that stage was equaled or exceeded for different stage heights 

provided some measure of river hydroperiod for comparison 

(fig. 11). While reports have been published to show that 

the Saluda Dam has effectively reduced the high flows and 

floodwaves of the Saluda River tributary (Whetstone, 1982; 

Patterson and others, 1985), stage relations showed that low 

and medium flows have actually increased following dam 

operation from more regulated discharge and flood storage 

capacity of Lake Murray (fig. 11). These data confirmed that, 

even though the dam was completed in 1930, several years 

0

50

100

150

200

250

300

350

1940

1950

1960

1970

1980

1990

2000

Number of days

Year

Figure 10. Stage records for the Congaree River at Columbia, S.C. A, Number and pattern of missing daily records illustrating less than 

5 percent of missing data from 1940 to 2000. B, Mean annual stage showing a slight rise since 1940 attributed in part by more evenly 

distributed flows released from the Saluda Dam for hydroelectric generation and likely changes in river gradient. C, Maximum stage 

height from 1940 to 2000 illustrating the year-to-year extremes and variability of high and low flows. D, Typical hydrographs from high 

(1975) and low (1988) flow years differentiated more by increased frequency of singular flood events than sustained water flow of higher 

or lower stage.

A
Results 17

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

1940

1950

1960

1970

1980

1990

2000

Mean

Slope =0.018

Year

Relative stage height, in meters

0

1.5

3.0

4.6

6.1

7.6

9.1

10.7

19401950

1960

1970

1980

1990

2000

Year

Maximum stage height, in meters

1100200300

0

1.5

3.0

4.6

6.1

7.6

Juliandate

Relative stage height, in meters

1975

1988

BCD
18 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

0

20

40

60

80

100

120

140

160

180

1892 1902 1912 1922 1932 1942 1952 1962 1972 1982 1992

Days above 36.0 m

0

10

20

30

40

50

60

70

80

1892 1902 1912 1922 1932 1942 1952 1962 1972 1982 1992

Year

Days above 36.6 m

Figure 11. The number of days that water levels equaled or exceeded select stage heights for the 

Congaree River at Columbia, S.C., over the period of record from 1892 to 2000. Stage differences pre- and 

post-Saluda Dam operation circa 1934 illustrate an overall increase in low flows following dam installation. 

A, Stage height of 35.3 m. B, Stage height of 36.0 m. C, Stage height of 36.6 m.

C

0

50

100

150

200

250

300

350

1892 1902 1912 1922 1932 1942 1952 1962 1972 1982 1992

Days above 35.3 m

B

A
Results 19

were required to accumulate the streamflow and rainfall to 

reach permanent pool stage in Lake Murray by 1934. Analyses 

for testing flood effects on forest resources therefore utilized 

streamflow data preceding from before 1930 and after 1934 to 

represent predam and postdam streamflow conditions.

Water quality and flooding are major factors affecting 

wetland composition, structure, and productivity of flood 

plain ecosystems (Carter, 1986; Mitsch and Gosselink, 1993). 

The elevation of a given tract of forest or park trail and the 

degree to which the flood plain slope is coupled with the river 

stage determine the flood potential and local hydroperiod. 

Some systematic studies have been conducted in Congaree 

Swamp to describe the forest structure and composition in 

relation to land elevation and hydrologic forcing (Rikard, 

1988); however, given the subtle local relief of this flood plain, 

greater vertical resolution was needed to model the flood 

contingencies of park trails and to calibrate a flood inundation 

model for predicting flood effects and forest response.

Park trails and water level gages within the Congaree 

River and flood plain were surveyed into a common datum 

to determine the degree of hydrological coupling with river 

stage. All park trails were explicitly surveyed with differential 

leveling by using laser level equipment and methods to 

establish surface elevations of soils and ponded water on a 

10-m horizontal resolution and 1-mm vertical resolution (fig. 

12). Elevation circuits inclusive of the entire trail system of 

the park met closure error and standards exceeding third-order 

vertical leveling criteria. The combined elevation circuits 

inclusive of park trails were resolved into a digital elevation 

model. The digital elevation model of the trail system was then 

used in conjunction with hydrological surface models to create 

a flood plain inundation model for forecasting flood potential 

for current and next day conditions based on river stage at 

Columbia. 

Flood Plain Inundation Model

A typical hydrograph of Congaree River stage and the 

Boggy Gut stage in the backswamp for the 4-year period 

(2002–5) demonstrated synchronous stage heights and 

coupling when river flows reached full bank and breached 

natural levee height (fig. 13). Some gages were damaged by 

extreme floods during 2003 and thereafter by fallen trees or 

debris flows and were not replaced. Spring and summer floods 

of 2003 also destroyed many trail boardwalks over sloughs and 

channels, which affected park access and visitation (fig. 14). 

Regression models were applied to establish autocorrelation 

in flood stage between gages at Columbia and those in the 

upper, middle, and lower reaches of the river and backswamp 

within the park. Flood relations between backswamp gages 

demonstrated greater flood retention and delayed recession 

behavior within the lower reach compared to the upper and 

middle reaches of Congaree Swamp.

A flood plain inundation model was developed for the 

park trail system on the basis of relations and regressions of 
20 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Figure 12. Photograph of trail at Congaree National Park and laser level equipment used to establish vertical 

elevations of ground and water surfaces.

