U.S. Department of the Interior

U.S. Geological Survey

Simulated Effects of Impoundment of Lake Seminole 

on Ground-Water Flow in the Upper Floridan Aquifer 

in Southwestern Georgia and Adjacent Parts of 

Alabama and Florida

Prepared in cooperation with the

Georgia Department of Natural Resources

 Environmental Protection Division

 Georgia Geologic Survey

Scientific Investigations Report 2004-5077
Cover: Northern view of Jim Woodruff Lock and Dam from the west bank of the Apalachicola River. Photo by Dianna M. Crilley, U.S. 

Geological Survey. 

A. Map showing simulated flow net of the Upper Floridan aquifer in the lower Apalachicola-Chattahoochee-Flint River basin under 

hypothetical preimpoundment Lake Seminole conditions.

B. Map showing simulated flow net of the Upper Floridan aquifer in the lower Apalachicola-Chattahoochee-Flint River basin under 

postimpoundment Lake Seminole conditions.
Simulated Effects of Impoundment of Lake Seminole 

on Ground-Water Flow in the Upper Floridan Aquifer 

in Southwestern Georgia and Adjacent 

Parts of Alabama and Florida

By L. Elliott Jones and Lynn J. Torak

Prepared in cooperation with the

Georgia Department of Natural Resources

Environmental Protection Division

 Georgia Geologic Survey

Scientific Investigations Report 2004-5077

U.S. Department of the Interior

U.S. Geological Survey

Atlanta, Georgia
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

This report is available on the World Wide Web at http://infotrek.er.usgs.gov/pubs/ 

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the 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:

Jones, L. Elliott, and Torak, Lynn J., 2004, Simulated Effects of Impoundment of Lake Seminole on Ground-Water Flow 

in the Upper Floridan Aquifer in Southwestern Georgia and Adjacent Parts of Alabama and Florida: U.S. Geological 

Survey, Scientific Investigations Report 2004-5077, p. 22.
iii

Contents

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

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

Purpose and Scope ................................................................................. 3

Study Area and Physiography ..................................................................... 3

Geohydrology ....................................................................................... 5

Numerical Methods ....................................................................................... 8

Model Background and Assumptions ............................................................ 8

Vertical Hydraulic Conductance .................................................................. 9

Cauchy-Type Boundaries ........................................................................ 9

State-Line Flow .................................................................................... 10

Simulation Results ....................................................................................... 10

Uncertainty and Variability of Parameters ............................................................ 16

Summary ................................................................................................... 17

References Cited .......................................................................................... 17

Figures

 1–2. Maps showing:

1. Location of study area, lower Apalachicola–Chattahoochee–Flint (ACF) 

River Basin area, lower ACF River Basin model area, and physiographic 

districts of the Coastal Plain Province ............................................................................. 2

2. Southern part of the lower Apalachicola–Chattahoochee–Flint River 

Basin model area, showing physiographic districts, topography, major 

streams, lakes, and cities .................................................................................................. 4

3. Correlation chart of geologic and hydrologic units comprising the stream-lakeaquifer flow system near Lake Seminole ................................................................................. 6

4. Diagram of hydrogeologic units and conceptualization of ground-water and 

surface-water flow in the vicinity of Lake Seminole ............................................................. 7

5. Map showing boundary conditions of the lower Apalachicola–Chattahoochee–

Flint River Basin model in the Lake Seminole area, showing modifications for 

simulating hypothetical preimpoundment conditions ........................................................... 9

6. Graphs showing model-input river stage for Cauchy-type boundaries representing 

the Chattahoochee and Flint Rivers and Spring Creek, hypothetical preimpoundment 

conditions and postimpoundment conditions ....................................................................... 10

 7–12. Maps showing:

7. Preliminary simulated increase in ground-water level in the Upper Floridan 

aquifer due to the impoundment of Lake Seminole based on initial 

preimpoundment conditions ............................................................................................ 10
iv

8. Estimated decrease in input-head values along the southeastern external 

Cauchy-type boundary to account for lower hypothetical preimpoundment 

water levels in the Upper Floridan aquifer .................................................................... 11

9. Simulated increase in ground-water level in the Upper Floridan aquifer 

due to the impoundment of Lake Seminole and accounting for lower 

preimpoundment water levels along the southeastern external 

Cauchy-type boundary ..................................................................................................... 12

10. Simulated potentiometric surface and directions of ground-water flow in the 

Upper Floridan aquifer from simulation of (A ) hypothetical preimpoundment 

conditions and (B ) postimpoundment conditions ........................................................ 13

11. Simulated ground-water recharge or discharge rates at Cauchy-type 

boundaries from simulation of (A) hypothetical preimpoundment conditions 

and (B ) postimpoundment conditions ............................................................................ 14

12. Simulated flow rates in the Upper Floridan aquifer adjacent to zones of 

element sides and at Cauchy-type boundaries along the Georgia–Alabama 

and Georgia–Florida State lines: (A ) hypothetical preimpoundment conditions; 

and (B ) postimpoundment conditions ............................................................................ 16

Tables

1. Simulated water-budget components for the lower Apalachicola–

Chattahoochee–Flint River Basin model, hypothetical preimpoundment and 

postimpoundment conditions. ................................................................................................. 13

2. Simulated discharge to streams in the Lake Seminole area, hypothetical 

preimpoundment and postimpoundment conditions. .......................................................... 14

3. Simulated regional flow rates at external model-boundary zones in the Lake 

Seminole area, hypothetical preimpoundment and postimpoundment conditions. ....... 15

Conversion Factors and Datum

Factors for converting inch-pound units to the International System (SI) of units are given below: 

Multiply By To obtain 

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Flow

million gallons per day (Mgal/d) 0.04381 cubic meters per second (m3/s)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88). Historical data collected and stored as National Geodetic Vertical Datum of 1929 

have been converted to NAVD 88 for this publication.
Abstract

 Hydrologic implications of the impoundment of Lake 

Seminole in southwest Georgia and its effect on components 

of the surface- and ground-water flow systems of the lower 

Apalachicola–Chattahoochee–Flint (ACF) River Basin were 

investigated using a ground-water model. Comparison of 

simulation results of postimpoundment drought conditions 

(October 1986) with results of hypothetical preimpoundment 

conditions (a similar drought prior to 1955) provides a qualitative measure of the changes in hydraulic head and groundwater flow to and from streams and Lake Seminole, and across 

State lines caused by the impoundment.

Based on the simulation results, the impoundment of 

Lake Seminole changed ground-water flow directions within 

about 20–30 miles of the lake, reducing the amount of ground 

water flowing from Florida to Georgia southeast of the lake. 

Ground-water storage was increased by the impoundment, as 

indicated by a simulated increase of as much as 26 feet in the 

water level in the Upper Floridan aquifer. The impoundment 

of Lake Seminole caused changes to simulated components 

of the ground-water budget, including reduced discharge from 

the Upper Floridan aquifer to streams (315 million gallons per 

day); reduced recharge from or increased discharge to regional 

ground-water flow at external model boundaries (totaling 

183 million gallons per day); and reduced recharge from or 

increased discharge to the undifferentiated overburden (totaling 129 million gallons per day).

Introduction

Lake Seminole is a 37,600-acre (58.8-square-mile [mi2]) 

impoundment, located in extreme southwestern Georgia and 

northwestern Florida (fig. 1), that was formed from construction of Jim Woodruff Lock and Dam by the U.S. Army Corps 

of Engineers (USACE) from the late 1940s to mid-1950s. The 

dam is situated on the Apalachicola River about 1,000 feet 

(ft) downstream of where the Flint River and Chattahoochee 

River channels converge near the Georgia-Florida State line, 

approximately 107 miles (mi) upstream from Apalachicola 

Bay and the Gulf of Mexico. About 250 mi of lake shoreline 

are irregularly distributed along two major impoundment arms 

on the Chattahoochee and Flint Rivers and along two minor 

impoundment arms on Spring Creek and Fishpond Drain, both 

of which are tributary to the Flint River impoundment arm. 

Backwater conditions inundate natural courses and floodplains 

of these streams, extending about 47 mi from the dam up the 

Chattahoochee River and Flint River impoundment arms (U.S. 

Army Corps of Engineers and others, 1984), and extending 

lesser distances upstream of the dam along other tributaries.

Jim Woodruff Lock and Dam is a multiple-purpose structure operated by the USACE, the primary functions of which 

are to aid navigation in the Apalachicola River downstream of 

the dam and in the Chattahoochee and Flint Rivers upstream, 

and to generate hydroelectric power. Secondary benefits of the 

dam include public recreation, regulation of streamflow, and 

fish and wildlife conservation. Despite its size, Lake Seminole 

is a run-of-the-river impoundment, dependent on inflow from 

the Chattahoochee and Flint Rivers to maintain flow in the 

Apalachicola River downstream of the dam. No flood-control 

storage is available in Lake Seminole (U.S. Army Corps of 

Engineers and others, 1984).

Construction of the lock and dam began during September 

1947; the lock was opened to navigation during May 1954; 

and full pool (77 ft) was attained on February 4, 1957, at 

which time the power plant began operation (U.S. Army Corps 

of Engineers and others, 1984). At extreme low flow, the dam 

provides a 33-ft pool differential between tail water in the 

Apalachicola River (44 ft) and normal lake stage (full pool).

Releases from the dam and the impounded water behind it 

have become important local resources for shipping, hydroelectric power, and recreation. Outflow from the lake has a 

direct influence on the water resources, ecology, and economy 

of the Apalachicola River, its floodplain and estuary, and the 

Apalachicola Bay region, affecting navigation, hydroelectric 

power generation, flow regulation, and the supply of nutrients and detritus that support diverse aquatic biota, including 

Apalachicola Bay’s diverse shellfish population.

