WETLANDS, Vol. 16, No. 3, September 1996, pp. 279-287 © 1996, The Society of Wetland Scientists

HYDROLOGY OF THE BLACK SWAMP WETLANDS ON THE CACHE RIVER, ARKANSAS

R a y m o n d Walton2 Jack E. Davis, 2 Thomas H. Mart in , 3 and R a y m o n d S. C h a p m a n 4

WEST Consultants, lnc. 2101 4 'h Ave, Suite 1050 Seattle, WA 98121-2357

"- Coastal Engineering Research Center U.S. Army Engineer Waterways Experiment Station

Vicksburg, MS 39180-6199

3 Foster Wheeler Environmental Corporation 10900 NE 8th Street

Bellevue, WA 98004-4405

Ray Chapman & Associates 1725 MacArthur Place Vicksburg, MS 39180

Abstract: The hydrologic characteristics of the Cache River wetlands between Patterson and Cotton Plant, Arkansas, were investigated. The Cache River is an underfit stream with wetlands predominantly located in abandoned channels and backswamps. Much of the Cache River upstream of the study area has undergone extensive channelization to allow agricultural development in the basin. Hydrologic measurements included streamflow gages at the upstream and downstream limits of the study area. water-level recorders inside the study area, a nest of deep and shallow ground-water wells that monitored variations in the underlying aquifer, a meteorological recording station that collected precipitation, air temperature, and solar radiation data inside the study area, and regional precipitation data. Analysis of the wetland's water budget showed that the system is dominated by river discharges and that the magnitudes of other water-budget components are less than the error associated with well-maintained streamflow gages (5-10%). The system is characterized by floods occurring from late fall to late winter and again in mid-to-late spring. Peak flood discharges are approximately 185 m3/s for a 2-year event and 270 mVs for a 5-year event. Peak discharges between the upstream and downstream gages are reduced by 10-20% with greater attenuation occurring when the system is initially drier. Peak discharge at the downstream gage lags the peak at the upstream gage by 4-8 days depending on antecedent conditions. The majority of overbank flooding is produced by backwater from several constrictions in the downstream reach of the study area, rather than from the forward movement of the flood wave. Flood- peak attenuation between the upstream and downstream gages is due mainly to floodplain storage, with flow resistance contributing minimally. Finally, the relationship between the hydroperiods at different water-sur- face elevation gages was examined to determine if a long-term record could be used to estimate long-term hydroperiods at interior gage locations (or perhaps at interior computer model locations) with shorter-term information.

Kay Words: wetland hydrology, water budget, flood storage, stream flows, flood peak attenuation, flood discharge attenuation, hydroperiod

I N T R O D U C T I O N

Research has shown that hydrology and the dynamic nature of water flowing through a wetland affect its veg- etation (composition, structure, and diversity), influence

its primary productivity, control its organic accumulat ion and transport, and drive its nutrient cycling (Gosselink and Turner 1978). Hydrology influences the homogene- ity or heterogeneity of the wetlands, from varied topo- graphic relief and vegetative cover to broad monotypi-

279

280 WETLANDS, Volume 16, No. 3, 1996

cally vegetated flats (Bedlinger 1979). Mitsch and Gos- selink (1986) state that "hydrology is probably the single most important determinant for the establishment and maintenance of specific types of wetlands and wetland processes," and Nestler and Long (1994) state that most significant wetland functions can be described complete- ly or in part by hydrologic factors. The impol~ant influ- ences of hydrology on the character of a wetland -are similarly echoed in almost all publications that address the topic of wetland hydrology.

Yet, while the importance of hydrology on wetland ecology seems appreciated, our ability to define the relationship is tenuous. The problem is rooted in the fact that subtle changes in hydrology can cause sub- stantial changes in wetland characteristics (Mitsch and Gosselink 1986). Even if a wetland's hydrology is well understood, the wetland's response to the hydrology would be hard to predict. In the bottomland hardwoods (BLH) of the lower Mississippi Valley, the problem of defining the relationship between hydrologic and other wetland characteristics is also rooted in the fact that relatively few wetlands of this type have been rigor- ously studied.

