Hydrogeology, Model Description, and Flow Analysis of the

Mississippi River Alluvial Aquifer in Northwestern Mississippi

Prepared in cooperation with the

MISSISSIPPI DEPARTMENT OF ENVIRONMENTAL QUALITY,

OFFICE OF LAND AND WATER RESOURCES

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01 4035
HYDROGEOLOGY, MODEL DESCRIPTION, AND FLOW 

ANALYSIS OF THE MISSISSIPPI RIVER ALLUVIAL 

AQUIFER IN NORTHWESTERN MISSISSIPPI

by J. Kerry Arthur

____________________________________________________________

U.S. Geological Survey

Water-Resources Investigations Report 01-4035

Prepared in cooperation with the

Mississippi Department of Environmental Quality

Office of Land and Water Resources

Pearl, Mississippi

2001
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information Copies of this report can be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey, WRD Branch of Information Services

308 South Airport Road Box 25286

Jackson, Mississippi 39208-6649 Denver, Colorado 80225
Contents iii

CONTENTS

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

Introduction ........................................................................................................................................... 2

Hydrogeology of the Mississippi River Alluvial Aquifer ..................................................................... 4

Geology......................................................................................................................................... 4

Aquifer Flow Boundaries.............................................................................................................. 10

Aquifer Characteristics.................................................................................................................. 11

Water-Level Measurements, Potentiometric Surfaces, and Direction of Flow............................. 12

Water Volume................................................................................................................................ 15

Water Quality ................................................................................................................................ 20

Pumpage........................................................................................................................................ 23

Description of Ground-Water Flow Model ........................................................................................... 24

Model Grid and Layers ................................................................................................................. 24

Model Boundaries ......................................................................................................................... 26

Calibration Strategy ...................................................................................................................... 30

Hydraulic Properties ............................................................................................................. 32

Model Sensitivity.................................................................................................................. 36

Ground-Water Flow Analysis................................................................................................................ 36

Pumpage........................................................................................................................................ 36

Recharge and Discharge................................................................................................................ 36

Lateral ................................................................................................................................... 36

Bluff Hills..................................................................................................................... 39

Mississippi River.......................................................................................................... 39

Vertical.................................................................................................................................. 40

Precipitation.................................................................................................................. 40

Surface Water ............................................................................................................... 40

Underlying Aquifers..................................................................................................... 42

Flow Budget.................................................................................................................................. 42

Model Limitations ................................................................................................................................. 44

Summary................................................................................................................................................ 45

Selected References............................................................................................................................... 47

FIGURES

1-3. Maps showing:

1. Location of study area, northwestern Mississippi................................................................. 3

2. Generalized thickness of the Mississippi River alluvium..................................................... 5

3. Geologic units underlying the Mississippi River alluvial aquifer......................................... 7
iv Contents

4. Geologic sections A-A’ and B-B’ from west to east across the Delta.................................... 8

5. Graph showing relation of water levels in the Mississippi River to water levels in the 

Mississippi River alluvial aquifer .......................................................................................... 10

6. Graph showing average depth to water during spring and fall in the Mississippi River 

alluvial aquifer ....................................................................................................................... 13

7. Maps showing lowest and highest potentiometric surfaces of the Mississippi River 

alluvial aquifer since 1980..................................................................................................... 14

8. Schematic showing hydrologic and climatic processes in the Delta..................................... 16

9. Map showing volume of water in the Mississippi River alluvial aquifer during fall 1999... 17

10. Graph showing estimated rate of gain and loss of water from the Mississippi River 

alluvial aquifer based on changes in potentiometric surfaces................................................ 21

11. Graph showing relation of specific conductance to dissolved solids in the Mississippi 

River alluvial aquifer ............................................................................................................. 21

12-16. Maps showing:

12. Specific conductance of water in the Mississippi River alluvial aquifer in 1992 and 

1998 ....................................................................................................................................... 22

13. Model grid orientation........................................................................................................... 25

14. Specified heads at eastern model boundary........................................................................... 27

15. Potentiometric surface representing specified heads at the lower boundary (Sparta and 

Cockfield aquifers) ................................................................................................................ 28

16. Location of river and lake head-dependent-flux cells (river package) in model................... 29

17. Hydrograph showing simulated and measured water levels at selected observation wells 

completed in the Mississippi River alluvial aquifer .............................................................. 31

18. Graph showing simulated and measured west-to-east water-level sections across the 

Delta for spring and fall 1994 ................................................................................................ 32

19. Map showing observed and simulated spring and fall 1996 potentiometric surfaces of 

the Mississippi River alluvial aquifer .................................................................................... 33

20. Map showing simulated transmissivity of the Mississippi River alluvial aquifer for fall 

1996 ....................................................................................................................................... 35

21. Graph showing model sensitivity to independent changes in selected calibration 

parameters.............................................................................................................................. 37

22. Graph showing annual pumpage rate from the Mississippi River alluvial aquifer used 

in the calibrated model........................................................................................................... 37

23. Map showing areas of concentrated pumpage from the Mississippi River alluvial aquifer 

in 1996 ................................................................................................................................... 38

24. Graph showing relation between average annual stage of the Mississippi River at 

Greenville, Mississippi, and simulated recharge to the Mississippi River alluvial 

aquifer from the Mississippi River ........................................................................................ 41

25. Graph showing simulated areal recharge to the Mississippi River alluvial aquifer, 

1988-96 .................................................................................................................................. 41

26. Graph showing simulated recharge to the Mississippi River alluvial aquifer from surfacewater bodies within the Delta ................................................................................................ 41

27. Map showing area where water levels in underlying aquifers are greater than water 

levels in the Mississippi River alluvial aquifer...................................................................... 43

28. Graph showing average flow budget of the Mississippi River alluvial aquifer for the 

simulation period (1988-96) .................................................................................................. 44
Contents v

TABLES

1. Description of the geologic units underlying the Mississippi River alluvial aquifer ............ 6

2. Rainfall during 1981-96 at Stoneville, Mississippi ............................................................... 13

3. Average monthly stage, in feet above sea level, at selected gage sites on the 

Mississippi River, 1988-96 .................................................................................................... 18

4. Root-mean-square error of simulated water levels in the Mississippi River alluvial 

aquifer .................................................................................................................................... 31

5. Summary of the principal hydraulic values used in the calibrated model............................. 34

CONVERSION FACTORS AND VERTICAL DATUM

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 4,047 square meter

 foot squared per day (ft2/d)) 0.09290 square meter per day

square mile (mi2

) 2.590 square kilometer 

Volume

gallon (gal) 3.785 liter

gallon (gal) 0.003785 cubic meter 

gallon (gal) 3.785 cubic decimeter 

Flow rate

cubic foot per second (ft3/d) 0.02832 cubic meter per second

foot per second (ft/s) 0.3048 meter per second

foot per day (ft/d) 0.3048 meter per day

foot per year (ft/yr) 0.3048 meter per year

gallon per minute (gal/min) 0.06308 liter per second

million gallons per day (Mgal/d) 0.04381 cubic meter per second

inch per year (in/yr) 25.4 millimeter per year

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer
vi Contents

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F - 32) / 1.8

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD

of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both

the United States and Canada, formerly called Sea Level Datum of 1929.

Altitude, as used in this report, refers to distance above or below sea level.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times

foot of aquifer thickness [(ft3

/d)/ft2]ft. In this report, the mathematically reduced form, foot squared

per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L),

micrograms per liter (μg/L), or picograms per kilogram of water (pg/kg).

1 liter of water by volume is approximately 1 kilogram by mass.

1,000 grams (g) = 1 kilogram (kg)

1,000 milligrams (mg) = 1 gram

1,000 micrograms (μg) = 1 milligram
Abstract 1

Hydrogeology, Model Description, and Flow Analysis of the 

Mississippi River Alluvial Aquifer in Northwestern 

Mississippi

By J. Kerry Arthur

ABSTRACT

The Mississippi River alluvial aquifer 

underlies a 7,000-square-mile area of the Mississippi River alluvial plain in northwestern Mississippi, an area locally known as the Delta. The 

alluvial aquifer is the most heavily pumped aquifer 

in Mississippi, and wells yielding more than 2,000 

gallons per minute are common. About 98 percent 

of the pumpage from the alluvial aquifer is for 

agriculture. The sand and gravel that form the 

alluvial aquifer averages about 110 feet in thickness. The aquifer is confined over most of the 

Delta, and the upper confining unit averages about 

25 feet in thickness. The average depth to water in 

the alluvial aquifer during fall 1999 was about 25 

feet.

The alluvial aquifer receives lateral 

recharge at the western boundary from the Mississippi River and at the eastern boundary from aquifers that directly underlie the Bluff Hills. The 

alluvial aquifer receives water vertically from precipitation, internal streams and lakes, and locally 

from the Cockfield and Sparta aquifers where they 

directly underlie the alluvial aquifer. The alluvial 

aquifer also discharges water to the underlying 

aquifers, and during extended periods with no surface runoff, to the Mississippi River and to the 

internal streams and lakes. The magnitude of 

recharge from the Mississippi River, precipitation, 

and internal lakes and streams can vary greatly 

depending upon hydrologic and climatic conditions.

The U.S. Geological Survey modular threedimensional finite-difference ground-water flow 

model, MODFLOW, was used to simulate the 

Mississippi River alluvial aquifer flow system in 

northwestern Mississippi. The model uses one 

layer with a rectangular-grid and 1-mile square 

cells to represent the alluvial aquifer. The model 

was calibrated and verified by using spring and fall 

water-level measurements from January 1988 

through December 1996. The values of selected 

model calibration-derived parameters for the alluvial aquifer are hydraulic conductivity, 425 feet 

per day; specific yield, 0.32; and storage coefficient, 0.016.

The model showed that the aquifer lost 

water from storage at an average rate of 404 cubic 

feet per second during the 9-year simulation 

period. During this period, the average pumpage 

rate was 1,270 million gallons per day (1,980 

cubic feet per second). Simulated areal recharge 

from precipitation averaged 2.6 inches per year 

(1,360 cubic feet per second). Vertical recharge 

from the internal streams and lakes and lateral 

recharge from aquifers underlying the Bluff Hills 

averaged 113 and 108 cubic feet per second, 

respectively. Model results indicated that net 

recharge from the Mississippi River and from 

aquifers directly underlying the alluvial aquifer 

was small.
2 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

INTRODUCTION

The Mississippi River alluvial aquifer in northwestern Mississippi is part of the large aquifer system 

formed by the Mississippi River and its tributaries that 

underlies the Mississippi River alluvial plain in parts of 

six States in the Mississippi embayment. In northwestern Mississippi, the Mississippi alluvial plain is a lensshaped area that includes all or part of 19 counties covering about 7,000 square miles (fig. 1). Locally this 

area is known as the Delta. The Delta extends from the 

Mississippi-Tennessee border at Memphis, Tennessee, 

about 200 miles southward to Vicksburg, Mississippi. 

At the widest point, about midway between Memphis 

and Vicksburg, the Delta is about 70 miles wide. The 

western extent of the Delta is the Mississippi River, and 

the eastern extent is the loess-capped Bluff Hills. The 

Delta land surface has very little relief, and slopes gently at about 1/2 foot per mile from about 200 feet above 

sea level at the northern end near Memphis to about 80 

feet above sea level near Vicksburg. The Bluff Hills 

escarpment provides an abrupt transition in topography 

from the alluvial plain by rising 100 to 200 feet above 

the alluvial plain. The present day Mississippi River 

channel, which generally follows the western Mississippi State boundary in the Delta, penetrates, in most 

locations, about the entire thickness of the Mississippi 

alluvial plain. The river is a hydraulic as well as a 

physical boundary of the Delta. The Yazoo-YalobushaTallahatchie-Yocona-Coldwater River system drains 

the eastern part of the Delta and a large upland area to 

the east of the alluvial plain. The Sunflower-Bogue 

Phalia River system drains most of the central and 

western part of the alluvial plain outside of the Mississippi River levee system. All the water drained by the 

Sunflower-Bogue Phalia River system originates 

within the Delta and flows into the Yazoo River just 

north of Vicksburg. The Delta has many oxbow lakes 

that store large quantities of water, but the largest of 

these crescent-shaped lakes are old meanders of the 

Mississippi River. Five of the largest lakes are Horn 

Lake in DeSoto County, Moon Lake in Coahoma 

County, Lake Bolivar in Bolivar County, Lake Washington in Washington County, and Eagle Lake in Warren County (fig. 1).

The climate in the Delta is humid subtropical. 

Average annual temperature ranges from 62 degrees 

Fahrenheit near Memphis to 66 degrees Fahrenheit 

near Vicksburg (Krinitzsky and Wire, 1964). Average 

annual precipitation in the Delta is about 52 inches with 

very little spatial variation. Most of the precipitation, 

about 62 percent, occurs in the winter and spring. The 

fall season has the least precipitation comprising about 

17 percent of the annual total.

The Mississippi River alluvial aquifer (hereafter 

referred to as the alluvial aquifer) is the most heavily 

pumped aquifer in Mississippi and supplies most of the 

water used for agriculture and industry in the Mississippi Delta. All the municipal and water association 

withdrawals in the Delta, except for the city of Vicksburg and the Eagle Lake Water Association, are derived 

from the deeper Tertiary aquifers. The city of Vicksburg uses the alluvial aquifer as its municipal water 

source, but the city’s annual use is less than 1 percent of 

the total water pumped from the alluvial aquifer. About 

98 percent of the withdrawal from the alluvial aquifer 

is for agriculture. The Delta is the economic center for 

agriculture in Mississippi, producing 99 percent of the 

rice, 96 percent of the catfish, 79 percent of the soybeans, and 72 percent of the cotton grown in the State. 

Water required for catfish and rice production accounts 

for most of the ground-water demand in the Delta. 

Humphreys, Sunflower, and Leflore Counties contain 

more than half of the catfish pond acreage in the Delta. 

Bolivar, Sunflower, and Washington Counties account 

for more than half of the rice produced in the Delta. 

Irrigation of cotton, soybeans, and corn during periods 

of deficient rainfall also contributes to the demand for 

water from the alluvial aquifer.

Since fall 1980, personnel from the U.S. Geological Survey (USGS), the Mississippi Department of 

Environmental Quality, Office of Land and Water 

Resources (OLWR), and the Yazoo Mississippi Delta 

Joint Water Management District (YMD) have made 

water-level measurements at more than 300 observation wells in the alluvial aquifer during the spring and 

fall of each year except in 1987 and 1988 when only 

fall measurements were made. These measurements 

indicate that water levels in the alluvial aquifer fluctuate seasonally with the highest levels occurring in the 

spring and the lowest levels occurring in the fall. The 

magnitude of the fluctuation depends upon rainfall, 

hydrologic conditions, and the amount of water 

pumped from the aquifer between the measuring periods. Also, the water-level measurements indicate a 

decreasing trend in the total volume of water in the 

alluvial aquifer since 1980.