Simulated Effects of Impoundment of Lake Seminole on 

Ground-Water Flow in the Upper Floridan Aquifer in

Southwestern Georgia and Adjacent Parts of 

Alabama and Florida

By L. Elliott Jones and Lynn J. Torak
of the dam. The remaining 2,000-mi2

 area is tributary directly 

to the Apalachicola River (U.S. Army Corps of Engineers, 1973).

Of local and regional importance is the roughly 5,000 mi2 of 

drainage area proximate to Lake Seminole within the Dougherty Plain and Marianna Lowlands divisions of the Coastal 

Plain physiographic province (fig. 1). This region is underlain 

by karst limestone of Eocene and Oligocene age that comprises 

the Upper Floridan aquifer. The lake interacts hydraulically 

with the limestone by direct leakage through sinkholes, cavities, and other dissolution features that were once exposed in 

the karstic floodplains of the Chattahoochee and Flint Rivers,

but since impoundment are part of the lake bed. A relatively 

thin mantle of chemically weathered limestone residuum and 

alluvium, collectively termed undifferentiated overburden, overlies the aquifer and allows indirect leakage to and from Lake 

Seminole where these sediments are present in the lake bed. 

Many springs originating in the limestone discharge into Lake 

Seminole along the lake bottom or adjacent to the impoundment 

arms, flowing into the lake from small channels or spring runs.

The Upper Floridan aquifer is one of the most productive 

aquifers in the United States, and in the Lake Seminole area is 

the primary source of ground water for agriculture, industry, 

and public supply. Agricultural irrigation is the major groundwater use in this heavily agricultural region of southwestern 

Georgia and northwestern Florida. Nearly 500,000 acres are 

irrigated with ground water from about 4,000 wells completed 

in the Upper Floridan aquifer in the lower ACF River Basin 

(James E. Hook, National Environmentally Sound Production 

Agricultural Laboratory, the University of Georgia, Tifton, 

Georgia, written commun., November 2002).

In recent years, Lake Seminole and the water released from 

it have become focal points in water-allocation negotiations 

between Georgia, Florida, and Alabama that resulted from the 

ACF River Basin Compact.1 Increases in population, agriculture, and industry and the drought of 1998–2002 have made 

water supply and use in the lower ACF River Basin a major 

concern for water-resource managers in the region, as the three 

States’ conflicting demands have created competition for the 

basin’s limited water resources. These concerns led the States 

to sign an interstate water compact during 1997, intended to 

ensure the equitable use and availability of water resources in 

the region while protecting river ecology.

To acquire information and increase the understanding 

needed by water-resource managers, the U.S. Geological 

Survey (USGS) and the State of Georgia agreed to conduct 

jointly a technical study of Lake Seminole and the surrounding 

area. The 3-year study, which began in 1999, had the following

four objectives:

Figure 1. Location of study area, lower Apalachicola–

Chattahoochee–Flint (ACF) River Basin area, lower ACF 

River Basin model area, and physiographic districts of 

the Coastal Plain Province (modified from Jones and 

Torak, 2003).

1

As adopted by: the Alabama Legislature on February 18, 1997, and signed by the Governor of Alabama on February 25, 1997, as Alabama 

Acts 97-67, Alabama Code, Title 33-19-1 et seq.; the Florida Legislature on April 14, 1997, and signed by the Governor of Florida on April 

24, 1997, as Chapter 97-25, Laws of Florida, Section 373.71, Florida Statutes (1997); the Georgia Legislature on February 11, 1997, as 

Georgia Acts No. 7, and signed by the Governor of Georgia on February 25, 1997, as Georgia Code Annual Section 12-10-100 et seq., and 

passed by the United States Congress on November 7, 1997, and signed by the President of the United States on November 20, 1997, as 

Public Law Number 105-104, 111 Statute 2219; http://www.acfcompact.alabama.gov/pdfs/ACFCompact.pdf, accessed August 1, 2003.

ALABAMA GEORGIA

FLORIDA

ACF River

Basin

Map

area

Map area shown in:

a. figures 2 and 7–11

b. figures 5 and 12 a

Physiographic-district

boundary

Lower ACF River

Basin

EXPLANATION

Base modified from U.S. Geological Survey 1:100,000-scale digital data

G EORG IA

F LOR ID A

G EORG IA

F LOR ID A

A L A B AMA

A L A B AMA

0 10 20 MILES

0 10 20 KILOMETERS

30°

31°

32°

85° 84°

Gu l f o f Me x i c o

F v l i n t R i e r

A a pa l chicola R iver

River

Chattaho che o e G U L F C O A S TA L L O W L A N D S

T I F T O N

U

A P L

N D

D O U G H E R T Y

P L A I N

L O W L A N D S

M A R I A N N A

FA L L L I N E

H I L L S

Lake Seminole

Lower ACF

River Basin

model area

Jim Woodruff

Lock & Dam

b

a

up

U

d

dip i l F imit pper or o a a f r l n quife

Approximate

The Apalachicola–Chattahoochee–Flint (ACF) River Basin 

encompasses a long, narrow area of about 19,200 mi2, mostly 

in western and southwestern Georgia and partly in southeastern 

Alabama and northwestern Florida (fig. 1). About 17,200 mi2 

of the drainage basin are tributary to Lake Seminole, being nearly 

equally divided between the Chattahoochee and Flint Rivers, 

which provide headwater to the Apalachicola River downstream 

2 Simulated Effects of Impoundment of Lake Seminole
• Develop a water budget for Lake Seminole that will 

promote a reasonable understanding of the effect of the 

lake on the overall flow system in the lower ACF River 

Basin, and that can be used to guide water allocations 

between Alabama, Florida, and Georgia;

• Compare current and pre-Lake Seminole ground- and 

surface-water flow regimes to determine whether the 

volume of water flowing out of Georgia has changed 

substantially after construction of Jim Woodruff Lock 

and Dam and filling of the lake;

• Evaluate the possibility of a substantial amount of 

water entering the ground-water regime from Lake 

Seminole, flowing beneath Jim Woodruff Lock and 

Dam, and entering Florida downstream of the dam; and

• Assess the likelihood of failure of dissolution features 

in the karst limestone of the lake bottom, such as sinkhole collapse, and the likelihood of sudden partial or 

complete draining of the lake. If such an occurrence is 

likely, then propose a data-collection system to monitor 

pertinent hydrologic features that could cause sudden 

draining of Lake Seminole, and to provide data that 

could warn of such an occurrence.

The three-year study investigated features of the hydrologic system near Lake Seminole that contribute directly to 

the ground- and surface-water flow regime of the lake. The 

study focused only on those elements of the hydrologic cycle, 

surface-water features, and hydrogeologic units that are in 

hydraulic connection with the lake. A multidiscipline investigative approach was used that involved acquisition, analysis, 

and interpretation of chemical, limnologic, hydrogeologic, and 

meteorologic information, quantification of uncertainty, and 

numerical simulation.

Purpose and Scope

This report is one of a series of reports documenting the 

3-year study to evaluate the effects of impoundment of 

Lake Seminole on water resources in the lower ACF River 

Basin. The second of four study objectives listed previously 

is addressed in this report, namely, to compare current and 

pre-Lake Seminole ground- and surface-water flow regimes to 

determine whether the volume of water flowing out of Georgia 

has changed substantially since construction of Jim Woodruff 

Lock and Dam and filling of the lake. The following technical 

tasks were performed to achieve this objective:

• Evaluate existing information on ground-water levels, 

hydraulic properties, and streambed characteristics to 

define current (postimpoundment) and pre-Lake Seminole (preimpoundment) ground- and surface-water 

flow conditions;

• Modify an existing numerical model of stream-lakeaquifer interaction developed by Torak and others 

(1996) near Lake Seminole, and simulate pre- and 

postimpoundment flow conditions; and

• Compare results of simulations representing pre- and 

postimpoundment conditions to evaluate changes in 

ground-water flow directions in the Lake Seminole area.

This study uses results from a modified version of a 

numerical model for the lower ACF River Basin (Torak and 

others, 1996) to evaluate pre- and postimpoundment conditions of the stream-lake-aquifer flow system near Lake Seminole. Descriptions of the geohydrology, conceptualization of 

the stream-lake-aquifer flow system, and of model implementation and calibration given for that study also apply to this 

study. Therefore, for brevity, full descriptions of these aspects 

of the study are not repeated herein, but can be found in the 

earlier report (Torak and others, 1996); only those details that 

are pertinent to the vicinity of Lake Seminole and to study 

results are presented and discussed herein.

Study Area and Physiography

The study area is located in the Coastal Plain physiographic 

province and consists of about 2,000 mi2 surrounding Lake 

Seminole in extreme southwestern Georgia and northwestern 

Florida, and about 250 mi2 located in the extreme southeastern corner of Alabama (fig. 2). In the study area, the Coastal 

Plain Province is subdivided into physiographic districts: the 

Fall Line Hills, Dougherty Plain, Marianna Lowlands, Tifton 

Upland, and Gulf Coastal Lowlands (figs. 1 and 2). Descriptions of these physiographic provinces are summarized from 

Cooke (1925), Puri and Vernon (1964), Sapp and Emplaincourt (1975), Clark and Zisa (1976), Brooks (1981), Wagner 

and Allen (1984), and Torak and others (1996).

The northern extent of the study area is in the Fall Line Hills 

district at the updip limit of the Ocala Limestone, which is the 

principal water-bearing unit of the Upper Floridan aquifer in the 

study area. The district is a highly dissected series of ridges and 

valleys that diminish in relief to the south and east (fig. 2) and 

has little level land except for marshy floodplains and adjacent 

better-drained, narrow stream terraces. Stream valleys range 

from 50 to 250 ft below adjacent ridges. In the study area, the 

southern boundary of the Fall Line Hills approximates the 250-ft

elevation that separates the district from the Dougherty Plain.