This paper not only serves as a backdrop for the other papers presented in this issue of Wetlands that refer to the hydrologic regime of the Cache River, but it also is a step toward characterizing the hydrology of lower Mississippi Valley bottomland forests. The anal- yses characterize the Cache River system, identity ma- jor hydraulic and hydrologic processes, and examine the mechanisms that control these processes. A num- ber of data analyses were performed to gain insight into how this system functions. The analyses included correlations between various streamgages, an exami- nation of the water balance of the wetlands, a study of the major hydrologic and hydraulic processes, de- termination of some overall basin properties, and an analysis of hydroperiods at interior locations based on upstream gage data.

BACKGROUND

The study area wetlands (also referred to as tile Black Swamp) lie along the Cache River in eastern Arkansas between Patterson and Cotton Plant (Figure 1). The Cache River is an underfit stream, flowing in an old channel of the present-day Black mad St. Francis rivers. It is located in the Western Lowlands region of the Mississippi River Alluvial Plain. between the Ozark Plateau and the Mississippi River. The wetlands in the basin are predominantly in abandoned channels and backswamps and are generally within 2 km of the Cache River channel.

Much of the Cache River upstream of the study area has undergone extensive ehannelization to allow ag-

ricultural development in the basin. The area of the upstream drainage basin is 2,686 km z (Neely 1987). The study area includes 350 km 2 of the lower part of the drainage basin. Approximately 60 km 2 of the study area are BLH forests and are typical of wooded wet- land systems in the lower Mississippi River Valley (Kleiss 1993).

THE DATA

The data used in this hydrologic analysis of the Black Swamp fall into five categories: system geom- etry, meteorology, surface-water hydrology (discharg- es and stages), ground-water hydrology, and soils and vegetation. The period of record used is from 1 Oc- tober 1987, the start of Water Year (WY) 1988, through September 1993, the end of WY 1993. Elapsed times are generally reported in days from I October 1987.

System Geometry

The Black Swamp (Figure 1) averages about 4 km in width but is constricted in the lower reach. Longitudinal profiles of the Cache River thalweg and channel banks are shown in Figure 2. The average channel slope is 0.15 m/kin. The channel thalweg slope is steepest in the upper and lower reach of the study area and flatter in the mid- dle portion. In the lower reach, the channel is relatively narrow with high banks. In the middle of the study area, the channel is wide with low banks, and water can easily overflow the channel banks in this area and travel lat- erally across and down the floodplain.

Meteorology

Daily rainfall data were obtained from National Oceanic and Atmospheric Administration (NOAA) weather stations at Augusta, Georgetown, Paragould, Wynne, and Brinkley, Arkansas. An unweighted arith- metic average of the measured data was used for the daily rainfall over the study area. A weather station was also deployed within the study area to measure hourly air temperature, solar radiation, wind speed, and wind direction. Measurements are generally sim- ilar between meteorological stations, including the study-area station, when the rain is produced by winter and spring frontal systems. However, during the sum- mer and early fall, local thunderstorms can cause con- siderable variability between stations (an example se- lies from October 1987 is shown in Figure 3).

River Discharges and Stages

The U.S. Army Engineer District, Memphis, main- tains a permanent stage-recording gage on the Cache

Walton et al., H Y D R O L O G Y OF THE BLACK SWAMP 281

Figure 1.

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Black Swamp Wetlands on the Cache River, Arkansas

River near Patterson, Arkansas (Figure 1). The records from the Patterson gage date back more than 60 years (Nestler and Long 1994). Outflow from the study area was gaged at a U.S. Geological Survey (USGS) station located at a Dridge crossing the Cache River about 9 km north of Cotton Plant (Figure 1). The Patterson and Cotton Plant gages are roughly 42 river kilometers apart and effectively mark the upstream and down- stream limits of the study area. Another gage was tern-

porally deployed by the USGS upstream from the Cot- ton Plant gage at James Ferry (Figure 1).

Four study transects (called A, B, C, and D) were established within the Black Swamp (Figure 1). Each transect was surveyed, and a number of soils and veg- etation studies performed. Continuous water-level re- corders (referred to as the B5 and B8 gages) were in- stalled on the B transect about 300 m and 800 m ov- erbank from the main channel (B5 is shown in Figure

282 W E T L A N D S , V o l u m e 16, No. 3, 1996

60

Part "Son 58 - ~ % .