From 1995 to 2000, the USGS, in cooperation 

with the OLWR, conducted a study to better understand 

the hydrogeology of the Mississippi River alluvial 

aquifer flow system in northwestern Mississippi and to 
Introduction 3

Ya e zoo Riv r

Co dl water i R ver

Yalobusha River

Yocona River

Big Black River

Sunf lower River

Bogue Phalia

Tallahatchie R r ive

River

Mississ pi pi

Study Area

(Delta)

(EASTERN EXTENT OF MISSISSIPPI RIVER ALLUVIAL AQUIFER)

Mississippi

W

L

ESTERN EDGE

OF

IL

BL H

UFF S

Horn Lake

Lake Washington

Lake Bolivar

Moon Lake

Eagle Lake

Memphis

Vicksburg

B012

A022

N046

0 10

10 20 KILOMETERS

20 MILES

0

EXPLANATION

B012 Location of observation well.

Lexington

Charleston

Carrollton

Grenada

Hernando

Senatobia

Sardis

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

33°

35°

90°

34°

91°

Base modified from U.S. Geological Survey digital data

1:250,000, 1972; 1:100,000, 1983

Lambert Conic Conformal projection

Standard parallels 33° and 45°, central meridian - 90°

ARKANSAS

LOUISIANA

TENNESSEE

Cleveland

Clarksdale

Greenwood

Belzoni

Rolling Fork

Yazoo City

Mayersville

Indianola

Greenville

Marks

Tunica

Figure 1. Location of study area, northwestern Mississippi.
4 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

construct a computer model of the flow system. The 

study of the hydrogeology and flow analysis of the 

alluvial aquifer may provide better insight into the 

recharge/discharge processes for the aquifer, and help 

determine the effects that different hydrologic conditions and pumpage have on the capacity of the aquifer 

to yield sufficient quantities of water for continuing 

agricultural and industrial growth in the Delta. Hydrologic information and field assistance was provided by 

the OLWR, the YMD, and the Natural Resources Conservation Service. This report describes the hydrogeology, the computer model, and the flow system of the 

alluvial aquifer as determined by field investigations, 

data review, and computer modeling results. 

HYDROGEOLOGY OF THE MISSISSIPPI 

RIVER ALLUVIAL AQUIFER

The Mississippi River alluvial aquifer flow system is a dynamic, rapidly responding flow system. The 

flow system is probably the most complex of the major 

aquifers in Mississippi with regard to changes in 

recharge and natural discharge as a result of fluctuations in hydrologic and climatic conditions. The aquifer is recharged by water from the Mississippi River 

and rivers within the Delta during periods of high river 

stages; by natural and man-made lakes, both inside and 

outside the Mississippi River levee system; by aquifers 

and sediments abutting the eastern edge of the alluvial 

plain; by rainfall that does not run off, evaporate, or 

transpire; and by water in aquifers directly underlying 

the alluvial aquifer. The aquifer discharges water to the 

Mississippi River and to streams within the Delta during periods of low river stages; to lakes during the summer and fall when lake levels are low; to aquifers 

underlying the alluvial aquifer when heads in the alluvial aquifer are greater than heads in the underlying 

aquifer; and to pumpage for agricultural, industrial, and 

municipal uses. The components of recharge and discharge vary greatly in time and space depending upon 

the depositional environment and climatic and economic conditions, not only within the Delta but also 

outside the Delta area.

The alluvial aquifer consists of sand and gravel 

deposits of Quaternary age. The alluvial sediments 

were deposited in an entrenchment into Tertiary-age 

rocks in the Mississippi River Valley. Pleistocene glaciation caused a lowering of sea level and increased 

stream gradients, which resulted in incisement of the 

Mississippi River Valley as water from the melting glaciers rushed toward the Gulf of Mexico. The valley 

was eroded more than 100 feet deeper than the present 

surface of the alluvial plain (Krinitzsky and Wire, 

1964). As sea level rose, stream gradients decreased 

and the entrenched valley was filled with sediment. 

Since the initial formation of the alluvial plain, the deposition of eroded Coastal Plain deposits and the continued erosion and deposition of materials by the 

Mississippi River and other streams within the alluvial 

plain has resulted in a diverse sequence of deposits 

forming the alluvial plain.

Geology

The Mississippi River alluvium in northwestern 

Mississippi was deposited on an erosional Tertiary-age 

surface having a system of north-south valleys (Fisk, 

1944). The average thickness of the alluvium in Mississippi is about 135 feet. Locally, the thickness ranges 

from 75 to greater than 200 feet (Arthur and Strom, 

1996); throughout most of the Delta, the thickness 

ranges from 120 to 160 feet (fig. 2). The coarsest sediments, consisting of gravel and coarse sand, generally 

occur at or near the base of the alluvium and tend to be 

thicker where the alluvium is thickest. The finer clay, 

silt, and sand sediments generally occur in the upper 

part of the alluvium, but these sediments can occur to 

varying degrees throughout the entire thickness of the 

alluvium. The sand and gravel that form the alluvial 

aquifer average about 110 feet in thickness. The aquifer is generally thickest in the central part of the alluvial 

plain and thinnest adjacent to the Bluff Hills and in 

west-central Washington County (Arthur and Strom, 

1996). The various combinations of sand, silt, and clay 

that occur near the surface of the alluvial plain make up 

a low permeability topstratum (Fisk, 1944). The topstratum consists largely of alternating interbedded silty 

sand and silty clay of natural levees along stream channels and sand bars. The topstratum also consists of clay 

plugs, which were formed by the deposition of finegrained, relatively impermeable sediments in abandoned river channels and sloughs. The topstratum, 

which will be referred to in this report as the upper confining unit, overlies the alluvial aquifer throughout 

about 99 percent of the Delta. Over most of the Delta, 

where present, the upper confining unit ranges from 10 

to 50 feet thick and averages about 25 feet in thickness. 

A detailed description of the spatial distribution of the 

thickness of the upper confining unit and sand and 

gravel in the Mississippi River alluvium in northwest-
Hydrogeology of the Mississippi River Alluvial Aquifer 5

Figure 2. Generalized thickness of the Mississippi River alluvium.

River

Mississ pi pi

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

COAHOMA QUITMAN

33°

35°

90°

34°

91°

Base modified from U.S. Geological Survey digital data

1:250,000, 1972; 1:100,000, 1983

Lambert Conic Conformal projection

Standard parallels 33° and 45° central meridian -90°

0 10

10 20 KILOMETERS

20 MILES

0

EXPLANATION

140 Line of equal thickness

of the alluvium. Interval 20 feet

Eastern extent of Mississippi

River alluvial aquifer (study area)

180
6 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

ern Mississippi is given in reports by Arthur (1994) and 

by Arthur and Strom (1996).

The geologic units of Tertiary age underlying the 

Mississippi River alluvial aquifer dip 15 to 40 feet per 

mile to the west in the northern part of the Delta and to 

the southwest in the southern part of the Delta toward 

the axis of the Mississippi River embayment trough. 

The axis of the embayment approximately parallels the 

Mississippi River. The underlying units from the oldest to the youngest are as follows: Zilpha Clay, Sparta 

Sand, Cook Mountain Formation, Cockfield Formation, Yazoo Clay, and the Forest Hill Formation. A 

map (modified from Jennings, 2001) showing the areas 

where these geologic units subcrop the alluvial aquifer 

is shown in figure 3.

Two geologic sections (modified from Jennings, 

2001) showing the relation of these units to each other 

and to the overlying alluvial aquifer are shown in 

figure 4. The sediments of these units in contact with 

the alluvial aquifer consist of unconsolidated sand, silt, 

and clay beds of varying thickness. In areas where the 

underlying-unit contact consists of clay or silty clay, 

the contact impedes the vertical movement of water 

between the alluvial aquifer and the underlying aquifer. 

The Zilpha Clay and Yazoo Clay are low permeability 

marine clays. The Cook Mountain Formation is mostly 

clay, but has substantial sand thickness in the northern 

half of the Delta. The Cockfield Formation and the 

Sparta Sand are mostly sand with clay beds separating 

individual sand beds. The sand beds within the Cockfield and Sparta form two of the major aquifers that 

supply drinking water in the State. In areas where sand 

beds of the Cockfield Formation or Sparta Sand are in 

contact with the alluvial aquifer, hydraulic connection 

exists between the alluvial aquifer and the deeper aquifer. The Forest Hill Formation underlies the alluvial 

aquifer in a small area (about 3 square miles) in Warren 

County at the edge of the Bluff Hills. The Forest Hill 

Formation consists mostly of sand and clay and is a 

minor aquifer in Mississippi. The geologic units 

underlying the Mississippi River alluvial aquifer are 

described in table 1.

Table 1. Description of the geologic units underlying the Mississippi River alluvial aquifer

Geologic unit Maximum thickness

(feet) Lithology Water-bearing

characteristics

Forest Hill Formation

Yazoo Clay

Cockfield Formation

Cook Mountain Formation

Sparta Sand

Zilpha Clay

50

500

500

170

700

150

Sand and clay

Sand and clay

Sand and clay

Clay

Clay and sandy clay

Mostly sandy clay

in northern part of

Delta

Clay; becomes

sandy northward

Minor aquifer

(Gandi, 1979).

Not an aquifer.

Not an aquifer.

Not an aquifer.

Major aquifer

(Spiers, 1977). In

the Greenville area, water

levels are lower than in

the alluvial aquifer.

In other areas, water

levels are about the

same as in the alluvial

aquifer.

Major aquifer

(Newcome, 1976). Water

levels are about the same as

in the alluvial aquifer except

near Bluff Hills where they are

higher.
Hydrogeology of the Mississippi River Alluvial Aquifer 7

Figure 3. Geologic units underlying the Mississippi River alluvial aquifer.

River

Mississippi

0 10

10 20 KILOMETERS

20 MILES

0

Cockfield

Formation

Sparta Sand

Zilpha Clay

Forest Hill Formation

Yazoo Clay

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

BOLIVAR LEFLORE

TUNICA

QUITMAN

COAHOMA

33°

35°

90°

34°

91°

Jennings, (2001)

BL

HI

UFF

LLS

B’

B

A’

A Yazoo

Clay

Formation

Cook

Mountain
8 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aqufier in Northwestern Mississippi Figure 4a. Geologic section A-A’ from west to east across the Delta. 500’ 

SEA LEVEL 

500’ 

1,000’ 

1,500’ 

2,000’ 

D33 Log 67 E120 Note: trace of section shown in figure 3. VERTICAL SCALE GR

Greenville

Bogue Phalia

WASHINGT

Indianola

Sunflower River

Yazoo River

Greenwood

ON COUNTY

SUNFLOWER COUNTY

SUNFLOWER COUNTY

LEFLORE COUNTY

LEFLORE COUNTY

CARROLL COUNTY

P128

Log 112

N36

Log 66

J18

Log 36

L257

Log 114

D11

Log 56

Alluvium

Jackson Group

Cockfield Formation

Cook

Mountain

Sparta Formation

Zilpha Clay

Winona-Tallahatta

Meridian Sand

00

5

5

A

WEST 

Jennings, (
Hydrogeology of the Mississippi River Alluvial Aquifer 9

500’

SEA LEVEL

500’

1,000’1,500’2,000’2,500’

Alluvium

Cockfield

Formation

Cook Mountain

Sparta Formation

Zilpha Clay

Winona/Tallahatta

Meridian Sand

Wilcox Group

Midway

C106

Log 77

??

Log 37 Log 3

Note: trace of section

shown in figure 3.

VERTICAL SCALE G

Bogue Phalia

Shelby

BOLIVAR COUNTY

SUNFLOWER COUNTY

Sunflower River

Parchman

SUNFLOWER COUNTY

TALLAHATCHIE COUNTY

Sumner

Tallahatchie River

E107

Log 72

B69

Log 73

H35

Log 48

K23

Log 59 Log 10

L

F31 Log 40 

00

5

5

B

WEST 

Jennings, (2001)

Figure 4b. Geologic section B-B’ from west to east across the Delta.
10 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Aquifer Flow Boundaries

The Mississippi River alluvial aquifer in northwestern Mississippi functions as an independent flow 

system from the Mississippi River alluvial aquifer west 

of the Mississippi River in Arkansas and Louisiana. In 

most locations, the present day Mississippi River channel penetrates nearly the entire thickness of the alluvium and serves as a hydraulic boundary that separates 

flow in the alluvial aquifer east of the river from flow 

in the alluvial aquifer west of the river. Stresses applied 

to the Mississippi River alluvial aquifer on the western 

side of the river have no effect on the alluvial aquifer 

underlying the Delta in Mississippi. The Mississippi 

River, the western flow boundary, serves as a variablehead hydraulic barrier and both a recharge source and 

discharge area for the alluvial aquifer. When the Mississippi River is at a high stage, water flows from the 

river into the aquifer; conversely, when the river is at a 

low stage, water flows from the aquifer to the river 

(fig. 5). The stage of the Mississippi River can vary as 

much as 50 feet from extreme flood to severe drought 

conditions. As a result, the alluvial aquifer water levels 

adjacent to the river are buffered within this range of 

river-stage changes, and the aquifer is recharged and 

drained locally by the Mississippi River seasonally. 

However, because of the cyclic recharging and draining 

of the aquifer adjacent to the river, the net long-term 

contribution to ground water from the river is probably 

small compared to the other recharge sources.

The eastern flow boundary of the Mississippi 

River alluvial aquifer occurs at the western edge of the 

loess-capped Bluff Hills. The western edge of the Bluff 

Hills marks the eastern extent of the Mississippi River 

alluvial plain and the alluvial aquifer (fig. 1). The Mississippi River valley is incised into the underlying Tertiary-age deposits along the eastern edge of the Delta 

adjacent to the Bluff Hills (fig. 2). At the eastern edge 

of the Delta, along the contact between the alluvial 

aquifer and the Cockfield and Sparta aquifers, a substantial upward hydraulic gradient between the Tertiary 

aquifers and the alluvial aquifer produces a correspondingly large lateral and upward flow into the alluvial aquifer. The water-bearing units underlying the 

western edge of the Bluff Hills are a major recharge 

source for the Mississippi River alluvial aquifer.

The upper boundary of the alluvial aquifer is the 

low permeability topstratum, which is present over 

about 99 percent of the Delta. The topstratum consists 

of interbedded clay, silt, and sand. A detailed description of the topstratum (upper confining unit) is given by 

Arthur (1994). Where present, the topstratum impedes 

vertical recharge to the alluvial aquifer from precipitation and from surface-water bodies (rivers, lakes, 

ponds, and agricultural applications). About 20 inches 

of the annual precipitation is runoff, leaving about 32 

inches for evaporation, transpiration by vegetation, and 

replenishment of the ground-water reservoir. Usually 

only about 5 percent of the total rainfall (about 2.6 

inches) replenishes the ground-water reservoir (Krinitzsky and Wire, 1964). Infiltration of precipitation is 

the chief source of recharge to the alluvial aquifer 

(Boswell and others, 1968).