Lake Seminole lies within the Dougherty Plain, an inner 

lowland having an irregular and undulating surface characterized by heterogeneous stream-channel development and karst 

topography. Many shallow sinks and depressions, ranging in 

size from a few tens of square feet to several hundred acres 

and some containing water year-round, speckle the landscape 

and provide evidence of active dissolution that is occurring at 

or below land surface in the limestone aquifer. Dissolution of 

the Ocala and Suwannee Limestones in the Dougherty Plain 

has created a subsurface, internal drainage (fig. 2), typical of 

karst topography; main-stem streams flow in terraced valleys 

and cut shallow channels across the Dougherty Plain, however, 

small tributary streams are scarce.

Introduction 3
Figure 2. Southern part of the lower Apalachicola–Chattahoochee–Flint River Basin model area, showing 

physiographic districts, topography, major streams, lakes, and cities. (See figure 1 for map area.)

4 Simulated Effects of Impoundment of Lake Seminole

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

Base from USGS 1:100,000-scale digital elevation model

and USGS 1:100,000-scale digital line graphs

F i s h

p o n d D r a i n

F l i n t R i v e r

B i g S l o u g h

S p r i n g C r e e k

C o w a r t s C r e e k

O c h o l o c k n e

e

R i v e r

S a w h a t c h e r e C e e k

M a e r s h a l l C er k

D r e y C r e k

A p

a

a l c h i c o l a

R i e v r

C h o i p i l a

R e v r

C

h a t

r

t a o

h

o c h i e e

R v e

31° ′ 30

31°

30° ′ 30

85° 84° ′ 30

G E O R G I A

F L O R I DA

A L A B A M A

Blakely

Headland

Marianna

Blountstown

Bainbridge

Colquitt

Donalsonville

Newton

Chattahoochee

Gretna

Quincy

Tallahassee

G U L F C O A S

D S

T A L L O W L A N

T I F T O N U P L A N D

L O W L A N D

M A

S

R I A N N A

D O U G H E R T Y

P L A I N

F A L L L I N E

H I L L S

Jim

Woodruff

Lock & Dam

La k e S l e mino e

P e l

a m

E s

h

c a

p r

m

e

n t

Altitude of colors used in digital

elevation model, in feet above

NAVD 88 0

230

460

Physiographic-district boundary

Lower ACF River Basin model

boundary

EXPLANATION
A prominent, steeply sloping karst area termed the Pelham 

Escarpment1 (Clark and Zisa, 1976) bounds the Dougherty 

Plain along the southern side of the Flint River impoundment 

arm of Lake Seminole (fig. 2). The Pelham Escarpment produces local relief as great as 125 ft, forming a topographic and 

surface-water divide in the study area between the Flint River 

Basin to the northwest and the Ochlockonee River Basin to the 

southeast (Torak and others, 1996). Many small streams flow 

northwestward down the Pelham Escarpment and into caves 

and sinkholes along the eastern edge of the Dougherty Plain 

(Sever, 1965).

In northwestern Florida and extreme southeastern Alabama, the Coastal Plain Province in the study area has been 

subdivided by Vernon (1951) into a minor geomorphic unit 

termed the Marianna River Valley Lowlands, or, as proposed 

by Cooke (1939), the Marianna Lowlands (fig. 2). Like the 

Dougherty Plain, the Marianna Lowlands consists of low, 

generally flat or rolling topography that resulted from a complicated sequence of stream erosion, deposition, and capture 

by several streams, including the Apalachicola, Chattahoochee 

and Chipola Rivers. The lowland has existed since the early 

Pleistocene, being formed by removal and dissection of Miocene clastic sediments, which expose underlying limestone 

units at the surface. As a result of this erosion, the normal 

topography of a coastal plain has been formed, rather than 

karst topography (Moore, 1955). The Chattahoochee River 

forms the eastern boundary of the Marianna Lowlands (fig. 

2), separating it from the physiographically similar Dougherty 

Plain. Small streams that are tributary to the Chipola River 

and extend to the east and northeast have produced a northfacing escarpment, which forms the southern boundary of the 

Marianna Lowlands.

The Tifton Upland is a region of relatively high hills composed largely of resistant clayey sands, silts, and clays, located 

to the southeast of the Pelham Escarpment (Arthur and Rupert, 

1989). A dendritic drainage pattern dissects the hills, forming 

V-shaped valleys. In northwestern Florida, the Tifton Upland 

is termed the Tallahassee Hills (Puri and Vernon, 1964). The 

Tifton Upland ends abruptly at the Apalachicola River in steep 

bluffs having relief of about 150–200 ft above the floodplain 

that exposes Miocene to Holocene sediments. Torak and others 

(1996) described additional details about the Tifton Upland 

and a similar series of remnant hills and sand-hill ridges 

located to the west of the Apalachicola River and southwest of 

Lake Seminole.

In northwestern Florida, south of the Tifton Upland and 

Marianna Lowlands is the Gulf Coastal Lowlands, a sandy, 

flat, seaward-sloping physiographic district. In the northern 

part of the Gulf Coastal Lowlands are weathered and partially 

eroded relics of deltaic deposits in late Miocene to Pliocene 

sandy and silty soils. South of these paleodelta features are 

river terrace deposits that have eroded alluvial plains. These 

features were shaped mostly by wave and current activity from 

high sea-level stands during the Pleistocene Epoch (Arthur and 

Rupert, 1989).

Geohydrology

Geologic units of the stream-lake-aquifer flow system in 

the Lake Seminole area consist of Coastal Plain sediments of 

middle Eocene to Holocene age; these sediments typically are 

cross-bedded clayey sands, sands, and gravels; clay; limestone; 

dolomite; and limestone residuum, occurring in an offlapping 

sequence that dips gently and thickens gradually to the southeast. These units are, in ascending order, Lisbon Formation; 

Clinchfield Sand; Ocala, Suwannee, and Tampa Limestones; 

Hawthorn Group; undifferentiated overburden (residuum); and 

terrace and undifferentiated (surficial) deposits (fig. 3).

The Lisbon Formation of middle Eocene age is thick 

and dense throughout most of the study area and functions 

as a nearly impermeable base to the Upper Floridan aquifer. 

The surface of the Lisbon Formation crops out in the Fall 

Line Hills physiographic district in the northwestern part of 

the study area, dips to the southeast and south, and is nearly 

1,000 ft below NAVD 88 at the southern boundary of the 

lower ACF River Basin model area (Miller, 1986).

The Clinchfield Sand of late Eocene age overlies the 

Lisbon Formation (fig. 3) and crops out less than 1 mi beyond 

the updip limit of the overlying Ocala Limestone (Herrick, 

1972). Downdip the sand grades into the Ocala Limestone 

(Herrick, 1972).

The Ocala Limestone of late Eocene age overlies the 

Lisbon Formation and the Clinchfield Sand (fig. 3), where 

it is present in the Dougherty Plain, and consists of a “white 

bioclastic limestone that is honeycombed with solution cavities” (Sever, 1965). Locally, the surface of the Ocala Limestone is irregular as a result of dissolution of limestone and 

development of karst topography. In some areas, the upper 

few feet of the limestone in the subsurface consists of soft, 

clayey residuum (Miller, 1986). In extreme southeastern 

Alabama, the Ocala Limestone thickens to about 300 ft (Torak 

and others, 1996, pl. 3). The Ocala Limestone is about 250 ft 

thick at Bainbridge, Ga., thins to about 100 ft to the northwest 

near Donalsonville, Ga., and is absent farther to the northwest at the Chattahoochee River. Beneath the Tifton Upland, 

the Ocala Limestone thickens to about 750 ft and becomes a 

brown saccharoidal dolomitic limestone containing gypsum 

(Sever, 1965). The Ocala limestone in the study area has been 

subdivided into units that have distinct geohydrologic characteristics, as described by Moore (1955), Sever (1965), and 

Miller (1986).

1

The Pelham Escarpment also has been called the Solution Escarpment by (MacNeil, 1947), Sever (1965), Hicks 

and others (1987), and Torak and others (1996).

Introduction 5
The Suwannee Limestone of Oligocene age overlies the 

Ocala Limestone in the southern part of the study area. The 

Suwannee Limestone crops out at the base of the Pelham 

Escarpment but is absent from most of the Dougherty Plain 

(Sever, 1965). In northwestern Florida, the Suwannee Limestone crops out west of Lake Seminole. The limestone forms 

part of the bed of Lake Seminole, extending from the dam 

about 9 mi up the Chattahoochee River impoundment arm and 

16 mi up the Flint River impoundment arm, bordering the Pelham Escarpment (Sever, 1965, pl. 2). Thickness of the Suwannee Limestone varies from about 10 ft in the western part of 

the study area in Florida, to about 115 ft in Florida to the west 

of Lake Seminole near the dam, to about 210 ft south of the 

lake (Moore, 1955). The cavernous nature of the Suwannee 

Limestone results in abundant water yields to wells that are 

completed in this unit and provides good hydraulic connection 

with streams and the lake.

The Tampa Limestone consists of early Miocene sediments that overlie the Suwannee Limestone and, in turn, are 

overlain by either clayey sands and gravels of terrace and 

undifferentiated deposits or the Hawthorn Group (fig. 3). 