90

92

94

96

98

100

102

104

106

108

110

22-Jan-2002

20-Feb-2002

22-Mar-2002

20-Apr-2002

19-May-2002

17-Jun-2002

17-Jul-2002

15-Aug-2002

13-Sept-2002

12-Oct-2002

11-Nov-200210-Dec-2002

08-Jan-2003

06-Feb-2003

08-Mar-2003

06-Apr-2003

05-May-2003

03-Jun-2003

03-Jul-2003

01-Aug-200330-Aug-2003

28-Sept-2003

28-Oct-2003

26-Nov-200325-Dec-2003

23-Jan-2004

22-Feb-2004

22-Mar-2004

20-Apr-2004

19-May-2004

18-Jun-2004

17-Jul-2004

15-Aug-2004

13-Sept-2004

13-Oct-2004

11-Nov-200410-Dec-2004

08-Jan-2005

07-Feb-2005

08-Mar-2005

06-Apr-2005

05-May-2005

04-Jun-2005

03-Jul-2005

Date

Surface water level, in meters

Figure 13. Typical hydrograph of Congaree River stage (gray line) and the Boggy Gut gage (red line) in the backswamp over a 4-year 

period (2002–5) showing synchronous stage heights and coupling when river flows reach full bank and breach natural levee height.
Results 21

Figure 14. Photograph of damaged trail bridge 

across a narrow drain in Congaree National Park 

following high floods in spring of 2003.

backswamp stage observations with Congaree River stage 

at the upper park boundary and its headwaters in downtown 

Columbia. Upstream and downstream gage relations were 

established between the Columbia gage and Congaree gage 

at the park boundary. Correlations of simultaneous records 

of stage heights showed flood loop relations of higher and 

lower stage heights relative to floodwave size, timing, and 

movement between gage locations (fig. 15A). The resultant 

data field is unsynchronized, showing a pattern of scattered 

points reflecting a range of gage relations below and above 

full-bank behavior. Time-step integration was used to account 

for a floodwave lag of 20 hours or more from Columbia to 

the upper park boundary that compresses the data field and 

more clearly shows gage coupling (fig. 15B). This envelope of 

stage relations allows a maximum and minimum flood relation 

of 0.3–0.6 m range for flood conditions below full bank and 

near 1:1 relations for overbank riverflow. The small variance 

of the flood envelope between gages is related to a number 

of factors, including antecedent conditions of rainfall and 

runoff contribution from uplands affected by recharge status 

and residual water storage within the Congaree Swamp and 

elsewhere upstream, and to the properties of the floodwave 

size and speed, which can vary depending on tributary 

contributions and timing of peak flows. The upper and lower 

cusps of the envelope defined the states where flooding effect 

is at a minimum and maximum, respectively (see fig. 15B). 

Local elevations below the lower cusp or maximum inundation 

are always expected to be inundated, whereas elevations above 

the upper cusp are always above potential flooding and dry. 

The same procedure was used for successive gages in 

the backswamp zone and downstream of the Congaree River 

gage involving lags of another 4 or more hours depending on 

distance downstream and from the river. Another example 

of the relation of the river gage at the Congaree gage and the 

Boggy Gut gage shows a best-fit of a 4-hour lag (fig. 16A). 

Dissecting the data into flood rises and falls shows how the 

river rises and falls first and the interior gut fills and drains 

at different stage relations (fig. 16B). Gage relations and 

functions were defined for all gage combinations and used to 

create a surface response model for each 3-cm increment of 

river stage at the Columbia gage. 

The flood plain inundation model determines whether 

trail surfaces are dry or wet by contrasting the flood response 

surface heights for different river stage at the Columbia gage 

and the elevation of 1-m segments of the park trail digital 

elevation model (fig. 17). When trail elevations are above the 

predicted flood envelope, trails are dry. When trail elevations 

are below the flood envelope, they are inundated. When trail 

elevations are within the upper and lower flood envelope, they 

are considered wet and potentially flooded. The flood model 

was developed and tested to run on a Web-based server to 

allow park staff and beta testers to review model function and 

utility. By default the model uploads real-time and previous 

daily gage heights from the USGS National Water Information 
22 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Figure 15. Relation of stage height records and flood loop patterns for the upstream gage of the Congaree River at 

Columbia, S.C., and the downstream in-channel gage at the western boundary of Congaree National Park. A, Unshifted, 

simultaneous hourly readings. B, Shifted hourly readings of 20 hours to account for floodwave travel and coupling. The 

upper (tan line) and lower (red line) cusps of the envelope define the elevation where flooding is minimal and maximal, 

respectively.

30.2

29.6

29.0

28.3

27.7

30.8

31.4

32.0

32.6

33.2

33.8

34.7 35.4 36.0 36.6 37.2 37.8 38.4 39.0 39.6 40.2 40.8 41.5 42.0

Congaree River at Columbia stage, in meters

Congaree River stage at Congaree National Park, in meters

27.7

34.7 35.4 36.0 36.6 37.2 37.8 38.4 39.0 39.6 40.2 40.8 41.5 42.0

Congaree River stage at Columbia, in meters

33.8

33.2

32.6

32.0

31.4

30.8

30.2

29.6

29.0

28.3

Congaree River stage at Congaree National Park, in meters

A

BCongaree River stage at Congaree National Park, in meters Congaree River stage at Congaree National Park, in meters

Congaree River at Columbia stage, in meters

Congaree River at Columbia stage, in meters
Results 23

Figure 16. Stage relation of hourly readings from the Congaree River gage and the Boggy Gut gage in Congaree 

National Park. A, Shifted hourly readings with a best-fit 4-hour lag in floodwave travel. The upper (tan line) and lower 

(red line) cusps of the envelope define the states where flooding is minimal and maximal, respectively. B, Sorted hourly 

readings of flood rises (blue plus signs) and falls (red minus signs) illustrating the effect of timing and stage during the 

filling and draining of backswamp floodwater. Legend shows select flood events segregated by date and by rise (up) and 

fall (down) of passing flood wave.