Figure 5. Relation of water levels in the Mississippi River to water levels in the Mississippi 

River alluvial aquifer.

70

75

80

85

90

95

100

105

110

115

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

WATER-LEVEL ALTITUDE, IN FEET

ABOVE SEA LEVEL

DISTANCE FROM MISSISSIPPI RIVER, IN MILES

April 1996 water levels

October 1996 water levels

Mississippi River water level

WEST EAST
Hydrogeology of the Mississippi River Alluvial Aquifer 11

The lower flow boundary of the Mississippi 

River alluvial aquifer is the contact between the underlying Tertiary-age deposits and the alluvial aquifer 

(fig. 2). The Sparta Sand and Cockfield Formation 

underlie most of the alluvial aquifer in the Delta. In 

these areas, flow between the alluvial aquifer and the 

two underlying aquifers is probable. The most likely 

area for flow upward into the alluvial aquifer is near the 

eastern edge of the Delta where water levels in the 

underlying aquifers are higher than in other parts of the 

Delta. The most likely area for flow from the alluvial 

aquifer downward into the underlying unit is in the 

western part of the Delta where the Cockfield Formation underlies the alluvial aquifer, and where heads in 

the Cockfield aquifer are lower than the heads in the 

alluvial aquifer. In areas where the Yazoo Clay, Zilpha 

Clay, and Cook Mountain Formation underlie the alluvial aquifer, the low permeability characteristics of 

these deposits impede vertical flow. In recent times, 

net recharge at the lower boundary is probably small, 

but could be important locally where the alluvium is in 

direct contact with the underlying water-bearing sand 

beds, and where the water level in the alluvial aquifer 

is lower than the water level in the underlying aquifer. 

Sumner and Wasson (1990) assumed that flow interaction between the underlying Tertiary aquifers and the 

alluvial aquifer was negligible during the model simulation period 1981-83, and they simulated the lower 

boundary as a no-flow boundary. Before extensive 

development of the Cockfield and Sparta aquifers, 

water levels in these two underlying aquifers were substantially higher than water levels in the alluvial aquifer, and recharge from the underlying aquifers was 

probably greater. Predevelopment upward flow from 

the Cockfield aquifer is indicated by the greater concentration of dissolved solids in the alluvial water in 

the southwestern part of the Delta.

Aquifer Characteristics

The Mississippi River alluvial aquifer is capable 

of storing and yielding large quantities of water almost 

everywhere in the Delta. The average thickness of the 

sand and gravel in the alluvial aquifer in the Delta is 

about 110 feet. Wells in the Delta commonly yield 

more than 2,000 gallons per minute from the alluvial 

aquifer. 

Aquifer characteristics that indicate the capacity 

of an aquifer to store and transmit water are specific 

yield, storage coefficient, hydraulic conductivity, and 

transmissivity. The specific yield of the aquifer is the 

volume of water that a unit volume of aquifer material 

will yield by gravity drainage from a saturated state. 

Storage coefficient is the volume of water that an aquifer releases or takes into storage per unit surface area of 

the aquifer per unit change in head. The size of the 

storage coefficient depends on whether the aquifer is 

confined (water level in tightly cased well is above the 

top of the aquifer) or unconfined (water level is at or 

below the top of the aquifer). If the aquifer is confined, 

the storage coefficient is small because the water 

released from storage when the head declines comes 

from expansion of the water and from compression of 

the aquifer. If the aquifer is unconfined, the predominant source of water is by gravity drainage from aquifer 

material with the decline of head. The storage coefficient of an unconfined aquifer is virtually equal to the 

specific yield of the aquifer. Hydraulic conductivity is 

a measure of the ability of the aquifer to transmit water 

through a unit cross-sectional area of the aquifer in 

response to a unit hydraulic gradient. Transmissivity is 

a measure of the ability of an aquifer to transmit water 

through a unit width of aquifer material in response to 

a unit hydraulic gradient. Transmissivity is equal to the 

hydraulic conductivity times the saturated thickness of 

the aquifer material.

Few aquifer tests are available for the alluvial 

aquifer in northwestern Mississippi. Slack and Darden 

(1991) reported the results of six aquifer tests for the 

alluvial aquifer in the Mississippi Delta. Three tests 

were conducted in Warren County, two in Washington 

County, and one in Coahoma County. The storage 

coefficients for the aquifer tests ranged from 0.0003 to 

0.016. Krinitzsky and Wire (1964) reported a specific 

yield of 0.15 in Tallahatchie County. West of the Mississippi River in Arkansas, the specific yield of the 

Mississippi River alluvial aquifer is reported to range 

from 0.27 to 0.38 based on laboratory tests of repacked 

samples (Krinitzsky and Wire, 1964). The range of 

transmissivity from the six tests in Mississippi was 

12,000 to 51,000 feet squared per day, and the range of 

hydraulic conductivity was 130 to 400 feet per day. 

Krinitzsky and Wire (1964) reported a hydraulic conductivity of more than 1,100 feet per day for alluvial 

aquifer material in Arkansas.
12 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Water-Level Measurements, Potentiometric 

Surfaces, and Direction of Flow

 Prior to extensive development of the Mississippi River alluvial aquifer as a source of water for 

agriculture and industry, water levels in the aquifer 

were within 10 to 20 feet of land surface over most of 

the Delta (Brown, 1947). Wells near the Mississippi 

River were reported to flow when the river was at high 

stage, as well as some wells adjacent to the Bluff Hills, 

indicating the aquifer in these locations was under confined conditions. Subsequent to the extensive development of the aquifer (mainly for agriculture), water 

levels have declined to as much as 45 feet below land 

surface in localized areas in the central part of the 

Delta. Outside the heavy use area in the central part of 

the Delta, water levels in the aquifer average less than 

25 feet below land surface, and some wells adjacent to 

the Mississippi River levee system flow when the Mississippi River is at high stage.

Since 1992, to supplement the spring and fall 

water-level measurements made by the YMD at more 

than 300 observation wells, the OLWR has been making monthly water-level measurements at selected 

observation wells during the fall and winter seasons 

along three west-to-east sections across the Delta. 

Water-level measurements indicate that water levels in 

the alluvial aquifer fluctuate seasonally with the highest water levels occurring during early spring and the 

lowest water levels occurring during early fall. Since 

1980 the fall water levels in the Delta has averaged 

about 3.7 feet lower than the preceding spring water 

levels. During the winter, water levels recover, and 

spring water levels average about 3.5 feet higher than 

the preceding fall water levels (fig. 6) resulting in about 

an average decline of less than 0.2 foot per year since 

1980. The range of the seasonal fluctuation in water 

levels is between about 1.0 and 5.5 feet depending on 

the amount and distribution of rainfall and pumpage. 

In summer 1988, rainfall was below average 

(table 2), and pumpage for agriculture was greater than 

normal. As a result, the fall 1988 average water level 

in the alluvial aquifer was the lowest on record (average 24.6 feet below land surface) until fall 1999 when 

the record average low water level was equaled. Water 

levels recovered substantially by spring 1989; however, during spring 1989, water levels (average 19.9 

feet below land surface) were still below the typical 

spring levels. During spring and summer 1989, precipitation was above average, and pumpage for agriculture 

was less than normal. As a result of the wet spring and 

summer and reduced pumpage during the 1989 growing season, the fall 1989 water levels (average 21.6 feet 

below land surface) were about equal to fall levels prior 

to the drought during summer 1988.

Since fall 1980, the regional potentiometric surface of the alluvial aquifer in the Delta was lowest in 

fall 1988 and fall 1999 and was the highest in spring 

1984 (fig. 7). Since fall 1980, the regional potentiometric surface has fluctuated seasonally, but the 

regional potentiometric surface has declined only about 

2 to 3 feet. Locally, the potentiometric surface has 

declined greater than 30 feet in the central part of the 

Delta near the Sunflower-Leflore County line and as 

much as 15 feet in central Humphreys County.

Historically, the regional potentiometric surface 

of the alluvial aquifer has sloped from north to south, 

from the west, and from the east toward the Sunflower 

River, which flows from north to south in the central 

part of the Delta. The shape of the potentiometric surface indicates that the regional flow direction in the 

aquifer is composed of two components--a north-tosouth axial flow reflecting the north-to-south slope of 

the alluvial plain and a periphery-to-interior lateral 

flow due to the influence of the high-altitude (Bluff 

Hills) recharge source to the east and the Mississippi 

River to the west. The shape of the potentiometric surface indicates that the Sunflower and Yazoo Rivers are 

regional drains for the alluvial aquifer during low river 

stages. Although the alluvial plain has very little topographic relief, the land surface is about 100 feet lower 

at the southern part of the Delta near Vicksburg than at 

the northern part near Memphis, and the land-surface 

slope is reflected in the regional potentiometric surface 

of the aquifer. East of the Delta, the water levels in the 

water-bearing units underlying the Bluff Hills are generally equal to or greater than the altitude of the surface 

of the Mississippi River alluvial plain. Flow from this 

high-altitude recharge source is reflected in an east-towest slope of the potentiometric surface toward the 

central part of the Delta. Flow from the western edge 

of the Delta is influenced by the Mississippi River. 

During the typical 3 to 4 months of the year when the 

Mississippi River is at a stage higher than the potentiometric surface of the alluvial aquifer adjacent to the 

river, flow direction is eastward from the river. Flow in 

the aquifer adjacent to the river varies, but an equivalent water level in the aquifer is established in most 

locations along the river about 3 to 6 miles from the 
Hydrogeology of the Mississippi River Alluvial Aquifer 13

Figure 6. Average depth to water during spring and fall in the Mississipi River alluvial aquifer.

0

5

10

15

20

25

30

AVERAGE DEPTH TO WATER, IN FEET

1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

YEAR

Fall Spring

Table 2. Rainfall, in inches, during 1981-96 at Stoneville, Mississippi

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Total

1.53

8.24

4.01

3.55

2.45

2.42

3.06

5.26

3.74

7.66

0.74

7.48

2.81

3.70

2.40

2.65

4.50

2.42

5.97

5.61

2.15

6.67

11.95

8.23

5.82

4.66

6.94

1.72

5.32

6.09

5.50

2.00

0.71

4.72

4.25

2.84

3.50

2.85

4.27

4.51

5.03

7.27

1.69

7.71

1.97

6.01

5.61

4.54

7.80

5.41

5.45

0.98

4.65

2.45

6.65

5.37

16.93

1.99

9.62

7.57

3.50

1.92

3.48

5.53

1.86

2.10

5.92

4.83

4.78

5.96

1.16

6.59

9.97

6.35

7.55

2.62

1.75

4.44

5.47

5.12

5.00

2.43

0.70

2.45

4.77

3.29

9.45

4.61

3.80

7.10

2.05

4.58

4.02

4.84

3.26

3.45

1.98

2.22

5.65

0.57

3.17

3.92

3.10

9.12

1.57

2.91

0.47

2.92

3.74

5.82

2.42

3.28

2.06

11.58

2.43

4.06

3.40

3.32

3.82

1.56

3.62

4.38

3.11

2.50

1.63

1.76

1.41

1.52

0.54

0.91

0.45

1.43

4.49

1.70

4.33

2.38

4.95

7.00

3.28

4.34

2.97

3.39

1.47

1.60

2.83

3.18

0.72

1.14

0.91

2.96

6.50

4.39

3.23

3.09

0.81

6.26

2.70

0.65

3.38

0.27

1.30

6.56

4.70

5.03

7.52

10.99

0.79

4.40

2.88

3.17

2.55

4.37

2.58

2.67

6.61

3.67

10.95

2.62

6.82

4.31

12.98

3.16

5.58

4.68

4.50

6.11

5.15

2.30

5.31

4.29

3.26

10.70

7.09

4.99

6.59

19.53

9.59

4.22

4.99

1.07

3.95

4.70

6.35

5.75

34.80

67.34

43.70

44.58

59.73

66.29

53.62

53.95

63.93

59.11

41.19

58.23

49.72

44.61

40.77

47.69

Average 52.05

(1915-96)
14 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Figure 7. Lowest (a) and highest (b) potentiometric surfaces of the Mississippi River alluvial aquifer since 1980.

River

Mississ pi pi

River

Mississ pi pi

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

190

180

170

160

160

150

150

140

140

130

130

130

120

120

120

120

110

110

110

110

100

100

100

100

100

100

100

90

90

90

90

90

90

90

80

80

80

80

80

80

80

70

70

60

170

170

180

190

160

160

160

160

150

150

150

140

140 140

130

130

130

130

120

120

120

110

110

110

110

110

100

100

100

100

100

90

90

90

90

90

90

90

90

80

80

0 10

10 20 KILOMETERS

20 MILES

0

EXPLANATION

Eastern extent of Mississippi

River alluvial aquifer (study area)

80 Potentiometric contour Shows

altitude at which water level would

have stood in tightly cased wells.

Interval 10 feet. Hachures indicate

water-level depression. Datum is

sea level

35° 35°

34° 34°

33° 33°

90° 90°

91° 91°

a. Lowest Potentiometric Surface (fall 1988) b. Highest Potentiometric Surface (spring 1984)
Hydrogeology of the Mississippi River Alluvial Aquifer 15

river. From this hinge point in the potentiometric surface, the potentiometric surface generally slopes to the 

southeast toward the central part of the Delta. Natural 

flow from the alluvial aquifer (discharge) provides 

water to internal streams and lakes in the Delta when 

the stages of the streams and lakes are below the level 

of the potentiometric surface of the aquifer; conversely, 

during periods when internal streams have sufficient 

runoff to produce high stream stages, flow is reversed 

locally and water flows from the streams into the aquifer (fig. 8). The magnitude of the flow between the 

aquifer and the internal surface-water bodies is determined by the extent of incisement of the surface-water 

body channel into the upper confining unit (topstratum), the head difference between the aquifer and the 

surface water, and the duration of the head difference. 

Due to the shorter duration of the high stages in the 

unregulated internal streams and the shallower incisement of the channels into the alluvium as compared to 

the Mississippi River, the internal streams at high 

stages have less effect on the regional flow pattern in 

the alluvial aquifer than does the Mississippi River. On 

a local scale, the internal water bodies are major 

recharge sources in most areas in the Delta, especially 

surface-water bodies near the heavy withdrawal areas 

in the central part of the Delta.

Water that enters the aquifer areally as direct 

infiltration from precipitation is a small part of the total 

annual rainfall (reported in 1964 by Krinitzsky and 

Wire to be about 5 percent), but is a significant flow 

component in the alluvial aquifer flow system. 

Recharge from precipitation can vary substantially on a 

local basis due to variations in permeability of the topstratum, but on a regional basis areal recharge is 

believed to be relatively uniform and, as a result, has 

little effect on regional flow direction. Water that 

enters the alluvial aquifer from underlying waterbearing units (Cockfield Formation and Sparta Sand), 

or water that flows from the alluvial aquifer into these 

units may locally have some impact on flow direction 

and shape of the potentiometric surface in areas where 

the alluvial aquifer is hydraulically connected to the 

underlying water-bearing units.