The Tampa Limestone is absent from the Dougherty Plain 

and crops out in a narrow band around the southern margin 

of the Marianna Lowlands, where it is from about 20 to 40 

ft thick (Sever, 1965). The Tampa Limestone underlies the 

high-relief region of the Tifton Upland and southern part of 

the study area in Florida. South of the Tifton Upland and east 

of the study area in Georgia, the Tampa Limestone is nearly 

250 ft in thickness. On the Tifton Upland, most domestic and 

some industrial wells are completed in the Tampa Limestone 

because the depth to older, more productive water-bearing 

limestones of the Upper Floridan aquifer is greater than 400 ft 

(Sever, 1965). The foundation of Jim Woodruff Lock and Dam 

is emplaced in the Tampa Limestone, which is about 170 ft 

thick and thins to about 100 ft in the western part of the study 

area in Florida (Reves, 1961). The valley walls near the dam 

are composed of Tampa Limestone, although the lithology is 

chalky (U.S. Army Corps of Engineers, 1948). The limestone 

also is exposed in large streams that dissect the Tifton Upland 

(Sever, 1965).

The Hawthorn Group of middle to late Miocene age consists of sediments that overlie the Tampa Limestone in part 

of the study area (fig. 3). Near Jim Woodruff Lock and Dam, 

the Hawthorn Group is about 40 ft thick and consists of sandy 

clay and fine- to medium-grained sand. The Hawthorn Group 

crops out in the valleys of large streams in the Tifton Upland. 

Sands in the upper part of the formation yield water to dug and 

bored wells (Sever, 1965).

Undifferentiated overburden (residuum) consisting of late 

Miocene alluvial deposits and chemically weathered limestone 

remnants overlies the Hawthorn Group and limestone units 

of the Upper Floridan aquifer (fig. 3). The residuum ranges 

in thickness from a few feet to as much as 100 ft. Although 

the thickness of the residuum is quite variable, in areas where 

it overlies the calcareous parts of the Suwannee and Tampa 

Limestones, the irregular topographic surface conforms to the 

surface of the underlying limestone, a result of the dissolution of the underlying soluble limestone (Reves, 1961, p. 42). 

Hayes and others (1983) and Hicks and others (1987) noted 

that approximately the lower half of the residuum is more 

clayey than the sandy upper part, perhaps due to its origin as 

a weathering product of the underlying limestone. The clayey 

lower part of the residuum is a semiconfining unit to the 

underlying Upper Floridan aquifer; and, where present, the 

upper sandy part can contain a water table. Hydraulic connection of the Upper Floridan aquifer with the water table in the 

sandy upper part of the overburden, or with terrace and undifferentiated deposits, is indirect by vertical leakage through the 

clayey residuum overlying the limestone.

Terrace and undifferentiated deposits of Holocene and 

Pleistocene age (fig. 3) consist of marine terrace deposits in 

the Marianna Lowlands to the south and west of Lake Seminole, and lowland terraces and floodplains along the principal 

streams, namely the Apalachicola and Chattahoochee Rivers. 

These deposits directly overlie the residuum and limestone 

units of the Upper Floridan aquifer, which are exposed in river 

valleys as a result of dissection and removal of the Miocene clastic formations (Moore, 1955). Thickness of terrace 

deposits ranges from 30 to 50 ft (Moore, 1955); however, near 

Figure 3. Correlation chart of geologic and 

hydrologic units comprising the stream-lakeaquifer flow system near Lake Seminole 

(modified from Torak and others, 1996).

SERIES

GEORGIA

Geologic unit Hydrologic

unit

HOLOCENE

AND

PLEISTOCENE

Terrace and

undifferentiated

(surficial) deposits

MIOCENE

Hawthorn

Group

Tampa

Limestone

Suwannee

Undif

Limestone

ferentiated overburden

(residuum)

Semiconfining

unit

EOCENE

Ocala Limestone

Clinchfield

Sand

Lisbon Formation

Upper

Floridan

aquifer

Lower

confining

unit

OLIGOCENElower middle middle and upper upper

6 Simulated Effects of Impoundment of Lake Seminole
Jim Woodruff Lock and Dam, erosion and dissolution of the 

Tampa Limestone have deeply incised former channels of the 

Apalachicola River, and these ancient incisions have been 

filled with alluvium that varies in thickness from at least 30 ft 

to nearly 80 ft in some places (U.S. Army Corps of Engineers, 

1948). Terrace and undifferentiated deposits can contain a 

water table that, depending on the amount of clay or residuum 

that is present, either fluctuates with the adjacent river stage or 

underlying aquifer, or creates a perched water-table condition 

that fluctuates independently of the river or aquifer. Hydraulic 

connection of the terrace and undifferentiated deposits with 

the underlying Upper Floridan aquifer can be direct where 

sandy deposits overlie the limestone units, or indirect where 

fluvial deposits overlie clayey limestone residuum.

Geologic units contributing to the stream-lake-aquifer 

flow system around Lake Seminole are grouped into hydrologic units according to their hydraulic characteristics and 

are termed semiconfining unit, Upper Floridan aquifer, and 

lower confining unit (figs. 3 and 4). Karst processes, hydraulic 

properties, and stratigraphic relations limit stream-lake-aquifer 

interaction to geologic units that include and are younger than 

the late Eocene-age Ocala Limestone (fig. 3). Descriptions 

of the hydraulic properties of the semiconfining unit, Upper 

Floridan aquifer, and lower confining unit are given in Torak 

and others (1996).

Stream erosion and dissolution of carbonate sediments in 

the study area have produced a flow system in the Upper Floridan aquifer that can be characterized as having: (1) high rates 

of direct recharge through sinkholes, swallow holes, or similar 

depressions; (2) indirect recharge by vertical leakage through 

and/or from overlying terrace and undifferentiated deposits, 

Hawthorn Group sediments, and residuum; and (3) channel 

leakage to or from the aquifer across the streambed or lake 

bed (fig. 4). A lower confining unit consisting of the Lisbon 

Formation forms an impermeable base to the stream-lakeaquifer flow system.

Figure 4. Diagram of hydrogeologic units and conceptualization of ground-water and surface-water 

flow in the vicinity of Lake Seminole (modified from Torak and others, 1996).

? ?

80

80

80

GADSDEN CO

SEMINOLE CO DECATUR CO

JACKSON CO

GA

FL

Terrace and undifferentiated,

Hawthorn Group, residuum

Tampa Limestone

Suwannee Limestone

Ocala Limestone

Lisbon Formation

NOT TO SCALE

80 Typical postimpoundment groundwater-level contour, in feet above

NAVD 88

EXPLANATION

Ground-water flow direction

Section view

Plan view Surface-water flow direction

Upper Floridan aquifer

Lower confining unit

Semiconfining unit

N

Introduction 7
The Upper Floridan aquifer is a stepped sequence of 

carbonate sediments consisting of the Ocala, Suwannee, and 

Tampa Limestones and, locally, the Clinchfield Sand. The 

older sediments extend to the surface as the northernmost 

outcrop area of the sequence; younger sediments crop out 

successively to the south. Where limestone units of the Upper 

Floridan aquifer are near land surface, they are semiconfined by the overlying terrace and undifferentiated deposits, 

residuum, and Hawthorn Group. This occurs in extreme 

southeastern Alabama and southwestern Georgia, and in the 

western and central parts of the study area in Florida. In the 

Dougherty Plain, the Upper Floridan aquifer primarily consists 

of the Ocala Limestone, but includes the Suwannee Limestone 

along the Pelham Escarpment and on the Tifton Upland, and 

the Clinchfield Sand, where present. The Tampa Limestone is 

included in the aquifer south of the Florida-Georgia State line, 

where it overlies the Suwannee Limestone.

The function of the Tampa Limestone in transmitting 

ground water in the stream-lake-aquifer system varies, depending on juxtaposition of the limestone with the Apalachicola 

River and other surface-water drainage, and on areal extent 

and lithology of the limestone. West of the river, the combination of incomplete areal extent, sandy lithology, and welldeveloped surface-water drainage results in the Tampa Limestone being hydrologically similar to the underlying limestone 

of the Upper Floridan aquifer; thus, the limestone is included 

in the Upper Floridan aquifer in this area. West of the river, 

the semiconfining unit to the Upper Floridan aquifer consists 

of, where present, residuum, Hawthorn Group sediments, and 

terrace and undifferentiated deposits. By comparison, east of 

the Apalachicola River, the Tampa Limestone has a large areal 

extent and thickness, a dense, clayey lithology, and less-developed surface-water drainage. The Tampa Limestone east of 

the river also contains a higher hydraulic head than underlying limestone units of the Upper Floridan aquifer. This higher 

head, combined with less-transmissive hydraulic characteristics than the deeper units, results in the Tampa Limestone east 

of the Apalachicola River functioning as a semiconfining unit, 

providing a mechanism for indirect downward vertical leakage 

to the Upper Floridan aquifer from the overlying residuum, 

Hawthorn Group, or terrace and undifferentiated deposits.

Numerical Methods

A previously published, calibrated numerical ground-water 

model of the lower ACF River Basin (Torak and others, 1996) 

was used to evaluate the effects that the impoundment of Lake 

Seminole had on ground-water flow and stream-aquifer interaction. The vertical hydraulic conductance of the semiconfining unit overlying the Upper Floridan aquifer was modified 

slightly from the calibrated ground-water model to reflect 

changes in current understanding of the occurrence and thickness of the overburden and to represent the absence of Lake 

Seminole from the hydrologic system. Model boundaries were 

added to represent the Chattahoochee and Flint Rivers and 

Spring Creek, as these streams existed prior to the impoundment of Lake Seminole. Simulation results representing both 

pre- and postimpoundment conditions were analyzed to provide estimates of the change in volume of ground-water flow 

along the Georgia-Alabama and Georgia-Florida State lines.