E 56 % ' % " " \ . . . . / " ~ . " COtt~n Plant

0 5 10 15 20 25 30 35 40 45

Distance Downstream from Fat lerson (km)

Existing Bottom BankfuH

Figure 2. Cache River thalweg and bankful] profiles.

1). The hydrographs at B5 and B8 are very similar, as the time period of a flood event is generally large com- pared to the lateral time of travel across the flood plain. Only the longer B5 record was used in the analyses described here. Discrete water-surface elevations were also measured at a number o f transect locations throughout the study period.

Figure 4 shows the periods of record for these gag- es, Mean daily discharges for the USGS gage at Pat- terson and the local average daily rainfall are plotted in Figure 5. This figure shows the wet-dry seasonal cycle, including the relatively rapid rise and recession of the hydrograph at either end of each wet period. The data show that there is a wet season between late fall and late winter and a second wet season soon af- terward in mid- to late spring. During WY 1989 and WY 1991, distinct late-winter dry spells can be seen. In WY 1991, the discharge receded to summer base- flow levels (about 5 mVs) by the end of March. In

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James Ferry

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Figure 4. Stage gages period of record, Cache River, Ar kansas.

1989, the discharge did not fully recede until the end o f April.

To illustrate the runoff response of the watershed to rainfall, individual storm hydrographs and daily rain- fall were compared. Figures 6a-b show storm events occurring with dry antecedent conditions. The "'time base" for storm events (the difference in time between the initial rise in the hydrograph at Patterson and the time at which the hydrograph recedes to has°flow at Cotton Plant) ranged from 3 to 4 weeks. The lag time between the peaks of the hydrographs at Patterson and Cotton Plant ranged from 4 to 8 days. The mean re- duction in peak discharge between the two gages was about 20 %.

Wet season hydrographs show a cumulative re- sponse to frequent rainfall events. In the series from WY 1991 shown in Figure 7, the storm event of 19 December 1990 caused the initial rise, and then many periods of rainfall followed, with about 60 mm occur-

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[

1460 1825 2190

Figure 3. October 1987 precipitation measurements in the Figure 5. Discharge and rainfall for water years 1988- Cache River watershed. 1993, Cache River, Arkansas.

Walton et al., H Y D R O L O G Y OF THE B L A C K S W A M P 283

125 , 100

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Time (Days f~m 1-Ocl-90)

'v Rain + pa.er~rl CottotIPiani- 1

Figure 6. a. 18 April 1988 storm hydrograph, Cache River, Arkansas. b. 1 October 1990 storm hydrograph, Cache River, Arkansas.

ring in January 1991. Events with less rainfall, how- ever, also created a response in the observed hydro- graphs, unlike similar rainfall amounts falling during the dry season. The largest event during this period resulted in a 2-day lag between Patterson and Cotton Plant, with a peak discharge reduction of about 10%.

Time series of stage at the continuous gages at Pat- terson, B5, James Ferry, and Cotton Plant for WY 1990 (Figure 8) show that the floodplain at B5 can remain inundated throughout the winter and spring wet periods. The stage at the B5 gage remained at about ground elevation from January into June, During the 1990 dry season, about seven rainfall events occurred. Five of the events resulted in inundation at B5 lasting one to three weeks, while the remaining events were confined to the channel.

Ground-Water Hydrology

The USGS conducted a ground-water study of the Black Swamp and surrounding region (Gonthier and Kleiss 1994). Static heads were measured in the con-

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Figure 7. Wet season storm hydrograph, Cache River, Ar- kansas.

fining unit and underlying aquifer at 121 wells and 13 staff gages within the wetlands. In general, the stratig- raphy in the area consists o f surficial soil and leaf litter overlying a confining unit of clay and silt, which is 1.5 to 9 m thick. The base of the underlying sand aquifer is about 27 to 48 m in depth and consists o f sands and gravel.

Observed heads suggests that ground-water dis- charge occurs during the wet season when the water table is high, and that ground-water recharge occurs when the water table is low during the dry season. Cases where discharge and recharge occur in various parts of the system were also observed. At this point, ground-water flows within the wetlands cannot be ac- curately quantified because the vertical hydraulic con- ductivities of the soils were not measured.