Water Volume

 The sand and gravel deposits in the Mississippi 

River alluvium, which compose the alluvial aquifer, 

have the capacity to store an enormous volume of 

water. Assuming the alluvial aquifer has an average 

specific yield of 0.32, and ignoring the water stored as 

the result of compaction of water and the expansion of 

the aquifer with increasing head, the aquifer in the 

Delta has the capacity to store an average of about 

21,570 acre-feet of water per square mile (fig. 9). 

Based on twice yearly water-level measurements made 

since fall 1980, the least volume of water stored in the 

alluvial aquifer was determined to have occurred during fall 1999 with an average of about 20,690 acre-feet 

per square mile. Generally, areas in the Delta with the 

greatest volume of water are those where the sand and 

gravel beds are thickest. Most counties in the Delta 

have large volumes of alluvial water, but an exception 

is west-central Washington County where the sand and 

gravel beds are less than 50 feet thick. Bolivar and 

Sharkey Counties have the greatest average thickness 

of sand and gravel in the Delta and, consequently, have 

the largest volume of water per square mile of surface 

area. The greatest average total volume of water in the 

aquifer since fall 1980 was calculated to be about 

21,290 acre-feet per square mile during spring 1984. 

On a regional basis, only a small volume of water has 

been depleted from the alluvial aquifer. Since fall 

1980, the amount of water stored in the aquifer has 

ranged from about 96 to about 99 percent of the aquifer’s unconfined capacity, although localized areas in 

the central part of the Delta have experienced drawdowns greater than 30 feet.

The volume of water in the alluvial aquifer fluctuates yearly from spring to fall depending on climatic, 

hydrologic, and economic conditions that influence 

recharge, discharge, and pumpage. The volume of 

water in the aquifer increases from fall to spring and 

generally decreases from spring to fall. The magnitude 

of the volume change depends upon the amount of natural discharge from and recharge to the aquifer and the 

magnitude of pumpage. Most of the natural discharge 

and pumpage from the aquifer occurs during summer 

and early fall, whereas most of the recharge occurs 

from late fall through spring. Within about 3 to 6 miles 

of the Mississippi River, the volume of water in the 

alluvial aquifer fluctuates due to the typically large seasonal stage changes in the Mississippi River. From 

1988 to 1996, the average monthly stage of the Mississippi River at Vicksburg ranged from 46.3 to 91.9 feet 

above sea level (table 3). Based on water-level measurements made during fall and spring from 1980 to 

1999, assuming a specific yield of 0.32, and that the 
16 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aqufier in Northwestern Mississippi

Figure 8. Hydrologic and climatic processes in the Delta.

Miss i iss ppi River

Miss i iss ppi River

Jackson

Jackson

Group

Group Mississippi River Alluvium Mississippi River Alluvium

Deep infiltration

to aquifer

Deep infiltration

to aquifer

Irrigation

well

Irrigation

well

Irrigation

well

Irrigation

well

Water level Water level

Deep infiltration in area

along foot of hills

Evapotransporation Evapotransporation

Stream flow

from area

Strea

from 

Catfish ponds

and wells

Surface water

lift pump for

irrigation

Su

li

Cockfield Formation Cockfield Formation

Cook Mountain Formation

Loess 

BLUFF HILLS

Sunflower River

Sunflower River

Yazoo River

Precipitation about

52 inches annually

Winter 17 inches

Spring 15 inches

MISSISSIPPI

MISSISSIPPI

LOUISIANA

LOUISIANA

Co

A Winter and Spring Seasons Summer and Fall Seasons B

Modified from 

EXPLANATION

Flow path of water in aquifer
Hydrogeology of the Mississippi River Alluvial Aquifer 17

Figure 9. Volume of water in the Mississippi river alluvial aquifer during fall 1999.

River

Mississippi

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

33°

35°

90°

34°

91°

EXPLANATION

Acre-feet of water per square mile in the

alluvial aquifer during fall 1999

25,000

20,000

15,000

10,000

5,000

0
18 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Table 3. Average monthly stage, in feet above sea level, at selected gage sites on the Mississippi River,

1988-96

1988

1989

1990

1991

1992

Date Memphis Helena Rosedale Arkansas

City

Greenville Lake

Providence

Vicksburg

January 198.1 164.3 133.5 120.6 109.9 91.8 74.9

February 199.8 165.2 132.0 119.0 108.7 90.4 75.0

March 194.7 159.7 127.6 115.4 105.1 86.3 70.7

April 197.4 162.3 131.3 118.8 108.6 89.8 74.0

May 186.0 150.3 116.4 105.6 95.7 76.0 59.8

June 177.1 141.3 105.1 95.8 86.4 65.3 49.4

July 174.9 139.4 103.5 93.6 84.0 63.5 46.5

August 174.9 139.1 103.0 93.5 83.7 63.5 46.5

September 175.8 140.0 103.4 93.7 83.8 63.5 46.3

October 176.8 140.8 104.7 94.7 84.7 65.2 48.0

November 185.3 149.4 112.6 101.4 90.9 71.0 53.7

December 186.1 151.3 117.1 105.8 95.9 76.0 60.5

January 202.2 167.7 132.5 119.1 108.8 90.4 75.6

February 202.2 167.5 133.4 120.1 109.9 91.2 75.7

March 208.8 175.8 142.7 128.5 118.7 101.1 86.1

April 205.4 171.8 138.6 124.9 115.2 97.7 82.7

May 198.3 163.2 129.3 116.1 106.4 87.4 72.3

June 200.9 166.0 132.3 118.9 109.4 90.7 75.4

July 195.7 161.1 127.4 115.0 105.8 87.5 73.5

August 184.9 148.3 112.6 102.1 93.0 72.9 57.3

September 192.2 156.1 120.6 108.8 99.3 79.3 63.1

October 192.4 156.6 120.2 108.1 98.7 78.6 62.5

November 190.2 153.9 117.2 105.7 96.2 75.1 60.3

December 185.1 149.1 112.1 101.6 92.3 73.3 56.4

January 195.3 158.9 121.8 109.6 99.4 80.1 64.4

February 210.8 176.0 140.6 126.2 115.9 98.5 82.9

March 205.6 172.2 141.2 127.4 117.2 100.3 85.4

April 197.8 163.4 134.0 121.2 111.2 93.5 79.1

May 204.9 170.4 140.3 126.5 116.1 98.7 83.2

June 208.4 174.7 142.2 128.5 118.5 101.9 86.7

July 198.4 163.2 128.5 115.5 105.5 87.1 71.3

August 191.6 155.7 120.6 108.2 98.8 79.4 62.8

September 188.5 152.5 118.0 105.7 96.2 76.2 60.1

October 188.8 152.7 118.1 105.4 95.9 75.2 59.0

November 187.7 151.6 116.5 104.3 94.7 74.1 58.2

December 199.4 163.4 126.7 113.3 103.3 83.2 67.1

January 216.3 182.7 147.6 144.0 122.2 105.3 89.5

February 204.7 170.2 136.1 133.7 112.2 94.5 80.0

March 208.3 173.9 138.5 136.1 114.3 97.1 82.6

April 210.1 176.6 142.5 139.7 117.9 100.9 85.7

May 204.1 170.2 138.3 136.9 115.1 98.8 85.2

June 198.4 163.5 129.2 128.6 106.8 88.7 74.3

July 189.7 153.3 118.3 119.2 97.4 77.9 62.1

August 183.4 145.9 110.7 112.0 90.2 69.9 54.7

September 181.1 143.1 108.4 109.8 88.0 67.7 52.5

October 180.0 141.9 106.8 108.7 86.9 66.7 51.0

November 185.4 148.5 116.4 117.7 95.9 75.0 59.3

December 201.7 166.8 133.6 131.8 110.0 92.0 76.7

January 194.7 159.3 126.6 113.4 103.5 84.9 70.0

February 189.7 153.4 118.5 106.6 96.5 76.8 61.3

March 201.8 166.8 131.5 117.9 107.6 89.2 73.6

April 197.8 162.5 127.7 114.4 104.4 85.9 69.5

May 193.8 158.6 124.6 111.8 102.0 83.2 67.3

June 189.4 153.5 121.1 109.3 99.4 80.1 63.8
Hydrogeology of the Mississippi River Alluvial Aquifer 19

Table 3. Average monthly stage, in feet above sea level, at selected gage sites on the Mississippi River,

1988-96--Continued

1992

1993

1994

1995

1996

Date Memphis Helena Rosedale Arkansas

City

Greenville Lake

Providence

Vicksburg

July 191.9 155.8 122.0 109.6 99.8 80.5 63.9

August 194.1 159.0 125.4 112.9 103.1 84.5 68.5

September 188.1 151.6 116.3 104.8 94.7 75.1 58.6

October 186.1 150.1 115.0 103.0 92.8 73.2 57.2

November 193.9 157.4 121.4 108.8 98.1 78.5 62.0

December 201.8 167.2 134.2 120.8 110.2 92.4 76.6

January 206.2 172.0 139.3 125.1 114.9 97.5 82.2

February 198.9 164.1 132.0 118.8 108.6 91.6 75.7

March 208.7 174.1 139.8 125.4 115.1 97.6 81.8

April 214.7 181.0 147.2 131.6 121.2 104.1 88.3

May 209.8 176.6 145.9 131.7 121.8 105.0 89.5

June 200.9 165.7 134.1 121.6 111.7 94.2 78.8

July 208.4 173.2 138.0 124.2 114.1 95.9 79.6

August 211.1 177.0 141.9 128.0 118.1 100.9 84.8

September 202.3 166.6 131.4 118.5 108.7 90.3 74.2

October 201.6 166.4 132.1 119.3 109.4 91.2 75.2

November 199.7 163.8 128.5 115.6 105.4 86.2 69.9

December 203.4 169.2 135.7 122.1 112.1 94.4 79.2

January 197.2 161.3 127.3 114.2 103.9 85.1 69.7

February 210.7 176.4 141.1 126.2 115.8 98.4 83.5

March 213.7 180.4 146.6 131.4 121.0 104.1 88.9

April 217.1 183.2 148.3 132.9 122.5 105.1 89.6

May 208.6 175.7 145.2 130.6 120.4 103.3 89.0

June 193.1 157.4 123.3 110.9 100.8 82.8 66.6

July 192.9 157.3 122.8 110.5 100.4 82.2 66.2

August 189.0 152.7 117.4 106.0 95.8 77.5 61.1

September 184.1 147.4 111.9 101.7 91.2 72.3 56.9

October 184.9 147.9 111.7 101.3 90.6 71.3 55.9

November 188.2 151.5 118.5 107.7 97.3 78.6 61.9

December 195.3 160.1 126.8 114.6 104.1 85.7 70.0

January 195.1 159.4 125.1 112.6 102.1 83.5 67.9

February 197.4 162.1 127.6 114.9 104.5 86.4 71.4

March 200.7 165.7 131.8 118.9 108.5 91.3 76.4

April 195.2 159.3 125.4 113.0 102.7 84.7 69.2

May 209.5 174.5 140.3 126.5 115.8 98.5 82.8

June 212.9 180.2 149.0 135.1 124.5 107.8 91.9

July 195.9 161.2 129.6 117.5 107.5 90.3 75.3

August 192.8 157.5 124.0 111.6 101.5 83.4 67.4

September 184.2 148.3 114.0 102.2 91.9 73.3 58.0

October 186.1 149.8 113.6 102.6 92.2 73.7 57.5

November 192.7 156.8 119.9 107.8 97.3 78.3 62.3

December 189.2 153.2 117.0 105.4 94.8 76.4 60.5

January 193.7 157.9 122.7 110.2 99.5 80.8 65.0

February 201.3 166.3 130.6 117.5 106.8 89.1 73.4

March 200.8 165.8 130.1 116.5 105.7 87.6 71.4

April 201.0 166.0 130.3 117.1 106.6 89.0 73.1

May 212.0 178.3 144.5 129.9 119.0 101.7 84.9

June 208.7 175.5 144.8 130.9 120.5 104.1 87.9

July 195.3 160.1 125.4 112.8 103.0 85.9 69.2

August 192.6 156.7 121.6 109.5 99.7 81.7 65.6

September 187.0 151.2 115.0 103.4 93.1 74.9 58.0

October 188.9 152.9 117.8 106.2 95.6 77.7 61.1

November 200.2 164.2 127.6 115.3 104.7 86.6 70.1

December 207.6 172.9 137.5 123.9 113.3 96.1 80.4
20 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

aquifer is under unconfined conditions, the estimated 

average rate of gain of water to the alluvial aquifer 

from fall to spring is about 1,500 million gallons per 

day (201 million cubic feet per day). The least gain of 

water from fall to spring since fall 1980 was about 380 

million gallons per day (51 million cubic feet per day) 

from fall 1985 to spring 1986 (fig. 10). From November 1985 through April 1986, only 13.22 inches of precipitation was recorded at Stoneville, as compared to 

the long-term average of 30.40 inches for the same 

6-month period (table 2). The average rainfall at 

Stoneville for this period since 1981 is 29.41 inches. 

The greatest gain of water from fall to spring since 

1981 was about 2,200 million gallons per day (294 million cubic feet per day) from fall 1988 to spring 1989. 

This recovery occurred after the severe drought in summer 1988. The volume of water in the alluvial aquifer 

has decreased from spring to fall every year since 1980 

except during 1989, when 35.96 inches of rainfall was 

recorded at Stoneville from May through October. The 

long-term average rainfall for this 6-month period at 

Stoneville is 21.29 inches. Including the gain in water 

from spring to fall 1989, the estimated average loss of 

water from the alluvial aquifer from spring to fall since 

1980 is about 1,800 million gallons per day (241 million cubic feet per day). The greatest loss of water from 

the aquifer from spring to fall since 1980 was about 

3,200 million gallons per day (428 million cubic feet 

per day) in 1999.

Water Quality

 The quality of water in the alluvial aquifer in 

the Mississippi Delta is generally well suited for irrigation, but less suited for municipal use and some industrial uses. Most of the industrial use is for cooling 

water since the supply is plentiful and the water temperature is stable at about the average annual air temperature (62 to 66 degrees Fahrenheit). The water is 

commonly a hard, calcium bicarbonate type, and usually contains appreciable amounts of manganese. Dissolved iron concentrations are usually greater than 3.0 

milligrams per liter making the water less desirable for 

municipal use. Dissolved solids concentrations in 68 

water samples from the aquifer ranged from 153 to 751 

milligrams per liter (Dalsin, 1978). Along the eastern 

edge of the alluvial plain adjacent to the Bluff Hills, the 

quality of alluvial water is better suited for personal use 

than water from other areas of the Delta. The relation 

between dissolved solids concentration and specific 

conductance is shown in figure 11.