Model Background and Assumptions

The numerical model code used to simulate groundwater flow with stream-lake interaction in the lower ACF 

River Basin was the USGS MODular Finite-Element model, 

MODFE (Cooley, 1992; Torak, 1993a,b). Torak and others 

(1996) applied MODFE to a conceptualization of the streamlake-aquifer flow system in the lower ACF River Basin to 

define better stream-aquifer relations and to evaluate the 

effects of ground-water pumping on streamflow. Details of 

MODFE formulation, simulation approach, conceptualization 

of the stream-lake-aquifer flow system, and application of 

MODFE to the study area are contained in these references. 

The resulting numerical model of the lower ACF River Basin 

(Torak and others, 1996) simulated ground-water flow in the 

Upper Floridan aquifer, linearly distributed flows to or from 

streams and external boundaries, and areally distributed vertical leakage to or from the overlying undifferentiated overburden and the bed of Lake Seminole. The model was calibrated 

using drought conditions of October 1986.

Linearly distributed ground-water flows to or from streams 

and external boundaries are represented in the existing 

numerical model using head-dependent Cauchy-type boundaries (Torak and others, 1996). Major and minor streams were 

discretized linearly as element sides, which were grouped into 

zones of Cauchy-type boundaries. These boundaries relate 

the flow rate across the streambed to the difference in head 

between the aquifer and stream and to hydraulic characteristics 

of the streambed (Torak and others, 1996, p. 38–40). Each end 

node was assigned a head value based on stream stage during 

drought conditions of October 1986. Element sides along the 

northeast, southeast, and southwest external boundaries of 

the model were grouped into zones of Cauchy-type boundaries, and each end node was assigned a head value based on 

the October 1986 potentiometric-surface map. In the bed of 

Lake Seminole, Cauchy-type boundaries were added along 

the impoundment arms of the Chattahoochee and Flint Rivers 

and Spring Creek to Lake Seminole to simulate these streams 

under hypothetical preimpoundment conditions.

Areally distributed vertical leakage to or from the overburden and Lake Seminole both are represented in the numerical 

model by a steady, nonlinear, head-dependent vertical-leakage boundary. The vertical-leakage boundary is distributed 

areally based on estimated vertical hydraulic conductance 

(vertical hydraulic conductivity divided by thickness) of the 

overburden or lake bed. In the postimpoundment simulation, 

vertical hydraulic conductance: (1) was zero where overburden 

is absent, (2) ranged from 8.4x10-10 to 9.8x10-4 feet per day 

8 Simulated Effects of Impoundment of Lake Seminole
per foot (ft/d/ft) in areas where undifferentiated overburden is 

present, and (3) was 8.0x10-3 ft/d/ft for the bed of Lake Seminole (Torak and others, 1996, table 4, pl. 5). In areas where 

overburden is present, head in the overburden was estimated 

from water levels in wells. For the area representing Lake 

Seminole, head was set equal to the pool elevation of Lake 

Seminole during October 1986 (75.66 ft) for the postimpoundment simulation and in the hypothetical preimpoundment 

simulation; vertical hydraulic conductance was zero, representing the absence of overburden.

Because the purpose of this study was to investigate only 

the effects of the impoundment of Lake Seminole on groundand surface-water flow, other model stresses unrelated to the 

impoundment were not changed. Between the postimpoundment and hypothetical preimpoundment simulations, the only 

input changes were the conditions of hydrologic boundaries 

to the Upper Floridan aquifer that represented: (1) vertical 

leakage from the bed of Lake Seminole (postimpoundment) 

or overburden sediments (hypothetical preimpoundment), 

(2) flows to preimpoundment streams (hypothetical preimpoundment), and (3) flow at external boundaries (hypothetical preimpoundment). Pumpage for the hypothetical preimpoundment simulation was input at the same rate as that used 

in the calibrated model of October 1986 conditions (Torak 

and others, 1996, p. 43–44), even though irrigation pumpage 

is known to have increased considerably during the period 

1957–86. Considering these basic assumptions, results of 

these simulations should be viewed as hypothetical, for purposes of hydrologic evaluation, and might not represent any 

real flow condition. 

Vertical Hydraulic Conductance

Minor modifications to values of vertical hydraulic 

conductance from those used in the existing numerical 

model (Torak and others, 1996) were made to reflect current 

understanding of the occurrence of undifferentiated overburden in the Lake Seminole area. Well records compiled for this 

study and recent field reconnaissance indicate that the undifferentiated overburden probably is absent near the channels 

of the Flint River, Spring Creek, and Fishpond Drain north of 

Lake Seminole. To reflect the absence of overburden in the 

numerical model, vertical hydraulic conductance in selected 

elements along these stream reaches was set to zero (fig. 5); 

whereas, vertical hydraulic conductance in element zones 

north of Lake Seminole had been nonzero and nearly uniform 

in the original numerical model. The effect of this change on 

simulated head in the Upper Floridan aquifer was minimal—

the maximum simulated change from the calibrated model 

was 0.04 ft (Jones and Torak, 2003). For preimpoundment 

conditions, the undifferentiated overburden also was assumed 

to be absent beneath Lake Seminole; in the numerical model, 

the vertical hydraulic conductance in the area representing 

Lake Seminole was set to zero (fig. 5).

Cauchy-Type Boundaries

Upstream of Lake Seminole, the Chattahoochee and Flint 

Rivers and Spring Creek are represented in the model by headdependent Cauchy-type boundaries. Near Lake Seminole, 

for the postimpoundment simulation, values of head at these 

boundaries were set to estimates of stage in these streams 

during October 1986, which were influenced by the impoundment. To represent preimpoundment conditions, historic 

streamflow data and associated stage-discharge rating curves 

from the Chattahoochee, Flint, and Apalachicola Rivers prior 

to 1955 (U.S. Geological Survey, unpublished records, Georgia 

District, Atlanta, Georgia) were used to estimate the stage that 

corresponds to streamflow along these three streams during 

a drought similar to that of October 1986. In the numerical 

model, head values assigned to each node of the Cauchy-type 

boundaries representing these streams were lowered accordingly (fig. 6). Head-dependent Cauchy-type boundaries representing the preimpoundment channels of these streams in the 

area of Lake Seminole, downstream to the site of Jim Woodruff 

Lock and Dam, also were added (fig. 5) and assigned head values in a similar manner (fig. 6). Simulated head change, due to 

the impoundment of Lake Seminole, near external Cauchy-type 

boundaries was examined to determine the appropriateness of 

assigning the same head values for both simulations.

Figure 5. Boundary conditions of the lower Apalachicola–

Chattahoochee–Flint River Basin model in the Lake Seminole 

area, showing modifications for simulating hypothetical 

preimpoundment conditions. (See figure 1 for map area.)

Added Cauchytype boundaries

Existing Cauchytype boundaries,

head modified

Existing Cauchytype boundaries,

head not modified

Added areas of no

overburden

Existing areas of no

overburden

Lake Seminole model

area (no overburden)

Lower ACF River Basin

model area

EXPLANATION

Apalachicola

River boundary

Flint River

boundary

Fishpond

Drain

channel

Chattahoochee

River boundary

Spring Creek

boundary

Sawhatchee

Creek

boundary

Ichawaynochaway

Creek ALA- boundary BAMA

FLORIDA

ALABAMA

GEORGIA

FLORIDA

GEORGIA

30 mi

20 mi

10 mi

30 mi

20 mi

10 mi

50

mi

40 mi

30 mi

20 mi

10 mi

0 5 10 MILES

0 5 10 KILOMETERS

N

Numerical Methods 9
State-Line Flow

To assess change in interstate ground-water flow under 

simulated postimpoundment and hypothetical preimpoundment conditions, model element sides closest to the Georgia-Alabama and Georgia-Florida State lines were selected 

and separated into six zones based on the neighboring State, 

hydrologic boundaries, and simulated ground-water flow 

directions. Details of the zonation are discussed in the next 

section. To quantify ground-water flow on each side of these 

State-line element sides, MODFE was modified to calculate 

specific-discharge vectors (equivalent to Darcy velocity) at 

the centroid of the elements containing the State-line element 

sides, each side having one element in Georgia and one in the 

neighboring State (Alabama or Florida). The component of an 

elemental specific-discharge vector that is normal to a Stateline element side (equivalent to a boundary flux) was assumed 

to represent the flow of ground water that is either moving 

toward or away from the State line on that side, depending on 

the direction of the normal component. The product of this 

normal component, average thickness of the aquifer along a 

State-line element side, and length of the element side yields a 

volumetric ground-water flow rate. For each element containing a State-line element side, this volumetric rate is either an 

inflow into Georgia or an outflow from Georgia. Inflows and 

outflows were summed by State-line zone for elements located 

in Georgia and for elements located in the neighboring States. 

Where State-line element sides correspond to Cauchy-type 

boundaries, model-boundary outflows (discharge to streams) 

also were summed.

Simulation Results

Initially, the only differences in model input data between 

the postimpoundment and the hypothetical preimpoundment 

simulations, were: (1) the elimination of leakage from the bed of 

Lake Seminole, which was replaced by an area without overburden; (2) the addition of Cauchy-type boundaries to represent the 

preimpoundment (channel) conditions of the Chattahoochee and 

Flint Rivers and Spring Creek; and (3) the reduction of inputhead values at Cauchy-type boundaries representing the lowest 

reaches of these streams, which had input-head values that were 

increased due to the impoundment. The preliminary simulated 

increase in ground-water level in the Upper Floridan aquifer due 

to the impoundment, based on these initial preimpoundment 

conditions (fig. 7), was examined to determine whether or not 

this result was reasonable. In an aquifer of nearly uniform thickness and transmissivity, the increase in ground-water level due 

to an impoundment such as Lake Seminole would be expected 

to be nearly symmetric at points located the same radial distance 

from the impoundment. In other words, the contours of simu40 30 20 10 0 40

50

60

70

80

50

Flint River

Existing Added

Spring Creek boundary

40 30 20 10 0 40

50

60

70

80

50

Existing Added Woodruff Dam

Chattahoochee River boundary

40 30 20 10 0 40

50

60

70

80

50

Spring Creek

Woodruff Dam

Existing Added

Flint River boundary

RIVER MILES

ALTITUDE, IN FEET ABOVE NAVD 88

Figure 6. Model-input river stage for Cauchy-type 

boundaries representing the Chattahoochee and Flint 

Rivers and Spring Creek, hypothetical preimpoundment 

conditions (red) and postimpoundment conditions (blue). 