Soils and Vegetation

The predominant soils in the area are listed as hydric (SCS 1968). They have poor drainage and low infil-

58

52

50 ---';'----'--'-J

0 30 60 90 120 150 4BO 210 240 270 300 330 360 Time (Days from 1-Oc~-198g)

[--~--- Pstte . . . . . B5 ~ James Ferry - - Cotton Plan' 1

Figure 8, River stages from water year 1990, Cache River, Arkansas.

284 WETLANDS, Volume 16, No. 3, 1996

Table 1. Annual water budget on the Black Swamp.

Annual Volume/ Variable Unit Area (m)

Inflow 14 Outflow 16 Rainfall 1 Evapotranspiration 1 Ground-Water Discharge < 1 Infiltration < I

r_ration rates. The soils are classified as Typic Fluva- quents, Typic Ochraqualfs, Albic Glossic Natraqualfs, and Vertic Haplaquepts.

An extensive database of vegetation types, distri- butions, densities, and other properties was collected for the study. In general, vegetation in the most flood- prone areas consists primarily of tupelo gum (Nyssa aquatica L.) and bald cypress (Taxodium distichium (L.) Richard.) At higher elevations, the vegetation is typical o f the oak-hickory communit ies in the bottom- land hardwood tk~rests of the lower Mississippi Valley. There are variations in stem densities and basal area between the vegetative communities, but from a hy- drologic viewpoint, the floodplain can be adequately treated as having uniform roughness and evapotrans- piration due to vegetation.

DATA ANALYSIS

Water Budget

A water budget for the Black Swamp was developed using the rainfall records, estimates of evapotranspi- ration (based Ritchie's [19721 method), estimates of infiltration and ground-water discharge (based on in- formation about the wetlands' soils [SCS 1968], ob- served heads from the USGS study [Gonthier and Kleiss 1994], and typical values of hydraulic conduc- tivity for these soils), and the gaged river discharges. Table 1 shows that river inflows and outflows domi- nate the other water-budget components.

The annual outflow minus inflow is estimated to be approximately 2 m of water over the wetlands area. While this value is consistent with the estimates for the other water-budget components, it is also consis- tent with the error in gaged river discharges, which can be on the order of 10%. Therefore, it is not pos- sible to accurately determine from this analysis of an- nual averages whether the differences between inflows and outflows are explained by the other water budget components, such as ground-water discharge or local rainfall/runoff, or by the errors in the gage values.

Flow exceedance curves at the Patterson and Cotton Plant gages (Figure 9) show that the distributions of

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Percent flow exceedence, Cache River, Arkansas.

flow are very similar, but that on average, the flow at Cotton Plant exceeds that at Patterson by about 2 mVs. This value is consistent with the estimate of local rain- fall over the wetlands, In addition, the figure shows that peak flood discharges are about 185 mVs for a two-year return event and 270 m-Vs for a 5-year event.

Finally, a comparison between the rainfall over the watershed above Patterson with the flows measured at the Patterson gage (Figure 10) indicate that approxi- mately 30 percent of the rainfall contributes to stream- flow at Patterson.

Major Hydro lo~c Processes

During the dry season, peak discharges are reduced between Patterson and Cotton Plant by an average of 10 to 20% (Figure 6). Lateral spreading and overbank storage of the flood wave are responsible for much of the flood-peak attenuation, particularly when the sys- tem is initially dry and the event brief. When the sys- tem is initially wet or when the duration of the event

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O 365 73D "1095 1460 1625 2190 Tir~e (Elays from 1 October 1987)

==--- Flow ~t Patterson "Rainfall ]

Figure 10. Cummmulative flow and rain at Patterson, Cache River, Arkansas.

Walton et aL, H Y D R O L O G Y OF THE BLACK S W A M P 285

6 0 • -

~ 5 4

I I 2 0 4 0 6 0 8 0 4 0 0

Percent E x e e e d s n c e Of S t a g e

o P a t t e r s o n - - B 5 ~ James Fecry ~ C o t t o n P l a n t

Figure II. Stage duration--Patterson, Cotton Plant, B5, and James Ferry, Cache River, Arkansas.

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~ 0 0 0 6 . . . . . . . ~ = . - . . . . . . . .