29.0

29.3

29.6

29.9

30.2

30.5

30.8

31.1

31.4

31.7

32.0

32.3

32.6

32.9

28.3 28.7 29.0 29.3 29.6 29.9 30.2 30.5 30.8 31.1 31.4 31.7 32.0 32.3 32.6 32.9 33.2

Congaree River stage at Congaree National Park, in meters

Boggy Gut stage, in meters

30.5

30.8

31.1

31.4

31.7

32.0

32.3

32.6

32.9

31.4 31.7 32.0 32.3 32.6 32.9 33.2 33.5 33.8

12-15-02 up

12-15-02 dow n

12-25-02 up

12-25-02 dow n

02-24-03 up

02-24-03 dow n

03-08-03 up

03-08-03 dow n

03-17-03 up

03-17-03 dow n

04-08-03 up

04-08-03 dow n

04-19-03 up

04-19-03 dow n

04-25-03 up

04-25-03 dow n

06-09-03 up1

06-09-03 dow n1

06-09-03 up2

06-09-03 dow n2

06-09-03 up3

06-09-03 dow n3

06-20-03 up

06-20-03 dow n

Congaree River stage, in meters

Boggy Gut stage, in meters

A

BBoggy Gut stage, in meters Boggy Gut stage, in meters

Congaree River stage at Congaree National Park, in meters

Congaree River stage, in meters
24 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

System Web site to allow public access to online gage data 

and graphics at the Columbia gages and park gages on the 

Congaree River and Cedar Creek (fig. 18). This function 

allows users to check current river stages in the Congaree 

basin and allows the flood inundation model to forecast the 

24-hour flood condition on the trail network within the park. 

Features were added to allow users to manually insert stage 

Figure 17. Graphic 

user interface of the 

Congaree National Park 

flood plain inundation 

model illustrating 

functional buttons for 

predicting current and 

24-hour forecasts of 

flood conditions on park 

trails.

Congaree National Park. Click the buttons at the 

heights for the Columbia gage to see how the various trails 

undergo predicted inundation with higher river stages (fig. 

19). Users can also select a specific trail system of interest to 

enlarge the map view that also highlights trail information and 

photo stops that show actual digital photos of trail stops in 

winter and summer in an online virtual hike (fig. 20).

Figure 18. Graphic user interface of the Congaree National Park flood plain inundation model illustrating concurrent gage heights 

from the U.S. Geological Survey (USGS) National Water Information System Web site for the Congaree River at Columbia, S.C., and 

downstream park gages on the Congaree River and Cedar Creek.
Results 25

Figure 19. Illustrations of park trail condition (dry is indicated by brown, wet by yellow, and flooded by red) 

predicted by the Congaree National Park flood plain inundation model across the trail network at select 

stages of the Congaree River at Columbia, S.C.
26 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Figure 20. Features of the Congaree National Park flood plain inundation model highlighting specific park trails of interest with an 

enlarged map view of trail information (top panel) and photo stops in winter and summer (bottom panel) in an online virtual hike format.
Results 27

Tree-Ring Analysis

Forest Age and History

Tree cores had not heretofore been taken from Congaree 

National Park except to age select trees of loblolly pine and 

virgin specimens of baldcypress (Pederson and others, 1997; 

Laary Cushman and Joy Young, oral commun.). Remnant 

virgin-growth baldcypress trees are present in the park but 

were not cored for this study to avoid complications with 

growth analysis of millennial-aged trees. Results from this 

study showed that ages generally ranged from about 50 to 200 

years old for the collective group of sampled trees and species 

(table 1). Most hardwood trees sampled were less than 100 

years of age, though some individuals were nearly twice as 

old. Tree sizes were commonly more than 50 cm in diameter, 

with the largest individuals exceeding 1 m and a few as large 

as 1.5 m in diameter. Overall, the maximum growth rate for 

all species was fairly robust (greater than 1 cm/yr), excepting 

water tupelo. Species collections were generally on the higher 

elevations near full-bank elevation for backswamp zones on 

rich sandy, silt-loam soils that routinely receive continuous 

deposition following higher floods, excepting water tupelo and 

some baldcypress. Water tupelo was generally sampled from 

low-elevation sites along with some baldcypress collections 

where soil conditions are mostly saturated or semipermanently 

flooded from groundwater seepage, rainfall, or intermittent 

flood events. 

Water tupelo sizes were much smaller than all other 

species with ages commonly between 100 and 200 years, 

even though some tupelo trees exceeded 300 years based 

on ring counts. Water tupelo specimens contained faint ring 

boundaries and suppressed ring series that complicated 

accurate dating and measurement. Suppressed ring series 

in water tupelo appeared to be quite abrupt and related to 

disturbance or stand development (crowding) rather than to 

age. Wind damage from hurricanes or thunderstorms may be 

primary causes leading to branch breakage and topping out of 

water tupelo canopies that match long suppressed ring series 

of 20 or more years before eventual recovery of detectable 

ring sizes and variation. In almost all cases, water tupelo were 

older than neighboring baldcypress of near equal size, which 

is perhaps related to baldcypress resistance to wind storms 

(Doyle and others, 1995; Conner and others, 2007; Doyle 

and others, 2007). The two primary tree species, loblolly pine 

and baldcypress, were sampled intensively across Congaree 

Swamp to construct growth chronologies for relating any 

effects of changing river stage that are due to stream regulation 

and operation of Saluda Dam. 
28 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Loblolly Pine Chronology Development and 