Specific conductance measurements were made 

during August 1992 and August 1998 on water samples 

collected from about 300 irrigation wells pumping 

from the alluvial aquifer in the Mississippi Delta. The 

specific conductance studies were made to determine 

an estimate of the areal distribution of dissolved solids 

in the aquifer, and to help determine the sources and 

relative magnitudes of recharge to the aquifer. The 

results of the 1992 and 1998 specific conductance studies indicate that the specific conductance of the alluvial 

aquifer water increases from east to west with the largest values between 1,400 and 1,600 microsiemens per 

centimeter occurring in west-central and south-central 

Washington County (fig. 12). In parts of this area, the 

alluvial aquifer water may have dissolved solids concentrations greater than the desired level for some 

crops. The smallest values measured were between 

200 and 400 microsiemens per centimeter; these were 

mostly in areas adjacent to the Bluff Hills along the 

eastern edge of the Delta. All irrigation wells sampled 

were outside the Mississippi River levee system, so no 

information is available directly adjacent to the river; 

however, within about 2 miles of the river, most measured water samples had a specific conductance value 

ranging between 600 and 800 microsiemens per centimeter. Throughout most of the Delta, the specific conductance of water from the alluvial aquifer ranged 

between 400 and 800 microsiemens per centimeter. 

Water in streams at high stages in the Delta and 

in streams flowing into the Delta from the Bluff Hills 

naturally contains low concentrations of dissolved solids (specific conductance less than 200 microsiemens 

per centimeter). Water in the Mississippi River at high 

stages typically has a specific conductance of less than 

300 microsiemens per centimeter. Most water samples 

collected from the Sparta and Cockfield aquifers near 

the edge of the Bluff Hills have specific conductance 

values of less than 300 microsiemens per centimeter. 

In the southwestern part of the Delta, the dissolved solids concentrations in the Cockfield and Sparta aquifers 

are greater, and specific conductance values as much as 

3,000 microsiemens per centimeter were reported 

(Taylor and Thomson, 1971) in Washington County in 

water samples from wells screened in the Cockfield 

aquifer.

Prior to extensive development of the Sparta and 

Cockfield aquifers in the Mississippi embayment, the 

Mississippi River alluvial plain was a regional, down-
Hydrogeology of the Mississippi River Alluvial Aquifer 21

Figure 10. Estimated rate of gain and loss of water from the Mississippi 

River alluvial aquifer based on changes in potentiometric surfaces [No data 

available for 1986-87 and 1987-88].

Figure 11. Relation of specific conductance to dissolved solids in the 

Mississippi River alluvial aquifer.

0

500

1,000

1,500

2,000

2,500

3,500

3,000

2,500

2,000

1,500

1,000

500

0

-500

Rate of water gain from fall to spring

Rate of water loss from spring to fall

1981

1982

1983

1984

19851986

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

WATER GAIN,

IN MILLION GALLONS PER DAY

WATER LOSS,

IN MILLION GALLONS PER DAY

1980-81

1981-82

1982-83

1983-84

1984-85

1985-86

1988-89

1989-90

1990-91

1991-92

1992-93

1993-94

1994-95

1995-96

1996-97

1997-98

1998-99

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

0 100 200 300 400 500 600 700 800

DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER
22 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Figure 12. Specific conductance of water in the Mississippi River alluvial aquifer in 1992 and 1998.

River

Mississ pi pi

River

Mississ pi pi

EXPLANATION

1,600

1,400

1,200

1,000

800

600

400

200

Specific conductance,

in microsiemens per centimeter

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

35° 35°

34° 34°

33° 33°

90° 90°

91° 91°

1992 1998
Hydrogeology of the Mississippi River Alluvial Aquifer 23

dip discharge area for these two aquifers. In the Delta, 

the predevelopment potentiometric surfaces of the 

underlying Sparta and Cockfield aquifers were substantially higher than the potentiometric surface of the 

alluvial aquifer. In areas where the underlying aquifers 

were in contact with the alluvial aquifer, water flowed 

upward from the Sparta and Cockfield aquifers into the 

alluvial aquifer. As pumpage increased in the Sparta 

and Cockfield aquifers, the potentiometric surfaces of 

the two aquifers declined, and the rate of discharge into 

the alluvial aquifer decreased to the point where the net 

regional discharge to the alluvial aquifer in the Mississippi Delta probably is very small. Greater specific 

conductance measured in water in the alluvial aquifer 

in the west-central part of the Delta may be the result of 

upward movement of water from the Cockfield aquifer 

during predevelopment time. It is also possible that the 

higher specific conductance water was more extensive 

in the alluvial aquifer during predevelopment time than 

during the 1992 and 1998 study periods. The average 

specific conductance of water in this area of the alluvial 

aquifer may be decreasing as a result of decreasing 

recharge from the underlying aquifers and increasing 

recharge of lower specific conductance water to the 

alluvial aquifer. The two maps shown in figure 12 represent only snapshots in time, but indicate that the average specific conductance of alluvial water in 

Washington County may be decreasing. Additional 

specific conductance studies are needed to verify this 

interpretation.

The influence of Mississippi River water on the 

specific conductance of the water in the alluvial aquifer 

outside the levee system seems to be small; directly 

adjacent to the river, water in the aquifer is probably 

more reflective of the quality of the water in the river at 

high stage. The results of the specific conductance 

studies give support to the assumption that long-term 

net recharge from the Mississippi River to the alluvial 

aquifer is probably small (Boswell and others, 1968); 

however, the possibility exists that water from the Mississippi River is influenced by geochemical processes 

as the water moves through the aquifer resulting in 

increased dissolved solids concentrations. 

The areal distribution of specific conductance 

indicates that the water with a low concentration of dissolved solids that flows into the alluvial aquifer along 

the eastern edge of the alluvial plain has a substantial 

influence on the quality of water in the aquifer. The 

areal distribution of specific conductance also indicates 

that recharge at the eastern edge of the Delta is a major 

source of water for the alluvial aquifer (fig. 12). 

Recharge near the eastern edge of the Delta consists of 

infiltration from precipitation, flow from the YazooYalobusha-Tallahatchie-Yocona-Coldwater River 

drainage system, runoff from the Bluff Hills area, seepage from water-bearing units underlying the loesscapped Bluff Hills onto the surface of the alluvial plain 

and laterally into the alluvial aquifer, and upward flow 

from the two underlying aquifers, predominately from 

the Sparta aquifer. 

Pumpage 

 Pumpage from the alluvial aquifer for irrigation began on a small scale in the Delta in about 1910 

when a few large-capacity wells were installed to provide water for rice production. Interest in rice production soon declined because of an unfavorable market, 

and from 1912 until 1948 few irrigation wells were 

drilled in the Delta. In 1950, a favorable rice market 

returned, and rice acreage increased substantially with 

most of the acreage located adjacent to streams and 

lakes to facilitate surface-water usage. The drought 

during 1951-54 reduced surface-water availability and 

forced many rice growers to drill wells to save their 

crops. From 1950 to 1954, the number of largecapacity alluvial wells in the Delta increased from 35 to 

480 (Harvey, 1956). Few new large-capacity wells 

were installed in the aquifer from 1954 until the early 

1970’s when rice and catfish production increased substantially. Presently (2000) more than 12,000 largecapacity alluvial wells are permitted in the Mississippi 

Delta (J.H. Hoffmann, OLWR, oral commun., 1999).

 More than 98 percent of the pumpage from the 

alluvial aquifer is for agriculture and is concentrated in 

the central part of the Delta. Most of the water used for 

agriculture is for rice and catfish production. Prior to 

1948, fewer than 5,000 acres of rice were planted annually in the Delta. By 1954, rice acreage had increased 

to 79,000 acres and pumpage from the alluvial aquifer 

that year was estimated by Harvey (1956) to be 

334,000 acre-feet (298 million gallons per day). Rice 

acreage remained steady at about 55,000 acres until 

1973. Since 1973, rice acreage has increased and fluctuated widely with the maximum annual acreage 

planted being greater than 300,000 acres. Catfish pond 

acreage has increased from about 18,000 acres in 1977 

to more than 100,000 acres in 1998. In 1983, pumpage 

from the alluvial aquifer was estimated to be 1,100 million gallons per day (Sumner and Wasson, 1990). 
24 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Water application rates for rice and for maintaining 

water levels in catfish ponds can vary widely, depending mainly on rainfall amounts during the rice growing 

season and water management of the catfish ponds. 

Sumner and Wasson (1990) reported water application 

rates between 3.3 and 4.2 feet per year for rice and 5.1 

and 7.3 feet per year for catfish production. Currently, 

application rates for rice and catfish production are 

probably lower than the rates reported by Sumner and 

Wasson (1990) because Delta farmers have made a 

concerted effort to incorporate the most up-to-date 

farm research and technology to conserve water. 

A major need in the Delta is to establish a Deltawide program to accurately estimate ground-water 

pumpage at permitted wells completed in the alluvial 

aquifer. In addition to estimating withdrawals at individual large-capacity wells, an understanding of the 

actual net removal of water from the aquifer is necessary to determine the true stress on the aquifer system. 

Prior studies, such as the Mississippi Delta Management Systems Evaluation Areas (MSEA) project, have 

quantified irrigation runoff from various farm management practices. Some of the excess irrigation runoff 

water likely is returned to the alluvial aquifer as it flows 

through small channels toward larger surface-water 

bodies. As a result, the current estimated pumpage at 

individual irrigation wells may be an overestimate of 

the net removal of water from the alluvial aquifer. A 

report by Heimes and others (1987) indicated that from 

28 to 76 percent of the irrigation pumpage from the 

High Plains aquifer in three counties in Nebraska was 

resupplied to the aquifer. Although the soil characteristics in Nebraska are different from characteristics of 

Delta soils, the possibility of water from agricultural 

pumpage being resupplied to the alluvial aquifer needs 

to be considered.

DESCRIPTION OF GROUND-WATER 

FLOW MODEL

The modular three-dimensional finite-difference 

ground-water flow model (MODFLOW) developed by 

the U.S. Geological Survey (McDonald and Harbaugh, 

1988) was used to simulate flow in the alluvial aquifer 

in the Mississippi Delta. MODFLOW is the most 

widely used model code in the world for simulating 

ground-water flow (Leake, 1997). The modular design 

of the computer code for individual flow packages 

facilitates the use and application of the model for a 

variety of real-world conditions. The model code uses 

a finite-difference method to numerically solve partial 

differential equations that describe ground-water flow. 

The model-generated solutions to differential equations provide simulated heads (water levels) and flow 

budgets for aquifers for specified boundary conditions, 

hydraulic characteristics, and aquifer stresses. The following MODFLOW packages were use to construct 

the model of the Mississippi River alluvial aquifer: 

• BAS - Basic 

• BCF - Block-Center Flow 

• RCH - Recharge 

• RIV - River 

• WEL - Well 

• GHB - General-Head Boundary 

• CHD - Time-Variant Specified-Head 

• SIP - Strongly Implicit Procedure 

• OC - Output Control

Model Grid and Layers

To implement the finite-difference model, the 

Delta was discretized into a rectangular grid of 1-mile 

square cells by row and column. The grid is oriented 

north-south and is divided into 184 rows of 73 columns 

(fig. 13). The part of the grid covering the Delta area 

has 7,101 active cells representing 7,101 square miles. 

The north-south orientation of the grid is consistent 

with the alignment of the lateral flow boundaries, and 

allows cell faces to be generally perpendicular to the 

regional flow directions. Lateral anisotropy ratio of 1:1 

was used for simulation, and unique hydrogeologic 

properties were assigned to the center of each cell, 

defined as a node, by mathematical interpolation from 

observed point values. 

The model is vertically discretized into one 

active layer simulating the sand and gravel of the Mississippi River alluvial aquifer. The areal extent of the 

layer reflects the areal extent of the alluvial aquifer in 

the Mississippi Delta. Because the topstratum (confining unit) extends over most of the Delta, the model 

layer is simulated to allow for both confined and watertable conditions. Under water-table conditions, the 

model recomputes transmissivity values for each cell 

as changes occur in the saturated thickness. The underlying aquifers are represented as a boundary condition. 

Although the underlying aquifers extend beyond the 

Delta area, they are not simulated beyond the areal 

extent of influence on the alluvial aquifer. The vertical 

impedance to flow between the alluvial aquifer and the 
Description of Ground-Water Flow Model 25

Figure 13. Model grid orientation.

River

Mississippi

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

33°

90°

34°

91°

184Rows

1

1

Columns 73
26 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

underlying aquifers is implemented in the model by 

using an areal distribution of leakance values of the 

model layer based on the thickness of the clay beds 

(determined by Arthur, 1994) separating the alluvial 

aquifer from the underlying aquifer. 

Model Boundaries

Proper representation of model boundary conditions is one of the most important functions in the simulation of an aquifer system. Model boundaries are 

selected to simulate the actual hydrologic boundaries 

as realistically as possible. The lateral and upper and 

lower boundaries in a model control the simulated flow 

system by determining the location and the magnitude 

of the water that naturally enters and leaves the model. 

The lateral boundaries of the alluvial aquifer are 

quite distinct, as the lens shape of the Delta results in 

the aquifer having only a western and an eastern lateral 

boundary. The western boundary is the Mississippi 

River, which penetrates nearly the full thickness of the 

aquifer in most areas along the river, and as a result, is 

in almost complete hydraulic connection with the aquifer. Aquifer heads along the western boundary are virtually equal to the stage of the river and are simulated 

in the model as time-variant specified heads. The specified heads are equal to the average stage in the Mississippi River for each stress period simulated in the 

model. The average water level for each month at 

seven gaging stations on the Mississippi River between 

Memphis, Tennessee, and Vicksburg, Mississippi, were 

used to estimate the specified heads at the western 

boundary (table 3). The eastern model boundary coincides with the western edge of the Bluff Hills. Underlying the Bluff Hills are water-bearing deposits 

(predominately the Cockfield and Sparta aquifers) that 

abut the alluvial plain. The eastern model boundary is 

simulated as a general head boundary with the heads at 

the boundary representing the potentiometric surfaces 

of the Cockfield and Sparta aquifers and terrace deposits underlying the Bluff Hills (fig. 14). The potentiometric maps of the Cockfield and Sparta aquifers 

constructed by Darden (1986, 1987) and Oakley and 

Burt (1994a, b) indicate that the water levels in the 

Cockfield and Sparta aquifers at the eastern boundary 

have changed little with time. The heads are assumed 

to be constant throughout the simulation period, but 

can be varied if hydrologic conditions change. The 

hydraulic conductances at the eastern boundary (general head boundary) reflect the hydraulic conductivity 

of the abutting aquifers and terrace deposits, the area of 

contact between the alluvial aquifer and the adjacent 

aquifer, and the length of the flow path. 