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5t 5~.5 52 525 53 53.5 ~ 545 55 55.5 5o 5{}.5 57 S t a g e (m)

Figure 12. Cotton Plant stage, and gradient between James Ferry and Cotton Plant, Cache River, Arkansas.

is relatively long, flood-peak attenuation is smaller be- cause overbank storage does not completely drain be- tween storm events (Figure 7). Peak attenuation is in- fluenced most by aboveground, floodplain storage with overbank friction contributing to a lesser extent. This is because the time base for the flood events is long compared to the time required for floodwaters to travel laterally across the floodplain.

The most important mechanism causing overbank flooding in the Black Swamp is the natural topographic constrictions between the James Ferry and Cotton Plant gages. The effect of these constrictions is to in- crease river stages upstream so that the same discharge can be forced through the narrower width. This in- crease in stage causes extensive overbank flooding in the wetlands above James Ferry where the riverbanks are low and floodplains extensive.

This mechanism can be seen by examining the times series of the various gages in the study area (Figure 8). As river discharges increase, the water-surface el- evations at Cotton Plant, James Ferry, and B5 become more equal, hence the water surface gradient between James Ferry and Cotton Plant decreases. If the gage records are plotted as stage-duration curves (Figure 11), we can see that at high stages, the water-surface elevations at B5, James Ferry and Cotton Plant be- come more equal. The figure shows that the backwater effect does not extend upstream to Patterson. In fact, looking at the system geometry in Figure 2, it is likely that the backwater extends only to 10-15 km below Patterson and that overbank flooding further upstream is due to the elevation of the forward-moving flood wave from Patterson. Finally, if the stage at Cotton Plant is plotted against the water surface gradient be- tween James Ferry and Cotton Plant (Figure 12), the gradient clearly decreases as the stage at Cotton Plant increases to permit the higher flows to exit the system.

Hydroperiod

The length of time that an area is flooded is called the hydroperiod and is of particular interest to wetland scientists. For example, tree ring studies might need 50 or more years of hydroperiod data to correlate in- undation with tree growth. For the Cache River study, the data were examined to determine relationships be- tween observed stages and depths and durations of in- undation. Initial results suggested that the best predic- tor of overbank stages, such as at B5, would come from the stages at Cotton Plant rather than from the upstream stages at Patterson. This is consistent with the fact that the constrictions between James Ferry and Cotton Plant control the water-surface elevations up- stream. The gage at Patterson, however, has a much (decades) longer period of record than the other gages, and therefore, it would be useful to see if a relationship could be found between the stages at Patterson and the hydroperiods of overbank gages such as B5.

Figure 13 shows a linear regression between Patter- son and B5. The R 2 of 0.781 is smaller than the R 2 of 0.983 between Cotton Plant and B5. To improve the correlation, the time lag for the flood wave to travel between Patterson and B5 and the bankfull elevations were also considered, During the wet season, it was observed that the peak discharges at Cotton Plant lag Patterson by an average of about 4 days, As B5 is about at the midpoint, a lag time of 2 days was se- lected for this analysis. From the observations of stage at Patterson and B5 (Figure 8), the stage at Patterson reaches about 57.5 m National Geodetic Vertical Da- tum (NGVD) before overbank flooding is seen at B5, where the bank elevation is about 54.6 rn NGVD. Therefore, the data were reanalyzed using two-day- lagged stages at Patterson above 57.5 m and then com- pared to stages at B5 (Figure 14). The correlation im- proved to R 2 = 0.888.

286 WETLANDS, Volume 16, No. 3, 1996

57 .0 • - - - I

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~ 5 5 5

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0 5 = 8 . 6 6 3 + (0 .801"PT) W h e r e : R - s q u a r e d = 0 .781 BS = S t a g e a t 05

PT = S tage a t Pa t te rson

Figure 13. Regression analysis of stages at Patterson and B5 (no lag), Cache River, Arkansas.

As the stages at Patterson and B5 are correlated, their hydroperiods should also be correlated. Using the original gage records, the number of days that the stage at B5 exceeded each of 10 selected elevations (between ground elevation and the maximum observed water-surface elevation) were noted. The record at Pat- terson was then examined until 10 elevations giving t he s a m e number of days of stage exceedance were determined. Mean hydroperiods for each gage were then calculated by dividing the total number of days that the 10 elevations were exceeded by the number of "events" producing the exceedance. The results, shown in Table 2, indicate that a reasonable estimate of B5 hydroperiod total days can be made from ob- served stages at Patterson.