Productivity

Loblolly pine are common forest dominants in Congaree 

Swamp of appreciable age and size that can provide long-term 

growth records sufficient to span before and after operation 

of Saluda Dam and establishment of Lake Murray. Mean 

chronologies for pine specimens were constructed from seven 

sites or stands representing different hydrogeomorphic settings 

where flood history or hydroperiod was sufficiently different 

and unique to test for growth differences and relations. Three 

hydrogeomorphic settings were identified including upland 

sandhill (PinDry), flood plain seepage (PinWet), and flood 

plain full bank (PinBnk). Site names associated with each of 

these hydrogeomorphic classes are given along with age and 

diameter ranges of trees making up each chronology (table 2). 

Tree ages confirm that loblolly pine populations in Congaree 

Swamp are old growth up to 200 years old (table 2; Pederson 

and others, 1997). In all cases, sample trees are dominant 

and emergent canopy trees approaching 40 m in height based 

on actual ground measurements of fallen trees. This aging 

population of pines is subject to senescence and windthrow by 

tropical storms and other extreme wind events.

Mean growth chronologies of annual 

BAI were constructed from loblolly 

pine populations at seven sites within 

the park that were categorized into three 

hydrogeomorphic classes (fig. 21). Each 

chronology represents the common 

variance and mean growth by year for 10 

or more trees and 20 or more cores per 

site. Results showed highly synchronous 

growth behavior year to year and over the 

long term but showed different ranges of 

productivity potential. The PinDry group 

and site is an upland sandhill environment 

within the park’s bluff zone that has no 

influence of river flooding given that it is 

more than 9 m above Congaree Swamp. 

The wet flatwoods condition of this site 

is influenced by area rainfall and a sandy soil type lacking the 

soil quality and fertility to sustain high growth rates relative 

to flood plain soils of higher silt and clay composition. A 

similar site chronology and productivity ranging in annual 

BAI from 10 to 25 cm2 are found among the PinWet group 

(at site HQA) located on organic soils within the seepage 

zone that remains more or less saturated year round from 

groundwater drainage. Other seepage zone pine populations 

sustained intermediate stemwood production rates between 20 

and 70 cm2, comparable to some PinBnk sites of nearly equal 

site quality. The PinBnk group sustained the highest levels of 

stemwood productivity, approaching 100 cm2 for select years. 

Even though the range of stemwood productivity varies greatly 

with site, the temporal pattern of productivity change year to 

year and over the long term remained remarkably similar.

The year-to-year variation in tree-ring chronologies 

is usually attributed to climate influence and variability. 

Detrending filters are commonly used to reduce the lowfrequency or long-term variation that is often associated 

with changes in stand density or is disturbance related. In 

this study, long-term variation in the growth record could 

also be the result of changing flood patterns as a function of 

dam construction and operation dating back to the 1930s. A 

7-year running mean was applied to the mean chronologies 

to compare long-term changes and inflections in the growth 

record and the synchronicity or lack thereof between the 

different sites. Loblolly pine typically retains cohorts of 

needles from 5 to 7 years in age before being replaced with 

new needles. The combined needle sets contribute to tree 

productivity but to varying degrees on the basis of maturity. 

Developing needles of the current year and senescing needles 

may not add appreciably to stem growth potential as much as 

intermediate aged needles 2–4 years old do.

Results showed remarkable synchronicity of growth 

variation by time period among all sites, with some reduction 

in response amplitude with the least productive sites (fig. 

22). The greatest differences and incongruity between sites 

Table 1. Age and diameter ranges based on tree core analysis 

of select hardwood tree species at Congaree National Park.

Tree species 

(common name)

Age range,

in years

Diameter range,

in centimeters

Green ash 74–159 44–104

Laurel oak 70–129 48–104

Swamp chestnut oak 49–205 24–160

Sycamore 76–197 43–141

Water tupelo 97–170 49–63

Table 2. Site descriptions, site name abbreviations, and age and diameter 

ranges of loblolly pine tree core collections at Congaree National Park.

[Full names of sites are given in figure 9]

Hydrogeomorphic setting Site name

abbreviation

Age range,

in years

Diameter range,

in centimeters

PinDry GAR 95–135 43–62

PinWet EBW 161–198 70–114

PinWet HQA 190–208 69–93

PinWet HQB 152–181 89–123

PinBnk PIP 127–172 81–112

PinBnk SRV 132–193 83–131

PinBnk WAR 159–169 82–103
Results 29

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

10

20

30

40

PinDry at GAR

Year

Basal area increment, in square centimeters

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

20

40

60

80

100

120

Year

Basal area increment, in square centimeters

PinBnk at SRV

PinBnk at WAR

PinBnk at LTC

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

20

40

60

80

100

Year

Basal area increment, in square centimeters

PinWet at HQB

PinWet at EBW

PinWet at HQA

Figure 21. Mean growth 

chronologies of annual basal area 

increment (BAI) from loblolly pine 

populations at seven coring sites 

within Congaree National Park 

graphed by hydrogeomorphic 

association. A, Mean BAI 

chronology for upland sandhill 

(PinDry) at Garrick Road (GAR) 

site. B, Mean BAI chronologies 

for flood plain full bank (PinBnk) at 

Lower Toms Creek (LTC), Service 

Road (SRV), and West Access 

Road (WAR) sites. C, Mean BAI 

chronologies for flood plain seepage 

(PinWet) at Elevated Boardwalk 

(EBW), Headquarters A (HQA), and 

Headquarters B (HQB) sites.