The lower boundary represents the Sparta or the 

Cockfield aquifer, whichever of the two aquifers 

directly underlies the alluvial aquifer at a given location. The heads at the lower boundary represent the 

potentiometric surfaces of the Sparta and Cockfield 

aquifers (fig. 15). The heads at the lower boundary are 

unique for each cell. The potentiometric-surface maps 

constructed by Darden (1986, 1987) and Oakley and 

Burt (1994a, b) indicate little change in water levels in 

the underlying aquifers from 1984 to 1988. Consequently, the lower boundary water levels were held 

constant throughout the simulation period. The flow 

interaction between the alluvial aquifer and the lower 

boundary is determined by the head difference between 

the alluvial aquifer and the underlying aquifer and the 

thickness and hydraulic conductivity of the separating 

deposits. The thickness of the lower confining unit of 

the alluvial aquifer is described by Arthur (1994) and is 

shown in figure 2 of the referenced report. 

The upper or surficial boundary of the alluvial 

aquifer is the most complex to conceptually represent 

in the flow model. The upper boundary is simulated by 

using head-dependent-flux cells implemented by the 

river package (fig. 16) and specified flux cells implemented by the recharge package. The large stream 

channels and lake beds in the Delta are incised to varying depths into the sands, silts, and clays of the topstratum, and depending on the head difference between the 

alluvial aquifer and the surface-water body, water is 

either discharged from or recharged to the aquifer. The 

depth to which the major stream channels are incised 

into the topstratum was determined by using the thickness of the upper confining unit reported by Arthur 

(1994) and channel-bottom profiles constructed from 

fathometer surveys made during this study. During 

periods of high river and lake stages, these cells 

recharge water to the aquifer, and conversely, during 

periods of low stages, the head-dependent-flux cells 

represent aquifer discharge points. The magnitude of 

recharge and discharge is dependent upon the magnitude of the head difference between the river or lake 

and the aquifer, the area of the water body, and the 

thickness and hydraulic conductivity of the deposits 

between the surface-water body and the aquifer. Average monthly stages for the head-dependent-flux cells 

were determined by using the information presented in 

the U.S. Army Corps of Engineers annual data publica-
Description of Ground-Water Flow Model 27

River

Mississippi

193

193

193

192

192

192 191

191

190 190

190

190

191

191 190

191

193194

194

194

194

195 194

195

196

195

195

196

198

199

199

202

202

202

200

197

196

193189

184

181

180

178186

188

188194

198 193

192 187

186

182

182

181

181183

183

182

179

179

179

178

178

178

178

181

181

181

181

181

180

179

178

177

176

176

174

173

171

172

171

169

168

169

170

166

165

164 161

160

159

158

158

157 154

154

154

154

155

156

156

155

153

152

151 147

147

147

144

142 140 138

140

142

143

145

146

148

150

149

146

144

139 131

130

128

126

124

122

121 120 120

120 120

120

121 121

120 120

116 109

109 105

105 102

102

100

96 94

95 94

94 93

94 93

92

93 91

93 94 95 96

96 97 97 100 102 112 116

116

112

110 108

106

105 104

102

100 100

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

33°

35°

90°

34°

91°

EXPLANATION

Specified head in feet.

Datum is sea level

120

Figure 14. Specified heads at eastern model boundary.
28 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Figure 15. Potentiometric surface representing specified heads at the lower boundary.

River

Mississippi

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

33°

35°

90°

34°

91°

Bluff Hills

EXPLANATION

Potentiometric contour Shows altitude

at which water level would have stood

in tightly cased wells. Hachures indicate

depression. Interval 10 feet. Datum

is sea level

Sparta potentiometric

surface

Eastern boundary of study area

Cockfield potentiometric

surface

120

180

180

190

190

170

170

160

160

150

150

140

140

140

140

130

130

130

130

130

120

120

120

120

120

110

110

110

110

110

110

100

100

100

100

100

90

90

90

90

90

90

90

80

80

80

80

80

70

70

60

60

50
Description of Ground-Water Flow Model 29

Figure 16. Location of river and lake head-dependent-flux cells (river package) in 

model.

River

Mississippi

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

33°

35°

90°

34°

91°

EXPLANATION

Head-dependent-flux cell
30 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

tion "Stages and Discharges of the Mississippi River 

and Tributaries" published by the Vicksburg and Memphis Districts. 

Specified flux at the upper boundary simulates 

recharge to the alluvial aquifer from precipitation infiltrating through the topstratum. The complex depositional sequence of sands, silts, and clays that make up 

the topstratum form an upper confining unit that has a 

wide range of transmissive and storage characteristics. 

The temporal distribution of recharge from precipitation at the upper boundary is determined by the 

monthly rainfall totals at Stoneville, Mississippi and by 

the thickness of the clay beds in the topstratum. During 

the simulation period, monthly rainfall at Stoneville, 

Mississippi, ranged from 0.45 inch in August 1994 to 

16.93 inches in April 1991 (table 2). The stratigraphic 

thickness of the upper confining unit of the alluvial 

aquifer ranged from less than 10 feet to greater than 

100 feet (as shown in figure 1 in the report by Arthur, 

1994). Because of the complex sequence of sand, silt, 

and clay beds in the topstratum, the thickness of the 

upper confining unit was reduced 10 feet to better represent the aggregate clay thickness. The thickness of 

the alluvial aquifer was increased by 10 feet to represent the interbedded sand in the topstratum and to 

account for the uncertainty of the altitude of the base of 

the upper confining unit.

Calibration Strategy 

Model calibration is the attempt to minimize the 

difference between the model-generated results and the 

corresponding measured data by adjusting model input 

values. Calibration was accomplished for the alluvial 

aquifer model by adjusting model input values of horizontal hydraulic conductivity of the alluvial aquifer, 

vertical hydraulic conductivity of the lower confining 

unit, riverbed conductance of the internal streams and 

lakes, hydraulic conductance of the interface at the 

eastern boundary between the alluvial aquifer and the 

laterally adjacent aquifers, and areal recharge from precipitation and miscellaneous sources until an acceptable error was achieved between the model generated 

heads and the measured water levels in the alluvial 

aquifer. The model input values for each parameter 

were constrained within a range of values reported to 

be representative for the Mississippi River alluvial 

plain. 

The model was constructed and calibrated to 

simulate transient hydrologic conditions in the Delta 

from January 1988 through December 1996. Initial 

heads for transient simulation were determined by 

using the spring 1988 measured water levels. Steadystate construction and calibration was not considered 

acceptable for the Mississippi River alluvial model. 

Even in predevelopment time, water levels in much of 

the alluvial aquifer probably never approached equilibrium conditions. Fluctuating seasonal water levels 

caused by short-term variations in rainfall and large 

stage variations on the Mississippi River and internal 

streams and lakes resulted in the alluvial aquifer never 

approaching definable long-term equilibrium conditions. A seasonal average predevelopment potentiometric surface could not be determined with sufficient 

accuracy to develop and calibrate a predevelopment 

steady-state model. Consequently, development and 

calibration of a steady-state model accurately representing pumping conditions was even less likely. 

The dominant basis for transient model calibration was the water-level measurements made at more 

than 300 alluvial aquifer wells during May and October 

of each year of the simulation period. The seasonal 

change in hydrologic conditions in the Delta and the 

apparent sensitivity of the alluvial aquifer to these 

changes necessitated that time be discretized sufficiently to reflect this temporal sensitivity and to avoid 

significant temporal truncation error. Time was discretized in the model by having each stress period represent hydrologic conditions for 30.4 days (length of an 

average month). Thus, for each of the 108 stress periods in the model, pumpage, areal recharge, and river 

and lake stages were updated in the model. Water levels in the alluvial aquifer for each month of the 9-year 

simulation period were generated by the model. The 

model-generated May and October water levels for 

each year of the simulation (17 stress periods) were 

compared to the corresponding measured water levels. 

The average root-mean-square (RMS) error determined for the calibrated model was 4.55 feet (table 4). 

The RMS error ranged from 3.86 feet for stress period 

22 (fall 1989) to 5.58 feet for stress period 46 (fall 

1991). 

Simulated hydrographs and measured water levels in the alluvial aquifer for wells B012 in Coahoma 

County, N046 in Sunflower County, and A022 in Tallahatchie County (fig. 1) are presented in figure 17. Well 

B012 is less than 3 miles from the Mississippi River, 

and the water level in the well is greatly influenced by 

the stage of the river. Measured water levels in well 

B012 ranged from about 141 to 167 feet above sea 
Description of Ground-Water Flow Model 31

Figure 17. Simulated and measured water levels at selected 

observation wells completed in the Mississippi River alluvial aquifer.

Table 4. Root-mean-square error (in feet) of simulated water levels in the Mississippi River alluvial aquifer

Stress period

No. of wells

RMS error

Fall

88

10

391

4.81

Spr

89

17

387

4.76

Fall

89

22

364

3.86

Spr

90

29

389

4.32

Fall

90

34

340

4.43

Spr

91

41

354

4.31

Fall

91

46

356

5.58

Spr

92

53

383

4.15

Fall

92

58

425

4.23

Spr

93

65

439

4.42

Fall

93

70

446

3.88

Spr

94

77

467

4.81

Fall

94

82

449

4.71

Spr

95

89

461

5.05

Fall

95

94

467

4.83

Spr

96

101

478

4.75

Fall

96

106

451

4.56

85

90

95

100

105

110

130

140

150

160

170

1 11 21 31 41 51 61 71 81 91 101

Simulated

Water-Level

Hydrograph

Measured Water Level

Well B012

Coahoma county

STRESS PERIOD

WATER LEVEL, IN FEET ABOVE SEA LEVEL

Well N046

Sunflower county

Well A022

Tallahatchie county

Simulated

Water-Level

Hydrograph

Measured Water Level

Simulated

Water-Level

Hydrograph

Measured Water Level

140

145

150

155

160

165

170

175
32 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

level, due mainly to the fluctuation of the stage of the 

Mississippi River. The simulated hydrograph for well 

B012 is a reasonable representation of the measured 

water levels. Well N046 is located in south-central 

Sunflower County in the heavily-pumped central Delta 

area. Water levels in well N046 fluctuate much less 

than water levels in well B012. The seasonal waterlevel change in well N046 was less than 5 feet during 

the simulation period. The water-level trend in well 

N046 is downward about 1/2 foot per year, and the simulated hydrograph at this well presents a close representation of this trend. Well A022 at the eastern edge 

of the Delta in northern Tallahatchie County is within 2 

miles of the Bluff Hills. Measured water levels in well 

A022 fluctuate about 5 feet seasonally, and during the 

simulation period the water-level trend has been 

slightly downward. The simulated hydrograph for well 

A022 is a reasonable representation of the measured 

water levels for this well. 

The OLWR measures water levels in the alluvial 

aquifer on a section that runs from west to east from 

southern Washington County, across Humphreys 

County, and into Holmes County. Measured water levels for spring and fall 1994 and simulated water levels 

along this section are shown in figure 18. In 1994 the 

Mississippi River stage varied about 34 feet from 

spring to fall. The 1994 water levels were chosen to 

show the error of model-generated water levels along 

the section and to show the influence of the Mississippi 

River stage on water levels at the western edge of the 

Delta. Model results indicate that the western boundary of the alluvial model is properly simulated at the 

western edge of this section. 

The measured and simulated 1996 spring and fall 

potentiometric surfaces for the alluvial aquifer are 

shown in figure 19. The 1996 potentiometric surfaces 

are shown to illustrate the error between the measured 

and simulated surfaces; 1996 is the last year of the transient simulation and should show the cumulative 

results of any gross error. Both spring and fall simulated surfaces seem to be reasonable representations of 

the measured potentiometric surfaces.

Hydraulic Properties

Hydraulic properties that determine the transmissive, storage, leakage, and recharge capabilities of 

the alluvial aquifer were tested through a range of values and boundary conditions to generate the best-fit 

distribution between observed and simulated water levels. The tested values for each hydraulic property were 

constrained within a range of values considered reasonable as determined from examination of information 

presented in previous studies. A summary of the 

hydraulic values used in the calibrated model is presented in table 5. 

The calibration procedure resulted in a horizontal hydraulic conductivity of 425 feet per day for the 

alluvial aquifer, which is in close agreement with the 

400 feet per day reported by Sumner and Wasson 

(1990) for their model of the aquifer. Since the aquifer 

acts as both a confined and unconfined aquifer depending upon the altitude of the water level in relation to the 

top of the aquifer, the transmissivity of the aquifer 

depends upon the saturated thickness as well as the 

total thickness of the aquifer. The thickness of the aquifer used in this calibrated model is based on the thickness of the coarse sand and gravel reported by Arthur 

and Strom (1996). The areal distribution of simulated 

transmissivity for October 1996 is shown in figure 20. 

The simulated transmissivity is between 30,000 and 

50,000 feet squared per day over most of the Delta. 

Because the aquifer is under confined conditions at 

times, mostly adjacent to the Bluff Hills and near the 

Mississippi River when the river is at a high stage, a 

storage coefficient and a specific yield were required 

for model simulation. A storage coefficient of 0.016 

and a specific yield of 0.32 were used in the calibrated 

model, as compared to values of 0.001 and 0.30, 

respectively, used by Sumner and Wasson (1990). 

Figure 18. Simulated and measured west-to-east water-level 

sections across the Delta for spring and fall 1994.

50

60

70

80

90

100

110

120

130

11 14 17 20 23 26 29 32 35 38 41 44 47 50 53 56 59

COLUMN NUMBER

WATER LEVEL, IN FEET ABOVE SEA LEVEL

50

60

70

80

90

100

110

120

11 14 17 20 23 26 29 32 35 38 41 44 47 50 53 56 59

Fall 1994

Spring 1994

WEST EAST

Simulated Water Levels

Simulated Water Levels

Measured Water Levels

Measured Water Levels
Description of Ground-Water Flow Model 33

Figure 19. Observed and simulated spring and fall 1996 potentiometric surfaces of the Mississippi River alluvial aquifer.

River

Mississ pi pi

River

Mississ pi pi

80

80

EXPLANATION

Simulated water-level contour

Shows altitude at which water level

would have stood in tightly cased wells.

Hachures indicate water-level depression.

Interval 10 feet. Datum is sea level

Observed water-level contour

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

35° 35°

34° 34°

33° 33°

90° 90°

91° 91°

Spring 1996 Fall 1996

190 180

170

150

140

130

130

120

120

120

110

110

100

100

90

90

90

90

70

80

80

80

80

80

70

80

190

180

170

160

160

150

150

140

140

130

130

120

120

120

110

110

100

100

100

100

90

90

90

90

90

90

80

80

80

80 80

80

190

180

170

160

150

140

140

130

120

120

110

110

110

110

100

100

100

100

90

90

90

90

90

80

80

80

80

190

180

170

170

160

150

150

140

140

130

130

130

120

120

120

110

110

110

110

100

100

100

100

90

90

90

90

90

80

80

80

80
34 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

The average annual areal recharge for the calibrated model was 2.6 inches per year. Areal recharge 

was varied monthly depending upon the magnitude of 

the corresponding monthly rainfall totals at Stoneville, 

Mississippi. 