SUMMARY

Research has made clear that the characteristics of wetlands are largely controlled by the wetlands' hy- drologic regime. Hence, a hydrologic analysis of the Cache River in eastern Arkansas was conducted as part of a comprehensive field investigation of the processes and functions of bottomland hardwood forests. The analyses provide a backdrop to the other research con- ducted during the study, some of which is presented in this issue of W e t l a n d s .

A water budget was developed for the study area. The water budget showed that the system is dominated by river infows and outflows and that the estimated contribution of the other water-budget components (e.g., precipitation, ground-water discharge, evapo- transpiration) is lena than 10% of the river discharge. Since errors associated with streamflow gages are also about 10%, it is difficult to determine whether diffEr- ences between measured river inflows and outflows can be accounted for by the other water-budget com-

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Figure 14. Regression analysis between stages at Patterson and B5 (2-day lags, Cache River, Arkansas.

portents. However, differences between inflows and outflows are consistent with rainfall-runoff within the wetlands.

The analysis showed that the system is characterized by floods occurring from mid-fall to midwinter and again in mid-to-late spring. Peak flood discharges are about 185 m3/s for a two-year return event and 270 mVs tbr a 5-year event. The peak discharges between the upstream and downstream gages are reduced by about 10 to 20% with greater attenuation occurring when the system is initially drier. The flood-peak at- tenuation between the upstream and downstream gages is due mainly to floodplain storage with flow resistance contributing little. The peak discharge at the down stream gage lags the peak at the upstream gage by 4 to 8 days depending on antecedent conditions.

Analyses of stages at system gages showed a strong correlation between interior wet/and locations and the downstream Cotton Plant gage. This is consistent with the conclusion that flooding (and hence stage) is con-

Table 2. Hydroperiods (in days) at Station B5 and Patter- son.

B5 Patterson

Elevation Total Event Elevation Total Event (m) Days Mean (m} Days Mean

54.6 511 170 56.7 510 510 54.8 382 42 57.8 379 31 55.0 303 60 58.2 300 33 55.2 258 43 58.5 249 24 55.4 221 36 58.5 218 18 55.6 175 29 58.7 171 17 55.8 134 26 58.8 129 16 56.0 102 25 58.9 100 16 56.2 57 14 59.0 50 10 56.5 23 II 59.3 19 6

Walton et aL, HYDROLOGY OF THE BLACK SWAMP 287

trolled by natural constrictions in the downstream reach of the study area. Additional correlations were made between the B5 gage near the middle of the study area and the upstream Patterson gage. The re- lationship was not as strong as that between B5 and Cotton Plant. However, when the data were manipu- lated to account for the travel time between Patterson and B5, the correlation was improved. This suggests that it might be possible to develop hydroperiod rela- tionships between relatively short periods of record measured within the wetland to a longer period of rec- ord maintained at a streamflow gage (.such as the 60 years of record at Patterson). The correlation could provide a reasonable approximation of stage (and therefore hydroperiod) at the interior wetland site for the same period of record as the streamflow gage.

A C K N O W L E D G M E N T S This work was funded by the U.S. Army Corps of

Engineers Wetland Research Program. Permission to publish this paper was granted by the Chief of Engi- neers.

LITERATURE CITED Bedlinge~; M.S. 1979. Relation Between Forest Species and Flood-

ing. R 427-435. In P.E, Gr¢cson, J.R. Clark, and J.E. Cblrk (eds.)

Wetland Functions and Values: The Slate of Our Understanding. American Water Resources Association, Minneapolis, MN, USA+

Gonthier, G.J. and B.A. Kleiss. 1993. Ground-water flow patterns and water budget of a bottomland forested wetland: Black Swamp, Eastern Arkansas. U.S. Geological Survey Water Resources In- vestigations Report 95-4192.

Gosselink, J,G. and R.E. Turner. 1978. The role of hydrology in freshwater wetland ecosystems, p_ 63-78. In R_P~'. Good, I).E Whigham, and R.L. Simpson (eds/. Freshwater Wetlands: Ecolog ical Processes and Management Potential. Academic Press, New York, NY, U'SA.

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Manuscript received 28 March 1995; revision received 20 November 1995: accepted 17 January 1996.