A

B

C
30 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

20

40

60

80

100

HQB

LTC

SRV

HQA

WAR

EBW

GAR

Basal area increment, in square centimeters

Year

Figure 22. A 7-year running mean of basal area increment (BAI) chronologies among the different loblolly pine populations 

and sampling sites.

occurred early in the record between young and mature 

chronologies until convergence was achieved during the 

mid-1910s. This result indicates that all chronologies are 

responding to an overriding influence, climate or flood regime, 

that is ubiquitous across the park flood plain and uplands, 

rather than to disturbance or land-use effects that are more 

likely to have occurred at different places and at different 

times. The synchronous rises and falls during the postdam era 

from 1935 to present do not support the likelihood of flood 

effects or other land-use effects as probable causes. Logging 

or high-grading activities can foster growth releases, but 

always over a short term of a few years, and usually result in 

threefold to fivefold increases in growth rate, neither of which 

are observed here. The relatively gradual and characteristic 

patterns of positive and negative growth inflections appear 

more climate related than dam or disturbance related. 

Long-term climate patterns of favorable or less favorable 

temperature or precipitation patterns have been associated with 

long-term growth variance.

Loblolly Pine Climate-Growth Relations

Division climate data of mean monthly temperature, 

precipitation, and Palmer Drought Severity Index (PDSI) 

were used to associate short- and long-term relations with 

these loblolly pine growth chronologies. The PDSI and 

precipitation data are highly intercorrelated because of the 

weighted use of precipitation to derive the PDSI. Temperature 

and precipitation are more independent of each other and can 

be used individually or together to test model fit for explaining 

growth as a function of climate. Single factor relations are 

illustrated here to demonstrate the degree to which certain 

climate parameters and data reductions did or did not correlate 

with loblolly pine growth over the last century. To determine 

whether growth is correlated with frequency of rainfall during 

the spring/summer growing season months (March–August), 

the monthly precipitation record was reduced by counting the 

number of months with above average rainfall for the period of 

record. Results showed a slight but poor correlation between 

precipitation frequency and growth of flood plain loblolly 

pine (PinBnk) by year (fig. 23A). There was, however, no 

discernible correlation with quantity of rainfall for the spring/

summer growing season (fig. 23B). The results are the same if 

the annual record is averaged over a 7-year period to capture 

the long-term precipitation pattern; however, averaging 

the annual air temperature over a 7-year period including a 

2-year lag explains more than 50 percent of the variation in 

mean loblolly pine growth (fig. 23C). The 2-year lag may 

be related to the maturity and function of intermediate-aged 

needles that contribute more to current growth than do current 
Results 31

year needles. Warmer spring/summer temperature appears 

to foster greater growth potential for flood plain loblolly 

pine where available water at depth may not be limiting. 

Moreover, warmer temperatures and droughts may dry out 

some saturated soils which might otherwise limit loblolly pine 

growth potential. In contrast, there is no correlation of number 

of flood days by year for stage above 36.6 m at the Columbia 

gage of the Congaree River with flood plain growth of loblolly 

pine (fig. 23D).

Baldcypress Chronology Development and 

Productivity

Baldcypress were sampled in the park to develop growth 

chronologies and to determine any effects of changing 

climate and flood history. As a flood-tolerant species, 

baldcypress provides a contrast with loblolly pine, which is 

only moderately tolerant of flooding and thrives on relatively 

higher ground not subject to permanent flooding. Mean 

growth chronologies of baldcypress were constructed from 

six sites representing different hydrogeomorphic settings 

where flood history or hydroperiod were sufficiently different 

and unique to test for growth differences and relations. Two 

hydrogeomorphic settings were identified including riverbank 

(CypDry) and backswamp (CypWet) environments of higher 

and lower elevations and shorter and longer hydroperiods, 

respectively. Site designations associated with each of these 

hydrogeomorphic classes are given along with age and 

diameter ranges of trees making up each chronology (table 

3). Tree ages confirmed that baldcypress populations cored 

for this study are old growth, with some approaching 300 

years old. Trees sampled along riverbank (CypDry) settings 

were generally of younger cohorts and codominant canopy 

trees with other hardwood species more typical of seasonally 

flooded forests. Trees sampled in backswamp environments 

included low-elevation sites dominated by baldcypress and 

its common associate water tupelo, known to persist in 

semipermanently flooded forests. The size range of sampled 

trees in the CypWet sites was generally smaller for the same 

relative age, indicative of slower growth rates typical of 

flooded versus drained conditions.

Six growth chronologies of annual BAI were constructed 

from baldcypress sites within Congaree Swamp from 

riverbank (CypDry) and backswamp (CypWet) settings (fig. 

24). Chronology comparisons showed highly synchronous 

growth behavior year to year and over the long term 

but showed different ranges of productivity potential by 

site condition. The CypDry group occupies riverbank 

environments that flood infrequently and only when river 

flow nears peak flood stages. Mean chronologies for the two 

groups showed a difference in productivity range of 10–40 

cm2 BAI for the CypWet group in contrast to 20–60 cm2 BAI 

for the CypDry group. Other tree growth studies have shown 

that baldcypress grows better on aerated drained soils than 

in anaerobically saturated or deeply flooded sites. Similar 
32 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

50.8 55.9 61.0 66.0 71.1 76.2 81.3

20

40

60

80

Total spring/summer precipitation, in centimeters

Basal area increment, in square centimeters

B

Figure 23. Relation between mean annual basal area increment (BAI) of loblolly pine growth with various climate and 

hydrology parameters. A, Growth related to number of months of above average spring/summer (March–August) precipitation. 