The riverbed properties used in the river package 

of the calibrated model have a substantial effect on the 

amount of water that flows between the alluvial aquifer 

and surface-water bodies simulated in the model. The 

model incorporates 557 cells of the 7,101 active cells 

into the river package to represent the interaction of the 

internal rivers and lakes with the alluvial aquifer. Channel fathometer data collected during this study, and 

river channel cross sections from the U.S. Army Corps 

of Engineers and from previous U.S. Geological Survey studies were used to help determine channel incisement, channel widths, and channel bottom consistency 

(Harbeck and others, 1961). For stream channels that 

did not penetrate the entire thickness of the topstratum, 

the calibrated riverbed hydraulic conductivity is 0.005 

foot per day. For surface-water bodies that penetrated 

the entire thickness of the topstratum, a streambed sediment depositional thickness of 5 feet between the 

flowline of the channel and the aquifer was assumed. 

The hydraulic conductivity of the 5-foot thickness of 

riverbed sediments in the calibrated model was determined to be 0.15 foot per day. 

The eastern boundary of the model represents the 

water-bearing units (Cockfield Formation, Sparta 

Sand, and terrace deposits) adjacent to the alluvial 

aquifer and underlying the Bluff Hills at the eastern 

edge of the Delta. The hydraulic conductance at the 

eastern boundary was computed using the thickness of 

the alluvial aquifer at the general head boundary cell, a 

unit width, a unit flow length, and an average hydraulic 

conductivity of the adjacent sediments determined during the calibration process. The average horizontal 

hydraulic conductivity of the sediments at the eastern 

boundary used to compute the conductance at the general head boundary in the calibrated model is 26 feet 

per day. 

The clay beds directly underlying the alluvial 

aquifer represent the lower confining unit. The thickness of the clay beds of the lower confining unit ranges 

Table 5. Summary of the principal hydraulic values used in the calibrated model

Alluvial aquifer properties

River bed properties

Eastern boundary property

Lower confining unit property

Average areal recharge

Horizontal hydraulic conductivity

(feet per day)

425

Topstratum vertical conductivity

(foot per day)

0.005

Average horizontal hydraulic conductivity

(feet per day)

26

Vertical hydraulic conductivity

(foot per day)

0.000001

Storage coefficient

0.016

Bottom sediment thickness

(feet)

5.0

Specific yield

0.32

Bottom sediment vertical

conductivity

(foot per day)

0.15

2.6 inches per year (1988-96)
Description of Ground-Water Flow Model 35

Figure 20. Simulated transmissivity of the Mississippi River alluvial aquifer for fall 1996.

River

Mississippi

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

33°

35°

90°

34°

91°

Bluff Hills

EXPLANATION

70,000

90,000

Simulated transmissivity,

in feet squared per day

50,000

30,000

10,000

5,000
36 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

from less than 10 feet to greater than 500 feet (Arthur, 

1994). As the result of the calibration process, the vertical hydraulic conductivity of the underlying clay beds 

was determined to be 0.000001 foot per day.

Model Sensitivity

To determine the sensitivity of the model simulation results to changes in hydraulic properties, a 

selected property was varied independently from 0.2 to 

5 times its calibrated value. Because water levels are 

the most easily quantified aquifer characteristic, the 

RMS error of simulated water levels compared to measured water levels was used to assess the sensitivity of 

the model to a particular change in value. The sensitivity of model results to independent changes in areal 

recharge, horizontal hydraulic conductivity of the alluvial aquifer and water-bearing units at the eastern 

boundary, and to changes in vertical hydraulic conductivity of river bed sediments is shown in figure 21. 

Sensitivity analysis indicates that of the four calibration parameters shown in figure 21, the model is most 

sensitive to changes in areal recharge to the alluvial 

aquifer. The model is least sensitive to changes in 

riverbed vertical hydraulic conductivity. The model 

was more sensitive to an increase in areal recharge than 

to a corresponding decrease in areal recharge. This 

characteristic held true with changes in general head 

boundary horizontal hydraulic conductivity.

GROUND-WATER FLOW ANALYSIS

The calibrated model simulates the Mississippi 

River alluvial aquifer flow system in the Mississippi 

Delta from January 1988 through December 1996. The 

flow analysis represents the results of 108 monthly 

stress periods. The analysis includes a brief discussion 

of the magnitude and distribution of pumpage from the 

alluvial aquifer and a detailed discussion of the sources 

and magnitudes of recharge to and natural discharge 

from the alluvial aquifer. The analysis concludes with 

a flow budget showing sources and magnitudes of flow 

into and out of the alluvial aquifer flow system.

Pumpage

Pumpage rate estimates for the alluvial aquifer 

used in the calibrated model were totaled monthly from 

January 1988 through December 1996. Pumpage rates 

for agriculture in the Delta were determined by the 

YMD. Pumpage rates for nonagricultural uses were 

provided by the OLWR. Almost all of the pumpage 

from the alluvial aquifer during the model simulation 

period was for agriculture and for catfish production. 

The annual pumpage rates used in the calibrated model 

are shown in figure 22. The greatest annual pumpage 

rate from the alluvial aquifer used in the calibrated 

model was about 1,620 million gallons per day during 

1995. The smallest annual pumpage rate used in the 

calibrated model was about 620 million gallons per day 

during 1989. During 1989, rainfall in the Delta was 

about 15 inches greater than normal, and irrigation 

demand was at a minimum. The average annual pumpage rate from the alluvial aquifer used in the calibrated 

model for the simulation period was about 1,270 million gallons per day.

Most of the pumpage from the alluvial aquifer is 

concentrated in the central part of the Delta in Bolivar, 

Sunflower, Leflore, and Humphreys Counties. Bolivar 

County had the greatest annual pumpage rate from the 

alluvial aquifer of the 19 counties in the Delta during 

the simulation period with about 20 percent of the total 

annual pumpage. Bolivar County, covering 892 square 

miles, is the largest county in the Delta, and has the 

greatest rice acreage. Sunflower County has the second greatest annual pumpage rate accounting for about 

16 percent of the total annual pumpage in the Delta. 

Areas of concentrated pumpage where withdrawals 

were greater than 0.5 million gallons per day per square 

mile during 1996 are shown in figure 23.

Recharge and Discharge

Recharge to and natural discharge from the alluvial aquifer occur at lateral and vertical aquifer boundaries in the Mississippi Delta. The magnitude of the 

recharge and discharge is highly variable and is dependent upon hydrologic conditions both inside and outside of the Delta area.

Lateral

Lateral recharge and discharge are important 

flow components in the alluvial aquifer flow system. 

The lens shape of the Delta with the Bluff Hills to the 

east and the Mississippi River to the west limits lateral 

flow at the perimeter of the Delta to these two boundaries. 
Ground-Water Flow Analysis 37

Figure 21. Model sensitivity to independent changes in selected 

calibration parameters.

Figure 22. Annual pumpage rate from the Mississippi River alluvial aquifer used 

in the calibrated model.

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

1988 1989 1990 1991 1992 1993 1994 1995 1996

PUMPAGE,

IN MILLION GALLONS PER DAY

Average = 1,270

0.1 1.0 10.0 0.2 0.5 2.0 5.0

4.0

5.0

6.0

7.0

8.0

Alluvial Aquifer Horizontal

Hydraulic Conductivity

General Head Boundary

Horizontal Hydraulic

Conductivity (east boundary)

Areal Recharge

River Bed Vertical

Hydraulic Conductivity

MULTIPLIER OF CALIBRATED VALUE

ROOT-MEAN-SQUARE ERROR, IN FEET
38 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Figure 23. Areas of concentrated pumpage from the Mississippi River alluvial aquifer in 

1996.

River

Mississippi

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

COAHOMA QUITMAN

33°

35°

90°

34°

91°

Bluff Hills

EXPLANATION

Areas with pumpage greater than

0.5 million gallons per day per

square mile
Ground-Water Flow Analysis 39

Bluff Hills

The water-bearing units underlying the Bluff 

Hills (Cockfield Formation, Sparta Sand, and terrace 

deposits) have water levels that historically are greater 

than water levels in the alluvial aquifer at adjacent 

locations, resulting in water moving laterally from 

these units into the alluvial aquifer. Higher water levels 

in these units are the result of the relatively high altitude of their recharge areas as compared to the altitude 

of the alluvial plain. In the calibrated model, the water 

levels underlying the Bluff Hills were held constant for 

the simulation period (fig. 14). The potentiometric 

maps made by Darden (1986, 1987) and by Oakley and 

Burt (1994a, b) indicate that water levels in these units 

adjacent to the edge of the Delta changed very little 

from 1984 to 1988. No large pumping centers are withdrawing water from these units where these units 

directly underlie the Bluff Hills. Along the entire 

length of the eastern edge of the Delta, the alluvial 

aquifer receives recharge from these sediments. The 

magnitude of the recharge is dependent upon the head 

difference between the adjacent aquifer and the alluvial 

aquifer, the hydraulic conductivity of the adjacent sediments, and the area of lateral contact. The north-tosouth distribution of lateral recharge is relatively uniform, but less recharge occurs where the Yazoo Clay 

directly underlies the Bluff Hills (fig. 3), and greater 

recharge occurs from Vicksburg to about 20 miles north 

of Vicksburg due to large head differences. In the 

southern one-third of the Delta, a substantial part of the 

lateral recharge probably is intercepted by the Yazoo 

River due to the proximity of the river to the edge of the 

Bluff Hills. During the 9-year simulation period, the 

lateral recharge to the alluvial aquifer along the eastern 

edge of the Delta averaged about 108 cubic feet per 

second and ranged annually from 102 to 114 cubic feet 

per second. The result of long-term lateral recharge of 

water with low dissolved solids concentrations at the 

eastern edge of the Delta is indicated by the low specific conductance values of alluvial aquifer water in 

this area (fig. 12). Lateral recharge at the eastern edge 

of the Delta from water-bearing units laterally adjacent 

to the alluvial aquifer should be relatively stable for the 

foreseeable future if rainfall remains about normal and 

no large pumping centers are established in the waterbearing units underlying the Bluff Hills or in the alluvial aquifer adjacent to the Bluff Hills. Also, any large 

channel constructed in the Delta parallel to and adjacent to the foot of the Bluff Hills and deep enough to 

penetrate the saturated zone may intercept some water 

that would flow westward to the central part of alluvial 

aquifer.

Mississippi River 

The Mississippi River on the western boundary 

of the Delta is a lateral recharge/discharge area for the 

alluvial aquifer. The great depth of the river channel 

(as much as 100 feet deep) allows most of the area 

along the river to be in almost complete hydraulic connection with the aquifer. Depending upon the stage of 

the Mississippi River, the aquifer is either recharged by 

water from the river or the aquifer is discharging water 

to the river. Long-term net flow between the Mississippi River and the alluvial aquifer is probably small, 

but model simulations indicate that during periods 

when the river is at an extremely high stage or 

extremely low stage, the local recharge rate to and discharge rate from the alluvial aquifer in most areas along 

the river is large. 

During the 9-year simulation period of the calibrated model, the resultant net flow between the Mississippi River and the alluvial aquifer was small 

considering the large amounts of water that flowed 

locally back and forth between the aquifer and the river. 

The greatest average annual recharge to the alluvial 

aquifer from the Mississippi River during the simulation period was about 590 cubic feet per second during 

1993. The average stage of the Mississippi River in 

1993 was the highest during the simulation period. The 

average stage of the river at the Greenville, Mississippi, 

gage (about midway between Vicksburg and Memphis) 

during 1993 was 38.6 feet. The greatest average annual 

discharge from the alluvial aquifer to the Mississippi 

River during the simulation period was about 894 cubic 

feet per second during 1988. The average stage of the 

Mississippi River in 1988 was the lowest during the 

simulation period. The average stage of the river at the 

Greenville, Mississippi, gage during 1988 was 19.9 

feet. The relation between the average annual stage of 

the Mississippi River at the Greenville gage and simulated recharge to the alluvial aquifer from the Mississippi River is shown in figure 24. The relation 

indicates that on an annual basis, net recharge to the 

alluvial aquifer from the Mississippi River starts when 

the average annual stage at the Greenville gage reaches 

about 30 feet. The datum of the Greenville gage is 

74.92 feet above sea level. Simulation results indicate 

that a significant long-term change in the average stage 

of the Mississippi River will have corresponding 

changes in water levels in the alluvial aquifer adjacent 
40 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

to the river, and will affect recharge to the aquifer from 

the river and discharge from the aquifer to the river.

Model-simulated and measured water levels 

indicate that in most areas within about 6 miles of the 

Mississippi River, water levels in the alluvial aquifer 

are influenced by stages in the river (fig. 18). At high 

river stage, water is stored in the aquifer and will eventually discharge to the river during the next period of 

low river stage. If the timing was accurate, this water 

could be made available for many purposes within the 

Delta such as stream augmentation and other water 

uses. Although, currently (2000) the development of 

this source of water is probably not feasible, this 

resource is available for future consideration.

Vertical

Vertical recharge and discharge are important 

flow system components in the alluvial aquifer 

throughout the entire Delta, but more so in the central 

part of the Delta where lateral recharge and discharge 

have less effect on the flow system. The vertical 

recharge and discharge are the result of precipitation, 

flow to and from surface-water bodies, and flow to and 

from underlying aquifers and the alluvial aquifer.

Precipitation

Model simulation indicates that precipitation is 

the most important recharge component in the alluvial 

aquifer flow system in the Delta. The long-term average annual precipitation in the Delta is about 52 inches 

(table 2). Most of the rainfall runs off and flows into 

streams and lakes, evaporates, or transpires, but a small 

part of the annual precipitation infiltrates the ground 

and recharges the alluvial aquifer. The amount of infiltrated water depends on the amount and distribution of 

rainfall, the time of year, and the permeability of the 

topstratum. The amount of rainfall that recharges the 

alluvial aquifer is uncertain, and areal recharge has 

been reported and estimated over a wide range of values from about zero to about 5 percent of the annual 

precipitation. It is apparent that more study needs to be 

undertaken in the Delta to obtain a better understanding 

of the magnitude and distribution of recharge from 

rainfall. 