(See figure 5 for locations of streams in model.)

Figure 7. Preliminary simulated increase in groundwater level in the Upper Floridan aquifer due to the 

impoundment of Lake Seminole based on initial 

preimpoundment conditions. (See figure 1 for map area.)

GEORGIA

FLORIDA

GEORGIA ALABAMA

FLORID

A ALABAM

A

SEMINOLE

DECATUR

BAKER

MILLER

EARLY

HOUSTON

HENRY

MITCHELL

CALHOUN DOUGHERTY

JACKSON

GADSDEN

LEON LIBERTY

CALHOUN

GRADY 20

10

10

Simulated water-level

increase contour, in

feet. Contour interval

2 ft

10

Lake Seminole model

area

Lower ApalachicolaChattahoochee-Flint

River Basin model

area

EXPLANATION

N

0 10 20 MILES

0 10 20 KILOMETERS

0 10 20 MILES

0 10 20 KILOMETERS

10 Simulated Effects of Impoundment of Lake Seminole
lated increase in ground-water level would be roughly circular. 

The contours of ground-water level increase in figure 7; however, they are flattened near the external Cauchy-type boundary 

southeast of Lake Seminole. Flattened contours of groundwater level increase could be the result of input-head values 

that are too high along the external Cauchy-type boundary 

under hypothetical preimpoundment conditions. It is reasonable to expect that the impoundment of Lake Seminole would 

have raised water levels along this boundary, which at the closest point is only about 6 mi from the Flint River impoundment 

arm of the lake.

To account for the probable raising of water levels in the 

area of the southeastern external boundary of the model due to 

the impoundment of Lake Seminole, model input-head values 

for the Cauchy-type boundaries representing this external 

boundary were lowered systematically in a subsequent hypothetical preimpoundment simulation. By extending the contours of water-level increase past the southeastern boundary in 

a roughly circular pattern, a hypothetical increase in groundwater level along the boundary, due to the impoundment of 

Lake Seminole, was estimated that ranged from 0 to about 10 ft 

(see fig. 8). Reducing input-head values along the Cauchy-type 

boundary in this area produced a more reasonable estimation 

of hypothetical preimpoundment conditions than using the 

higher boundary heads caused by the impoundment. The result 

of reducing input-head values along the external boundary by 

this amount in shown in figure 9, which is a map of simulated 

water-level increase similar to that shown in figure 7. Although 

Jones and Torak (2003) reported preliminary results using the 

initial hypothetical preimpoundment simulation in this report, 

the modified simulation accounting for lower water levels 

in the Upper Floridan aquifer in the area of the southeastern 

boundary is considered to be a more accurate estimate of hypothetical preimpoundment conditions than previously reported.

The difference in simulated ground-water levels from hypothetical preimpoundment to postimpoundment conditions (fig. 9) 

indicates an increase in ground-water level in the Upper Floridan 

aquifer by as much as 26 ft in extreme southern Seminole County 

due to the impoundment of Lake Seminole. Ground-water storage apparently increased in several counties in southwest Georgia 

and adjacent parts of Florida and Alabama due to the impoundment—simulated water levels in the Upper Floridan aquifer 

increased at least 10 ft in south- and central-western Decatur 

County and throughout Seminole County in Georgia, and in 

eastern Jackson County and northwestern Gadsden County in 

Florida. These results also indicate, however, that the effect of 

the impoundment of Lake Seminole on ground-water level is 

relatively localized. Simulated ground-water level increased by 

less than 2 ft beyond linear distances from Jim Woodruff Lock 

and Dam of about 35 mi along the Chattahoochee and Flint 

Rivers, and about 20 mi along the Apalachicola River.

Comparison of maps showing simulated water levels and 

directions of ground-water movement in the Upper Floridan 

aquifer for hypothetical preimpoundment and postimpoundment conditions (fig. 10A and B, respectively) indicates that the 

impoundment of Lake Seminole locally altered the direction 

of ground-water flow. The greatest change in ground-water 

flow directions due to the impoundment occurred southeast of 

Lake Seminole and east of the Apalachicola River (labeled “a” 

in fig. 10). In this area, under hypothetical preimpoundment 

conditions, ground water flowed northwestward from Florida 

and discharged to the Flint River. Under postimpoundment 

conditions, ground water flows southwestward from Georgia 

and discharges to the Apalachicola River. There is a smaller 

area southwest of Lake Seminole (labeled “b” in fig. 10) where, 

under hypothetical preimpoundment conditions, ground water 

diverged from the regional southeastward flow direction, flowed 

eastward and finally northeastward from Florida, and discharged to the Chattahoochee River near its confluence with the 

Flint River. Under postimpoundment conditions, ground water 

flows southeastward and then southward, bypasses or flows 

under Lake Seminole, and discharges to the Apalachicola River. 

Contour of estimated

decrease in input

head, in feet. Contour

interval 2 ft

2

Simulated water-level

increase contour, in

feet. Contour interval

2 ft

10

Modified external

Cauchy-type

boundary nodes and

element sides

Lake Seminole model

area

Lower ApalachicolaChattahoochee-Flint

River Basin model

area

EXPLANATION

GEORGIA

FLORIDA

GEORGIA ALABAMA

FLORID

A ALABAM

A

SEMINOLE DECATUR

BAKER

MILLER

EARLY

HOUSTON

HENRY

MITCHELL

CALHOUN DOUGHERTY

JACKSON

GADSDEN

LEON LIBERTY

CALHOUN

GRADY 20

10

10 10 6 4

2

8

N

0 10 20 MILES

0 10 20 KILOMETERS

0 10 20 MILES

0 10 20 KILOMETERS

Figure 8. Estimated decrease in input-head values 

along the southeastern external Cauchy-type boundary 

to account for lower hypothetical preimpoundment 

water levels in the Upper Floridan aquifer. (See figure 1 

for map area.)

Simulated Results 11
Also, under hypothetical preimpoundment conditions, water 

levels in the Upper Floridan aquifer north of Lake Seminole near 

Spring Creek (labeled “c” in fig. 10) were below the streambed 

and very little ground water discharged to Spring Creek. Under 

postimpoundment conditions, water levels are higher than the 

streambed, and the aquifer discharges water to the creek, as indicated by the flow lines ending at Spring Creek and the bending 

of contours in the simulated potentiometric surface.

Regional effects of the impoundment of Lake Seminole 

are indicated by comparing water-budget components for the 

hypothetical preimpoundment and postimpoundment simulations (table 1). Recharge to the Upper Floridan aquifer was 

decreased by about 261 million gallons per day (Mgal/d), of 

which about 143 Mgal/d is decreased regional ground-water 

inflow across external model boundaries, about 115 Mgal/d 

is decreased downward leakage from the undifferentiated 

overburden, and about 3 Mgal/d is decreased infiltration near the 

updip limit of the aquifer, where it crops out. These recharge 

components mostly offset the decrease in discharge to streams 

and in-channel springs (about 314 Mgal/d). Other differences in 

discharge components due to the impoundment are an increase 

in regional ground-water outflow across external model 

boundaries (about 40 Mgal/d) and an increase in upward leakage to the undifferentiated overburden (about 13 Mgal/d).

Cauchy-type boundaries are used to represent the interaction 

of ground water and surface water along streams and in-channel 

springs, and were used to provide regional flow of ground-water 

in the Upper Floridan aquifer to and from (recharge and discharge) the model area at the external boundaries of the model. 

Among water-budget components in table 1, the largest differences between simulations of hypothetical preimpoundment 

and postimpoundment conditions involve Cauchy-type boundaries. The impoundment of Lake Seminole reduced discharge 

to streams and in-channel springs by about 314 Mgal/d, and 

reduced recharge from or increased discharge to regional flow 

at external model boundaries by about 143 Mgal/d and about 

40 Mgal/d, respectively, or by a total of about 183 Mgal/d. To 

assess more precisely which streams and boundaries account 

for these large water-budget differences, recharge and discharge 

rates at Cauchy-type boundaries in the Lake Seminole area for 

both simulations were plotted in figure 11A and 11B. The rate 

of recharge or discharge per length (in Mgal/day/mile) along 

each element side of a Cauchy-type boundary is indicated by 

color; ground-water recharge is shown in shades of blue and 

discharge is shown in shades of red. For each major stream and 

for zones of external model boundaries, the recharge or discharge rates are totaled along the length of the element sides.

The effects of the impoundment of Lake Seminole on 

stream-aquifer flow rates are indicated by comparing simulated 

ground-water discharge rates to nine major streams in the Lake 

Seminole area for hypothetical preimpoundment and postimpoundment conditions (table 2; streams listed in order of the 

absolute difference of simulated discharges). The impoundment caused discharge from the Upper Floridan aquifer to the 

Flint and Chattahoochee Rivers to be reduced substantially 

(about 267 Mgal/d and about 202 Mgal/d, respectively) along 

the lower sections of these streams where the model-input river 

stage was lowered for the hypothetical preimpoundment simulation. Although model-input river stage for Spring Creek also 

was lowered for the hypothetical preimpoundment simulation, 

ground-water levels between the Flint and Chattahoochee Rivers were low enough prior to the impoundment that almost no 

water discharged to the lower reaches of Spring Creek. Higher 

ground-water levels after the impoundment than before caused 

increased discharge to the lower reaches of Spring Creek 

by about 28 Mgal/d. Discharge to other streams near Lake 

Seminole (Apalachicola and Chipola Rivers, and Dry, Cowarts, 

Marshall, and Sawhatchee Creeks) for which model-input river 

stage was unchanged, was less for preimpoundment conditions 

than for postimpoundment conditions because of low preimpoundment ground-water levels were lower. The total difference in discharge to these nine streams (about 319 Mgal/d) is 

within 2 percent of the total reduction in discharge to streams 

and in-channel springs, due to the impoundment of Lake Seminole, for the entire model area (about 314 Mgal/d, table 1).