B, Growth related to total precipitation for spring/summer months (March–August). C, A 7-year growth period and temperature 

relation with a 2-year time lag offset indicating greater growth potential under warmer growing season temperatures. D, 

Growth related to number of flood days by year for stage above 36.6 m at the Columbia, S.C., gage.

0 2 4 6 8

20

40

60

80

Number of months above average spring/summer precipitation

Basal area increment, in square centimeters

A
Results 33

y = 10.78x - 726.36

R2 = 0.5284

25

30

35

40

45

50

55

60

21.1 21.4 21.7 21.9 22.2 22.5

7-year mean spring/summer temperature, lag of 2 years, in degrees Celcius

7-year mean BAI, in square centimeters

0 20 40 60 80

20

40

60

80

Number of days above 36.6 m at Columbia gage

Basal area increment, in square centimeters

C

D
34 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

to the loblolly pine chronologies there is a high degree of 

synchronicity of year-to-year variation, indicating that trees 

under wet or dry settings are responding to similar climate or 

environmental factors. Contrasting the two group chronologies 

demonstrated that the initial slope is a product of younger 

CypDry trees yet to establish maturity compared with the 

older set of CypWet trees, which achieved convergence by 

1920 and then had level growth to 1990 followed by a growth 

release after Hurricane Hugo (1989) damage to neighboring 

hardwood trees along the river’s edge (fig. 25). 

Baldcypress Climate-Growth Relations

A 7-year running mean was derived from the mean 

chronologies to compare long-term changes and inflections 

in the growth record and the synchronicity or lack thereof 

between the different hydrogeomorphic groups. Results 

demonstrated that the collective baldcypress chronologies also 

exhibit remarkable synchronicity between sites and groups, 

CypDry and CypWet, with the greatest difference in the 

range of productivity and a post-1989 hurricane effect (fig. 

26). This result indicates that all chronologies are responding 

Table 3. Site descriptions, site name abbreviations, and age and diameter ranges of baldcypress tree core collections at Congaree 

National Park.

[Full names of sites are given in figure 9]

Hydrogeomorphic setting Site name

abbreviations

Age range,

in years

Diameter range,

in centimeters

CypDry HRS 112–115 54–115

CypDry LCC 109–162 60–102

CypDry VCC 106–235 56–107

CypWet BWK 172–295 53–109

CypWet LTC 119–208 45–51

CypWet TCF 69–195 45–88
Results 35

Figure 24. Mean basal area increment (BAI) growth chronologies of baldcypress populations within Congaree National Park by 

hydrogeomorphic association. A, Riverbank baldcypress populations (CypDry) at sites Horsepen (HRS), Lower Cedar Creek (LCC), and 

Virgin Cedar Creek (VCC). B, Backswamp baldcypress populations (CypWet) at sites Boardwalk (BWK), Lower Toms Creek (LTC), and 

Toms Creek flood plain (TCF).

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

20

40

60

80

100

Year

Basal area increment, in square centimeters

CypDry at HRS

CypDry at VCC

CypDry at LCC

CypWet at BWK

CypWet at LTC

CypWet at TCF

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

20

40

60

Basal area increment, in square centimeters

Year

A

B
36 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

0

20

40

60

80

Year

Basal area increment, in square centimeters

CypWet

CypDry

Figure 25. Mean 

basal area 

increment 

(BAI) growth 

chronologies of 

baldcypress by 

hydrogeomorphic 

groupings 

contrasting 

productivity of 

riverbank (CypDry) 

sites versus 

backswamp 

(CypWet) sites.

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

10

20

30

40

50

60

Year

Basal area increment (BAI), in square centimeters

BAI of CypDry

BAI of CypWet

Figure 26. A 7-year 

running mean of 

riverbank (CypDry) 

and backswamp 

(CypWet) 

baldcypress 

chronologies 

showing overall 

synchronicity and 

difference in the 

range of productivity 

and a post-1989 

hurricane effect.
Results 37

to an overriding influence, climate or flood regime, that is 

ubiquitous across Congaree Swamp rather than to disturbance 

or land-use effects that are more likely to have occurred at 

different places and at different times. The synchronous rises 

and falls during the postdam era from 1935 to present do 

not support the likelihood of flood effects or other land-use 

effects as probable causes, a result that is similar to that of 

the loblolly pine analysis. Furthermore, there are differences 

in species productivity and pattern between loblolly pine and 

baldcypress chronologies that indicate different sensitivities to 

different climate variables.

Riverbank (CypDry) chronologies might be expected to 

be more sensitive to the frequency of precipitation events or 

high floods given the drained conditions and high percolation 

of the largely sandy soils typical of depositional levee 

environments; however, there was no apparent correlation of 

baldcypress growth corresponding to number of flood days 

above 36.6 m at the Columbia gage of the Congaree River (fig. 

27A). There was a slight but poor correlation of mean spring/

summer temperature, indicating that cooler temperatures 

enhance growth potential or account for increasing rainfall and 

cloudy days (fig. 27B). Similarly, there was a weak correlation 

of increasing spring/summer precipitation with greater growth 

potential (fig. 27C); however, correlating the 7-year running 

average of spring/summer PDSI (surrogate precipitation) 

with baldcypress growth explained more than 56 percent of 

the variation in growth (fig. 27D). Other tree-ring studies of 

baldcypress purport the importance of spring precipitation 

on species growth (Stahle and others, 1985, 1988; Stahle and 

Cleaveland, 1992, 1994; Cleaveland, 2000).

Saluda Dam Effects on River Stage and Tree 

Growth

Saluda Dam was completed in the early 1930s to create 

Lake Murray from the Saluda River near its confluence of the 

Broad River and headwaters of the Congaree River. Analysis 

of stage records for the Congaree River at Columbia dating 

back to 1892 indicated dramatic changes in the number 

of flood days above low stage at 35.3 m (mean sea level) 

following dam construction and operation (fig. 28). Gage 

relations showed that floodwaters circulate into Congaree 

Swamp when river stage at Columbia exceeds 36.6 m (mean 

sea level), flooding some of the park trail system and trees 

enough to potentially affect visitor safety and tree growth. 

Records of flooding and growth every 15 years prior to 

(predam) and following (postdam) dam operation were 

summed to test for differences in tree growth by species and 

hydrogeomorphic setting. Fifteen years was determined to be 

a sufficiently long period to allow for a flooding effect and to 

moderate the combination of wet and dry years of climate and 

flooding on growth to effectively determine an incremental 
38 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Figure 27. Relation between mean annual basal area increment (BAI) of riverbank baldcypress growth with various climate and 

hydrology parameters. A, Growth related to number of flood days by year for stage above 36.6 m at the Columbia, S.C., gage. B, 

Growth related to mean temperature for spring/summer months (March–August). C, Growth related to total precipitation for spring/

summer months (March–August). D, A 7-year growth period and spring/summer PDSI relation indicating an increase of tree growth 

with increasing wetness.

20.5 21.1 21.7 22.2 22.8 23.3

15

20

25

30

35

Mean spring/summer temperature, in degrees Celcius

Basal area increment, in square centimeters

0 6.1 12.2 18.3 24.4

15

20

25

30

35

Number of days above 36.6 m at Columbia gage

Basal area increment, in square centimeters

B

A
Results 39

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

55

50

45

40

35

30

PDSI

Basal area increment, in square centimeters

y = 4.481x + 43.215; R2 = 0.5683

50.8 55.9 61.0 66.0 71.1 76.2 81.3

15

20

25

30

35

Total spring/summer precipitation, in centimeters

Basal area increment, in square centimeters

C

D
40 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

or sustained effect of change in river stage from predam and 

postdam conditions.

A two-way analysis of variation (ANOVA) was 

used to test for differences in tree growth by species and 

hydrogeomorphic setting between successive periods prior 

to dam construction (1900–29) and in the postdam operating 

period (1935–79) (table 4). There were no differences between 

sampling periods, before or after operation of the dam, that 

had any effect on loblolly pine or baldcypress species on wet 

or dry sites (fig. 29). There were some minor differences 

mainly between species of higher and lower growth deviation 

among the postdam periods that are due to lower than normal 

spring/summer precipitation, negatively affecting baldcypress, 

and warmer than normal spring/summer temperatures, 

fostering greater loblolly pine growth (table 4). Because 

wet sites reduced species productivity for both loblolly pine 

and baldcypress in contrast to drained sites, it would be 

0

100

200

300

Stage 35.3m

Stage 36.0m

Stage 36.6m

Predam,

1900–14

Predam,

1915–29

Postdam,

1935–49

Postdam,

1950–64

Postdam,

1965–79

Postdam,

1980–94

Mean number of flood days above stage

Sampling period

Figure 28. The number of flood days above select stages at 35.3 m, 36.0 m, and 36.6 m mean sea level for the Congaree River at 

Columbia, S.C., for predam and postdam periods spanning 15 years each dating from 1900 to 1994 and illustrating a marked increase 

in low flow conditions with dam operation.

inconsistent to expect that increasing flood conditions would 

do likewise. Only climate variations and correlations suitably 

explain the divergent postdam growth differences between 

species, indicating that the increased daily stage from dam 

releases was not sufficient to suppress or augment forest 

growth beyond other climatic influences.

Summary

A forest growth and flood plain hydrology study was 

conducted in Congaree National Park, S.C., to determine 

the flood relations of backswamp forests and park trails as 

influenced by river stage and flood plain elevation by utilizing 

an integrated field and modeling approach. Water level gages 

were distributed across the length and width of “Congaree 
Summary 41

Figure 29. Mean annual basal area 

increment (BAI) of loblolly pine and 

baldcypress populations for different 

predam and postdam periods on more 

(PinWet, CypWet) or less (PinDry, CypDry) 

flooded sites in Congaree National Park.

Table 4. Contrasts of mean annual basal area increment (BAI, in square centimeters) over a 15-year period for five successive 

periods before (predam) and after (postdam) construction and operation of the Saluda Dam near Columbia, S.C.