Figure 9. Simulated increase in ground-water level in 

the Upper Floridan aquifer due to the impoundment of 

Lake Seminole and accounting for lower preimpoundment 

water levels along the southeastern external Cauchytype boundary. (See figure 1 for map area.)

Simulated water-level

increase contour, in

feet. Contour interval

2 ft

10

Lake Seminole model

area

Lower ApalachicolaChattahoochee-Flint

River Basin model

area

EXPLANATION

GEORGIA

FLORIDA

GEORGIA ALABAMA

FLORID

A ALABAM

A

SEMINOLE

DECATUR

BAKER

MILLER

EARLY

HOUSTON

HENRY

MITCHELL

CALHOUN DOUGHERTY

JACKSON

GADSDEN

LEON LIBERTY

CALHOUN

GRADY

10

10

20

N

0 10 20 MILES

0 10 20 KILOMETERS

0 10 20 MILES

0 10 20 KILOMETERS

12 Simulated Effects of Impoundment of Lake Seminole
Table 1. Simulated water-budget components for the lower Apalachicola–Chattahoochee–Flint River 

Basin model, hypothetical preimpoundment and postimpoundment conditions.

[All values are volumetric flow rates in million gallons per day. Difference indicates the simulated effect of the impoundment of 

Lake Seminole; decreases are negative, increases are positive. Due to rounding, differences and column totals may be inexact]

Water-budget component Hypothetical 

preimpoundment Postimpoundment Difference

Discharge, by budget component

Streams and in-channel springs 2,740 2,425 –314

Wells 475 475 0

Off-channel springs 333 333 0

Regional flow 263 304 +40

Undifferentiated overburden 36 49 +13

 Total 3,846 3,585 –261

Recharge, by budget component

Undifferentiated overburden 2,602 2,486 –115

Regional flow 1,075 933 –143

Upper Floridan aquifer outcrop 144 141 –3

Streams 25 25 –1

 Total 3,846 3,585 –261

Figure 10. Simulated potentiometric surface and directions of ground-water flow in the Upper Floridan 

aquifer from simulation of (A) hypothetical preimpoundment conditions and (B) postimpoundment conditions. 

(See figure 1 for map area.)

a,b,c Areas described in text

Simulated potentiometric-surface contour, in

feet above NAVD 88. Contour interval varies

7575

Simulated direction of ground-water flow

Cauchy-type boundary

Lake Seminole model area (A B; )

Lower Apalachicola-Chattahoochee-Flint

River Basin model area

EXPLANATION

GEORGIA

FLORIDA

ALABAMA GEORGIA

FLORIDA ALABAMA

35

40

45

50

55

60

65

70

75

75

80

80

85

85

90

90

90 100

100 110

110 120

120 130

130

140

140

150

150

160

160

170

170

180

180

190

190

200

200

210

210

220

230

c

b a

B. Postimpoundment

GEORGIA

FLORIDA

ALABAMA GEORGIA

FLORIDA ALABAM

A

35

40

45

50

50

55

55

60

60

60

65

65

65

70

70

70

80

80

90

90 90

100

100

110

110 120

120 130

130 140

140 150

150

160

160

170

170

180

180

190

190

200

200

210

210

220

230 240

c

b a

A. Hypothetical

Preimpoundment

A.

preimpoundment

N

0 10 20 MILES

0 10 20 KILOMETERS

0 10 20 MILES

0 10 20 KILOMETERS

N

0 10 20 MILES

0 10 20 KILOMETERS

0 10 20 MILES

0 10 20 KILOMETERS

Simulated Results 13
Table 2. Simulated discharge to streams in the Lake Seminole area, hypothetical 

preimpoundment and postimpoundment conditions.

[All values are volumetric flow rates in million gallons per day. Difference indicates the simulated effect 

of the impoundment of Lake Seminole; decreases are negative, increases are positive. Due to rounding, 

differences and column totals may be inexact]

Streams Hypothetical 

preimpoundment Postimpoundment Difference

Flint River 663 396 –267

Chattahoochee River 450 248 –202

Apalachicola River 207 266 +59

Chipola River 236 284 +48

Spring Creek 26 54 +28

Dry Creek (Florida) 47 57 +10

Cowarts Creek 16 19 +3

Marshall Creek 23 25 +2

Sawhatchee Creek 7 8 +1

 Total 1,675 1,356 –319

GEORGIA

FLORIDA

ALABAMA GEORGIA

FLORIDA ALABAMA

B. Postimpoundment

Southwestern boundary

Southern boundary

Northern

southeastern

boundary

Southern

southeastern

boundary

+439

–20

+186

–49

Marshall

Cr.

Cowarts Cr.

Dry Cr.

Sawhatchee

Cr.

Spring Cr.

Chipola R.

Chattahoochee R. Flint R.

Apalachicola

R.

–25

–54

–8

–19

–57

–284

–248

–396

–266

GEORGIA

FLORIDA

ALABAMA GEORGIA

FLORIDA ALABAM

A

A. Hypothetical

Preimpoundment

Southwestern boundary

Southern boundary

Northern

southeastern

boundary

Southern

southeastern

boundary

+462

–11

+256

+26

Marshall

Cr.

Chipola R.

Cowarts Cr.

Dry Cr.

Sawhatchee

Cr.

Chattahoochee R. Flint R.

Spring Cr.

Apalachicola

R.

–23

–16

–7

–236

–47

–207

–450

–26

–633

Total flow rate for stream or

external boundary, in

millions of gallons per day

–47

Dry Cr.

<–12

–12 to –6

>–6 to –2

>–2 to +2

>+2 to +4

>+4 to +8

>+8

Cauchy-type boundary flow, in millions

of gallons per day per mile

(positive - inflow; negative - outflow)

Simulated direction of ground-water

flow

Cauchy-type boundary

Lake Seminole model area ( ; A B)

Lower Apalachicola-ChattahoocheeFlint River Basin model area

EXPLANATION

N

0 10 20 MILES

0 10 20 KILOMETERS

0 10 20 MILES

0 10 20 KILOMETERS

N

0 10 20 MILES

0 10 20 KILOMETERS

0 10 20 MILES

0 10 20 KILOMETERS

Figure 11. Simulated ground-water recharge (shades of blue) or discharge (shades of red) rates 

at Cauchy-type boundaries from simulation of (A) hypothetical preimpoundment conditions and 

(B) postimpoundment conditions. (See figure 1 for map area.)

14 Simulated Effects of Impoundment of Lake Seminole
The effects of the impoundment of Lake Seminole on 

external model-boundary zones shown in figure 11 are indicated by comparing simulated inflow and outflow rates from 

and to regional flow (table 3). The compass-point designations of the external model-boundary zones (southwestern, 

southern, and southeastern) are those used in Torak and others 

(1996), except that the southeastern boundary is divided into 

a southern section and a northern section. Most of the change 

in regional flow between the two simulations occurs along 

the two sections of the southeastern boundary. The northern 

section of the southeastern boundary is a recharge boundary 

under hypothetical preimpoundment conditions (totaling about 

26 Mgal/d recharge), but it becomes a discharge boundary 

under postimpoundment conditions (totaling about 49 Mgal/d 

of discharge), a change of about 75 Mgal/d. The southern section is a recharge boundary in both simulations, but the total 

recharge rate decreases by about 70 Mgal/d (from about 256 

Mgal/d to about 186 Mgal/d) from hypothetical preimpoundment to postimpoundment conditions. Adding to these changes 

the total decrease of about 32 Mgal/d due to the southwestern 

and southern boundaries results in a total decrease in recharge 

or increase in discharge of about 178 Mgal/d along these four 

external model-boundary zones from hypothetical preimpoundment to postimpoundment conditions. This approximate 

178-Mgal/d change is within about 3 percent of the about 183 

Mgal/d in decreased recharge or increased discharge due to the 

impoundment of Lake Seminole (table 1).

For a discussion of across State-line flows, the GeorgiaAlabama-Florida State line in the model area was divided 

into six zones (fig. 12). Zone 1 corresponds to the GeorgiaAlabama State line along the Chattahoochee River from the 

external model boundary southward to the Alabama-Florida 

State line. Zone 2 corresponds to the Georgia-Florida State 

line along the Chattahoochee River from the Alabama-Florida 

State line southward to the Lake Seminole model area. Under 

both of the simulated conditions, zones 1 and 2 are Cauchytype boundaries that permit ground-water discharge from the 

aquifer to the Chattahoochee River. Zones 3 and 4 correspond 

to the Georgia-Florida State line along the channel of the 

Chattahoochee River in the Lake Seminole model area downstream to Jim Woodruff Lock and Dam. In the postimpoundment simulation, zones 3 and 4 were in the bed of Lake Seminole, and no Cauchy-type boundary condition was assigned 

to element sides in these zones; but, in the hypothetical 

preimpoundment simulation, zones 3 and 4 are Cauchy-type 

boundaries that were added to permit ground-water discharge 

to the Chattahoochee River in the same manner as zones 1 

and 2. Zones 5 and 6 correspond to the Georgia-Florida State 

line from Jim Woodruff Lock and Dam eastward to the model 

boundary; no hydrologic boundary condition is associated 

with element sides in these zones.