[Letters (a–h) signify statistical contrast by species groupings such that values with the same letters are not significantly different from one another]

Species group Predam

1900–14

Predam

1915–29

Postdam

1935–49

Postdam

1950–64

Postdam

1965–79

PinDry 71 a 63 b 64 b 66 b 47 b

PinWet 46 c 46 c 41 c 45 c 29 d

CypDry 30 e 42 fg 42 fg 38 eg 48 f

CypWet 28 h 28 h 30 h 25 h 29 h

0

20

40

60

80

Predam,

1900–14

Predam,

1915–29

Postdam,

1935–49

Postdam,

1950–64

Postdam,

1965–79

PinDry

PinWet

Mean basal area increment, in square centimeters

0

10

20

30

40

50

60

Predam,

1900–14

Predam,

1915–29

Postdam,

1935–49

Postdam,

1950–64

Postdam,

1965–79

CypDry

CypWet

Mean basal area increment, in square centimeters

A

B
42 Modeling Flood Plain Hydrology and Forest Productivity of Congaree Swamp, South Carolina

Swamp” (the flood plain portion of the park) to capture the lag 

and peak flood relations to river stage. A severe and prolonged 

drought at the study start in 2001 continued into 2002 before 

water levels were monitored over a 4-year period from 2002 

to 2006. Historical water level stage and discharge data from 

the Congaree River were collated and digitized from published 

sources and U.S. Geological Survey (USGS) archives to obtain 

long-term daily averages for an upstream gage at Columbia, 

S.C. Hydrological studies of the Congaree River demonstrated 

that the magnitude and frequency of floods had been greatly 

reduced following construction of Saluda Dam (Whetstone, 

1982; Patterson and others, 1985); however, daily stage was 

shown to actually increase rather than decrease after 1934 

as a result of dam operation. Flood volumes associated with 

the Saluda River are effectively stored behind Saluda Dam 

and released in operational pulses days and weeks later for 

hydropower.

Elevation surveys of ground surface were completed 

for all park trails and for additional circuits of roads and 

boundaries to create a digital elevation model for predicting 

inundation patterns and periods from floods. Regression 

models were applied to establish time lags and stage relations 

between gages at Columbia and in the upper, middle, and 

lower reaches of the Congaree River and backswamp within 

the park. Flood relations among backswamp gages showed 

that the retention and recession behavior between flood plain 

reaches was longer in the lower reach than in the upper and 

middle reaches of Congaree Swamp. Mathematical functions 

were developed to define the flooding envelope for position 

on the landscape gradient when the soil surface is either 

predictably wet or dry in relation to river stage. A Web-based 

flood plain inundation model was developed to predict critical 

river stages and potential inundation of hiking trails on a realtime basis and to allow the flood inundation model to forecast 

the 24-hour flood condition on the trail network within the 

park. Park officials have used the model to understand the 

process and pattern of trail flooding and to advise park visitors 

of current and advanced flood hazards.

Tree-ring analysis was used to evaluate the effects 

of flood events and flood history on forest resources at 

Congaree National Park. Tree core samples were collected 

from populations of loblolly pine (Pinus taeda), baldcypress 

(Taxodium distichum), water tupelo (Nyssa aquatica), green 

ash (Fraxinus pennslyvanica), laurel oak (Quercus laurifolia), 

swamp chestnut oak (Quercus michauxii), and sycamore 

(Plantanus occidentalis) within Congaree Swamp in shortand long-retention flood sites. Findings showed that all 

loblolly pine trees and nearly all baldcypress collections for 

this study are postsettlement and old growth, dating from 100 

to 300 years in age. Remnant virgin-growth baldcypress trees 

are present in the park but were not cored for this study to 

avoid complications with growth analysis of millennial-aged 

trees. Hardwood species demonstrated robust growth rates and 

relatively young cohorts, indicative of past logging activity 

and disturbance history. Growth chronologies of loblolly pine 

and baldcypress collections exhibited positive and negative 

inflections over the last century that corresponded with 

macroclimate patterns and residual effects of Hurricane Hugo 

(1989). Water level data demonstrated that operations of the 

Saluda Dam (post-1934) have increased average daily stage 

in the Congaree River and flood plain, whereas tree-ring data 

provided no evidence of long-term change or interannual 

effect in forest productivity related to dam operation or 

flood history. Stemwood production on average was lower 

for trees and species on sites with longer flood retention and 

hydroperiod, affected more by groundwater seepage or site 

elevation than by fluctuating river stage or floods. Overall, 

forest growth is fairly robust for all tree species and controlled 

largely by macroclimatic conditions over the long term, and to 

a limited degree in the short term, by intermittent disturbance, 

either logging or hurricane impact such as canopy tree 

removal and openings. Forest maturity and increasing canopy 

roughness in Congaree Swamp may sustain or increase forest 

turnover of canopy trees in coming years. New forest studies 

should focus on species recruitment in gaps and on the age and 

vigor of subcanopy trees to determine whether flood patterns 

have shaped the contemporary species distribution and 

regeneration that will shape future forests.

Acknowledgments

This study was funded under the joint U.S. Geological 

Survey and National Park Service Natural Resources 

Preservation Program. The author is grateful to Bobbi 

Simpson, former Natural Resource Manager, and Martha 

Bogle, former Superintendent, Congaree National Park, for 

promoting the need, vision, and logistics in support of this 

effort. The author is indebted to many people who contributed 

invaluable assistance with field work and model development 

including Jason Sullivan, Rick Putnam, Andy From, Marcus 

Melder, Ben Handley, Michael Cooper, and Kelia Fontenot. 

William Conner, Chris Swarzenski, Scott Zengel, Rassa 

Dale, and park staff provided valuable technical and editorial 

reviews. Guy Frantello provided land and river access across 

private lands managed under the Sanbar Hunting Club to 

remote park areas and water level recorders important to the 

success of this study. Logistical assistance was provided by 

park staff including Bill Hulslander, Theresa Yednock, and 

others for incidental help to aid field forays.

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