For both simulations that are compared in zones 1 and 

2, ground-water flow converges at the Chattahoochee River 

(fig. 12A,B) and results in outflow to the Chattahoochee River, 

which is represented by a Cauchy-type boundary. For the 

hypothetical preimpoundment simulation (fig. 12A) in zones 

3 and 4, ground-water flow converges and results in outflow 

to the Chattahoochee River at the added Cauchy-type boundary. For the postimpoundment simulation (fig. 12B) in zone 

3, ground water flows under a low hydraulic gradient from 

Florida southeastward into Georgia; and in zone 4, ground 

water flows under a slightly higher gradient from Georgia 

southward back into Florida (hydraulic gradients can be 

inferred from contour spacing in figure 10B). Under hypothetical preimpoundment conditions (fig. 12A), in zones 5 and 

6 ground water flows under a relatively high gradient northwestward from Florida into Georgia. Under postimpoundment 

conditions (fig. 12B) in zone 5, ground water flows under 

a low hydraulic gradient southwestward from Georgia into 

Florida; but in zone 6, ground-water flow generally is parallel 

to the Georgia-Florida State line, not across it.

Table 3. Simulated regional flow rates at external model-boundary zones in the 

Lake Seminole area, hypothetical preimpoundment and postimpoundment conditions.

[All values are volumetric flow rates in million gallons per day; positive values are inflow, negative values 

are outflow. Difference indicates the simulated effect of the impoundment of Lake Seminole; decreases are 

negative, increases are positive. Due to rounding, differences and column totals may be inexact]

Regional model 

boundaries

Hypothetical 

preimpoundment Postimpoundment Difference

Southwestern +463 +439 –24

Southern –11 –20 –9

Southeastern, lower +256 +186 –70

Southeastern, upper +26 –49 –75

 Total +734 +556 –178

Simulated Results 15
Uncertainty and Variability 

of Parameters

Records of ground-water levels and pumpage for the Upper 

Floridan aquifer prior to the construction of Jim Woodruff 

Lock and Dam and the impoundment of Lake Seminole are 

scarce, particularly in the area south of Lake Seminole. Consequently, the accuracy of the hypothetical preimpoundment 

simulation cannot be verified through a calibration procedure 

that matches simulated and measured water levels and stream 

baseflows. Also, under hypothetical preimpoundment conditions, simulated recharge to the aquifer from regional flow is 

greater than the recharge simulated for October 1986 postimpoundment conditions (table 1). The increased simulated 

recharge under hypothetical preimpoundment conditions 

mostly occurs along the external head-dependent Cauchy-type 

boundary located to the southeast of Lake Seminole (fig. 9). 

Head-dependent Cauchy-type boundaries usually are used 

where the aquifer extends beyond the boundary, but where 

storage effects in this external aquifer region can be ignored. 

Such boundaries are used where the contribution of flow 

across this mathematical boundary would not vary substantially from flow that would occur if the model were extended 

to include this external area. For the original numerical model, 

this boundary was placed at a ground-water divide that existed 

in that area during 1986. It is not known whether the divide 

existed in the same location prior to impoundment of Lake 

Seminole or whether preimpoundment water levels in this 

area were similar enough to those during postimpoundment 

to ensure that the hypothetical preimpoundment simulation 

does not violate any assumptions of the boundary. Results 

of a preliminary simulation of preimpoundment conditions, 

using the same input-head values as those used in the postimpoundment simulation, indicate that aquifer head along the 

southeast Cauchy-type boundary was probably lower prior 

Zones 5 and 6 (7.8 mi and 7.7 mi, respectively) From Jim Woodruff

Lock and Dam eastward along the Georgia-Florida State line to the

lateral model boundary

Zones 3 and 4 (6.4 mi and 5.8 mi, respectively) In the Lake Seminole

model area southeastward along the Chattahoochee River channel

to Jim Woodruff Lock and Dam. For hypothetical preimpoundment

conditions, these zones were a Cauchy-type model boundary

Zone 2 (13.3 mi) From Alabama-Florida State line southward along

the Chattahoochee River to the Lake Seminole model area

Zone 1 (13.9 mi) From the lateral model boundary southward along

the Chattahoochee River to the Alabama-Florida State line

ZONES OF ELEMENT SIDES

Inflow/outflow in elements

inside Georgia State line

Inflow/outflow in elements

outside Georgia State line

Outflow at Cauchy-type

boundary

Total flow rates, in millions

of gallons per day

(positive, into Georgia;

negative, out of Georgia

or model)

Simulated direction of

ground-water flow

Cauchy-type boundary

Lake Seminole model area

( ; A B)

Lower ApalachicolaChattahoochee-Flint

River Basin model area

1 Zone of element sides

EXPLANATION

6

5

4

3

2

1

Zone 6

–1 0

Zone 5

–24 –23

Zone 4

–10 –11

Zone 3

5 1

56 –39

–113

Zone 2

Zone 1

53 –38

–136

ALABAMA

FLORIDA

ALABAMA

GEORGIA

FLORIDA

GEORGIA

B. Postimpoundment

6

5

4

3

2

1

Zone 6

38 38

Zone 5

50 52

Zone 4

47 5

–53

Zone 3

54 –7

–85

98 –62

–174

Zone 2

98 –62

–174

Zone 2

67 –45

–144

Zone 1

67 –45

–144

Zone 1

ALABAMA

FLORIDA

ALABAMA

GEORGIA

FLORIDA

GEORGIA

A. Hypothetical

Preimpoundment

N N

0 5 10 MILES

0 5 10 KILOMETERS

0 5 10 MILES

0 5 10 KILOMETERS

0 5 10 MILES

0 5 10 KILOMETERS

0 5 10 MILES

0 5 10 KILOMETERS

Figure 12. Simulated flow rates in the Upper Floridan aquifer adjacent to zones of element sides and at Cauchy-type 

boundaries along the Georgia-Alabama and Georgia-Florida State lines: (A) hypothetical preimpoundment conditions; 

and (B) postimpoundment conditions. Black arrows represent ground-water discharge to the Chattahoochee River, 

where it is a Cauchy-type boundary. Blue and red arrows indicate ground-water inflows to Georgia (pointing right) 

and outflows from Georgia (pointing left) for each zone of element sides. Flow rates are totaled for elements outside 

Georgia (red) and elements inside Georgia (blue). Due to other flow components not shown, flow rates do not 

balance. (See figure 1 for map area.)

16 Simulated Effects of Impoundment of Lake Seminole
to the impoundment of Lake Seminole. For the hypothetical 

preimpoundment simulation, input heads for the Cauchy-type 

boundary in this area were lowered to more accurately depict 

historical conditions.

Extremely low observed and simulated hydraulic gradients 

in the Lake Seminole area during postimpoundment conditions cause the potentiometric surface near the lake to be flat 

and almost horizontal. Slight differences in computed hydraulic head at nodes in elements in this area are responsible for 

the ground-water flow paths shown in figure 10A; therefore, 

ground-water flow paths in the Lake Seminole area can only 

be considered approximate, because neither the model nor the 

hydrologic data on which the model was developed contain 

sufficient detail to identify accurately ground-water flow directions. Because the potentiometric surface is relatively flat near 

the lake, small changes to model input data—such as updated 

pumping estimates or improved estimates of aquifer properties—would alter the direction and magnitude of simulated 

ground-water flow vectors described herein.

Summary

Based on simulation results, the impoundment of Lake 

Seminole changed ground-water flow directions in the Upper 

Floridan within about 20–30 mi of the lake. The largest change 

occurred southeast of the lake, where, prior to impoundment, 

ground water flowed from Florida into Georgia, discharging to 

the Flint River downstream of Bainbridge, Ga. This water then 

flowed into the Apalachicola River. Following impoundment, 

ground water southeast of the lake flows from Lake Seminole 

to the Apalachicola River; however, hydraulic gradients and 

flow volumes are small. Also, discharge to the Upper Floridan 

aquifer from Lake Seminole has created a mound in the potentiometric surface of the aquifer near the lake, thus increasing 

the amount of ground water that is stored in this area. Aquifer water levels were increased by as much as 26 ft near Jim 

Woodruff Lock and Dam; the area in which water levels were 

increased at least 10 ft extends nearly 20 mi upstream along 

the Chattahoochee and Flint Rivers, and several miles downstream along the Apalachicola River.

Simulation results indicate that the impoundment of Lake 

Seminole changed ground-water budget components, including reduction of ground-water discharge from the Upper 

Floridan aquifer to streams by about 400 Mgal/d; reduction 

of inflow of ground water into the Upper Floridan aquifer in 

the region simulated by about 240 Mgal/d; and a reduction of 

ground-water recharge from the undifferentiated overburden 

by about 110 Mgal/d. Most of the reduction of ground-water 

discharge to streams was along the lowest reaches of the Chattahoochee and Flint Rivers that are now part of the lake. Prior 

to impoundment, stage in these streams was as much as 40 ft 

lower than postimpoundment conditions. Most of the reduction in regional inflow of ground water to the modeled area 

occurs at the model boundary southeast of the lake.

References Cited

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guidebook T185, Tampa to Tallahassee, Florida, July 1–7, 

1989: American Geophysical Union, p. T185: 10–14.

Brooks, H.K., 1981, Physiographic divisions of Florida: 

Institute of Flood and Agricultural Sciences, University of 

Florida, 6 p.

Clark, W.Z., and Zisa, A.C., 1976, Physiographic map 

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2,000,000.

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Summary 17
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18 Simulated Effects of Impoundment of Lake Seminole