The model calibration process indicated the 

importance of precipitation in providing recharge to the 

alluvial aquifer. Model simulation indicates that from 

1988 to 1996, about 86 percent of the recharge to the 

alluvial aquifer was from areal recharge. During this 

period, model simulation indicated that an average of 

about 2.6 inches per year of precipitation recharged the 

alluvial aquifer. The simulated recharge is about 5 percent of the average yearly precipitation. This value is 

substantially greater than the 0.5 inch per year areal 

recharge simulated for the alluvial aquifer from 1981 to 

1983 by Sumner and Wasson (1990). The areal 

recharge used in the model was distributed spatially for 

each monthly stress period based on the thickness of 

the upper confining unit and the monthly rainfall total 

reported at Stoneville, Miss. Using this method to distribute areal recharge in the model, areal recharge 

ranged from a low of about 2.1 inches during 1988 to a 

high of about 3.1 inches during 1989. During the 

9-year simulation period, the average annual rainfall at 

Stoneville was 53.6 inches. The annual rainfall at 

Stoneville was 40.77 inches during 1988 and 67.34 

inches during 1989. The rainfall in 1988 and 1989 

were the least and greatest, respectively, at Stoneville 

during the simulation period. Most of the areal 

recharge in the Delta occurs during the winter and 

spring months when precipitation is the greatest and 

evaporation and transpiration are at a minimum. The 

yearly areal recharge values used in the calibrated 

model during the simulation period are shown in 

figure 25.

Surface Water

Rivers, streams, lakes, and other surface-water 

bodies within the Delta are sources of vertical recharge 

and also discharge areas for the alluvial aquifer. Most 

of the recharge from the surface-water bodies occurs 

during winter and spring when streamflow is abundant, 

and most discharge from the alluvial aquifer occurs in 

late summer and fall as base flow to streams. 

During the simulation period, net recharge to the 

alluvial aquifer from surface-water bodies averaged 

about 113 cubic feet per second. The greatest average 

recharge was about 511 cubic feet per second during 

1989 when surface-water bodies were at higher than 

normal stages because of the large amount of rainfall 

during the year. In 1988, 1992, and 1995, the alluvial 

aquifer had a net loss of water to the surface-water 

bodies and discharged an average of about 21, 262, and 

51 cubic feet per second of water, respectively, to the 

surface-water bodies. The rate of simulated flow of 

water between the surface-water bodies within the 

Delta and the alluvial aquifer during the model simulation period is shown in figure 26. 
Ground-Water Flow Analysis 41

Figure 26. Simulated recharge to the Mississippi River alluvial aquifer from 

surface-water bodies within the Delta.

Figure 25. Simulated areal recharge to the Mississippi River alluvial

aquifer, 1988-96.

Figure 24. Relation between average annual stage of the Mississippi River at Greenville,

Mississippi, and simulated recharge to the Mississippi River alluvial aquifer from the

Mississippi River.

10

15

20

25

30

35

40

45

-1000 -800 -600 -400 -200 0 200 400 600 800

AVERAGE ANNUAL STAGE AT

GREENVILLE, IN FEET

RECHARGE TO ALLUVIAL AQUIFER, IN CUBIC FEET PER SECOND

1988 1989 1990 1991 1992 1993 1994 1995 1996

1.0

1.5

0.0

0.5

2.0

2.5

3.0

3.5

AREAL RE

CHAR

GE,

IN INCHES PER YEAR

-400

-300

-200

-100

0

100

200

300

400

500

600

1988 1989 1990 1991 1992 1993 1994 1995 1996

RECHARGE,

IN CUBIC FEET PER SECOND
42 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

Several of the major streams in the Delta have a 

large percentage of their drainage basins outside the 

Delta, and a large percentage of their discharges are 

regulated outflows from flood control reservoirs 

located outside the Delta. The Sunflower River is the 

only major river in the Delta with unregulated flow and 

a drainage basin contained totally within the Delta.

Underlying Aquifers

Model simulation results indicate that from 1988 

to 1996, net flow between the alluvial aquifer and the 

underlying aquifers (Cockfield and Sparta aquifers) 

was small. The average annual net flow for the simulation period for the entire Delta was about 0.2 cubic 

foot per second downward from the alluvial aquifer 

into the underlying aquifers. The small net interchange 

of flow at the lower boundary is attributed to small 

water-level differences between the alluvial aquifer 

and the underlying units over most of the Delta, and the 

low permeability of sediments separating the alluvial 

aquifer and the underlying aquifers over a large part of 

the Delta. The area where flow is upward (recharge) to 

the alluvial aquifer is predominantly in the eastern part 

of the Delta. Over much of the eastern part of the 

Delta, the Sparta and Cockfield aquifers directly underlie the alluvium and have water levels greater than 

water levels in the alluvial aquifer (fig. 27). 

Prior to development of the Cockfield and Sparta 

aquifers, water levels in the two aquifers were much 

greater (as much as 50 feet above land surface in the 

southern part of the Delta (Stephenson and others, 

1928)) than today (2000). As a result, much of the Mississippi River alluvial plain was a predevelopment 

regional discharge area for the Sparta and Cockfield 

aquifers, and net vertical flow was into the alluvial 

aquifer (recharge to the alluvial aquifer) from the 

underlying units. With the development of the Cockfield and Sparta aquifers, water levels in the underlying 

units decreased, resulting in a decrease in vertical flow 

and the reversal of net flow between the alluvial aquifer 

and the underlying units. The current (2000) net discharge from the alluvial aquifer into the underlying 

units will probably increase with increased development and decreased heads in the Sparta and Cockfield 

aquifers, but the increased net discharge from the alluvial aquifer probably will not have a substantial effect 

on the availability of water in the alluvial aquifer 

(Boswell and others, 1968).

Flow Budget

The year-to-year flow budget for the Mississippi 

River alluvial aquifer is extremely variable due to the 

complexity of the flow system and the fluctuation in 

agricultural pumpage. Simulation results indicate that 

the flow budget for any one month or for any one year 

can vary greatly from the previous month or year. 

Large volumes of water can be recharged to the alluvial 

aquifer or discharged from the aquifer depending on 

the stage of the Mississippi River and the stages of the 

streams and lakes within the Delta. Model results indicate that areal recharge from precipitation is the greatest source of water for the alluvial aquifer, but varies 

temporally and spatially. Pumpage for agriculture varies monthly and yearly depending upon the amount and 

distribution of rainfall. Pumpage is also dependent on 

the type and annual acreage of crops planted. During 

the simulation period, lateral recharge from the waterbearing units underlying the Bluff Hills was the most 

consistent source of water for the alluvial aquifer. If 

water levels in the water-bearing units underlying the 

Bluff Hills remain near present levels, then lateral 

recharge at the eastern edge of the Delta probably will 

not fluctuate substantially. The small net interchange 

of vertical flow between the alluvial aquifer and the 

underlying probably will not fluctuate greatly from the 

amount simulated unless water levels in the underlying 

aquifers are severely drawn down through time. Model 

results indicate that water from aquifer storage fluctuates yearly depending on availability of water from 

recharge sources and the magnitude and distribution of 

pumpage. Analysis of volumetric change of water in 

the alluvial aquifer indicates that a large amount of 

water is available from aquifer storage. 

In order to present a flow budget that would most 

nearly represent long-term flow conditions in the alluvial aquifer flow system, the average annual flow rates 

for the 9-year simulation period (1988-96) is presented 

in figure 28. These rates represent a window in time 

relative to historic and possible future hydrologic conditions in the Delta. 

Model results indicate that pumpage was the 

greatest average flow component in the alluvial aquifer 

flow budget during the simulation period. The average 

withdrawal of water from the aquifer was 1,965 cubic 

feet per second (1,270 million gallons per day). Areal 

recharge from precipitation was the second largest 

average flow component with 1,365 cubic feet per second of water entering the aquifer through the topstratum. Water supplied from aquifer storage was the third 
Ground-Water Flow Analysis 43

Figure 27. Area where water levels in underlying aquifers are greater than water levels in the 

Mississippi River alluvial aquifer.

River

Mississippi

0 10

10 20 KILOMETERS

20 MILES

0

DESOTO

WARREN

YAZOO

HOLMES

GRENADA

CARROLL

TATE

PANOLA

SHARKEY

ISSAQUENA

SUNFLOWER

WASHINGTON

HUMPHREYS

TALLAHATCHIE

LEFLORE

BOLIVAR

TUNICA

QUITMAN

COAHOMA

33°

35°

90°

34°

91°

Bluff Hills

EXPLANATION

Area where water levels in

underlying aquifers are greater

than water levels in alluvial aquifer
44 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

largest average flow component with 404 cubic feet per 

second. Recharge from the rivers and lakes within the 

Delta and lateral recharge from the Bluff Hills were 

almost equal with 113 and 108 cubic feet per second, 

respectively. An average of only 0.19 cubic foot per 

second was vertically discharged from the alluvial 

aquifer into the underlying aquifers during the simulation period.

MODEL LIMITATIONS

In order to effectively utilize flow model results, 

an understanding of the model limitations is essential. 

The flow model simulates only movement of freshwater through the alluvial aquifer flow system. The model 

is limited by simplification of the conceptual model of 

a complex flow system, by space and time discretization effects, by the availability of measurements for 

estimating spatial variation in hydraulic properties, by 

limitations in the accuracy of land-surface and waterlevel altitudes, by the availability of water-level measurements in the underlying aquifers, and perhaps most 

importantly, by limitations in the accuracy of pumpage 

estimates. 

Surface discretization of the study area into a 

rectangular grid of 1-mile square cells and vertical discretization of the alluvial aquifer into one layer forced 

an averaging of hydraulic properties. Each active cell 

represents a homogeneous block of some volumetric 

average of the aquifer medium. Each cell has a unique 

layer thickness determined by estimating aquifer thickness using drillers and geophysical logs from several 

thousand wells in the Delta, but a lack of hydraulic 

property measurements forced using uniform hydraulic 

conductivity, specific yield, and storage coefficient values. Discretization of time to 30.4 days for each stress 

period required using monthly average pumpage rates 

and monthly average stages for the Mississippi River 

and the streams and lakes within the Delta. Water levels for the aquifers underlying the alluvial aquifer and 

underlying the Bluff Hills were assumed constant for 

the 9-year simulation period, since published potentiometric maps in the Delta area indicated little change in 

water levels. If pumpage from the Cockfield and 

Sparta aquifers is substantially increased, then water 

levels will decrease and modification to the underlying 

and eastern boundary heads will be necessary for the 

model to be representative. 

Simplification of the model does not invalidate 

the model results, but does mean that model results 

should be interpreted at scales larger than the simulated 

elemental volume of homogeneous aquifer material. If 

model projections are made, special care should be 

taken in estimating the stages of the Mississippi River 

and the internal streams and lakes. Areal recharge during the calibration period was the largest recharge component, so proper model response is dependent upon 

reasonable estimates of areal recharge. Model simulation results and volumetric analysis indicate that areal 

recharge is a function of rainfall, so climatic cycles 

should be considered when making long-term model 

projections. One should also consider reviewing 

underlying and adjacent aquifer heads to allow for 

proper representation for long-term projections. 

Because pumpage (the largest flow component during 

the simulation period) will probably increase, consideration should be given to establishing a technically 

sound, intensive water-use data-collection program in 

the Delta to complement the long-term intensive waterlevel measuring effort that has been ongoing in the 

Delta since 1980. 

The longer the validation period of the model, 

the greater the probability of generating reliable model 

results. Model projections probably should not exceed 

the length of the calibration period, and the model 

should be periodically validated with updated data. 

Keeping the model as current as possible with the latest 

hydrologic information increases the length of the validation period and enhances the model’s capability to 

generate realistic projection results. Because the alluvial aquifer flow system is sensitive to hydrologic variables that are directly influenced by climatic and 

economic conditions both within and outside the Delta, 

one must cautiously select projected hydraulic parameter values for model input, and carefully analyze the 

Figure 28. Average flow budget of the Mississippi River 

alluvial aquifer for the simulation period (1988-96).

-2,500

-2,000

-1,500

-1,000

-500

0

500

1,000

1,500

2,000

Precipitation

Miss. River

Bluff Hills

Underlying

Aquifers

Storage

Pumpage

Rivers & Lakes

FLOW INTO THE ALLUVIAL AQUIFER,

IN CUBIC FEET PER SECOND
Summary 45

model results before formulating long-term waterresource management decisions.

SUMMARY

The Mississippi River alluvial aquifer underlies 

a 7,000-square-mile area of the Mississippi River alluvial plain in northwestern Mississippi locally known as 

the Delta. Farming and farm-related businesses are the 

major sources of income in the Delta. The Delta is a 

lens-shaped area with little topographic relief that 

slopes about 0.5 foot per mile from the MississippiTennessee border to near Vicksburg, Mississippi. The 

climate in the Delta is humid subtropical, and the area 

receives an average of about 52 inches of precipitation 

annually. The alluvial aquifer is the most heavily 

pumped aquifer in Mississippi and supplies most of the 

water used for agriculture and industry in the Delta. In 

1995, the U.S. Geological Survey began a study to better understand the hydrogeology of the alluvial aquifer 

and to construct a computer model of the alluvial aquifer flow system. 

The Mississippi River alluvial aquifer flow system is dynamic and complex in regard to changes in 

recharge and natural discharge in response to fluctuations in hydrologic and climatic conditions. The aquifer is recharged by the Mississippi River and internal 

rivers and lakes when at their high stages, by precipitation, by underlying aquifers, and by laterally adjacent 

aquifers underlying the Bluff Hills. The alluvial aquifer discharges water to the Mississippi River and internal rivers and lakes when surface-water stages fall 

below the water level in the alluvial aquifer. 

The alluvial aquifer consists of sand and gravel 

deposits of Quaternary age. The average thickness of 

the alluvium in the Delta is about 135 feet. The various 

combinations of sand, silts, and clays that occur near 

the surface of the alluvial plain make up a low permeability topstratum. The topstratum averages about 25 

feet in thickness and overlies most of the Delta. The 

sand and gravel of the alluvium that make up the alluvial aquifer averages about 110 feet in thickness. 

Tertiary-age deposits underlie the alluvial aquifer and 

dip to the west and southwest toward the axis of the 

Mississippi embayment. The Cockfield and Sparta 

aquifers underlie the alluvial aquifer and are two of the 

major drinking-water supply aquifers in Mississippi. 

The Mississippi River and the Bluff Hills are the 

lateral flow boundaries of the alluvial aquifer. The topstratum and the deposits that separate the alluvial aquifer from the underlying water-bearing units are the top 

and bottom flow boundaries of the alluvial aquifer. 

Hydraulic conductivity of the alluvial aquifer, determined from aquifer tests, ranges from 130 to 400 feet 

per day, and storage coefficients range from 0.0003 to 

0.016. Laboratory tests of aquifer material indicate that 

specific yield ranges from 0.27 to 0.38. Prior to extensive development, water levels in the alluvial aquifer 

were between 10 and 20 feet below land surface. During fall 1999, the average water level in the aquifer was 

about 24.6 feet below land surface. During fall 1999, 

the alluvial aquifer had the least volume of water since 

fall 1980 with an average of about 20,690 acre-feet per 

square mile. 

Water in the alluvial aquifer is generally well 

suited for irrigation, but less suited for municipal use 

and some industrial uses. The water is commonly a 

hard, calcium bicarbonate type with appreciable 

amounts of manganese and dissolved iron. 

A modular three-dimensional finite-difference 

ground-water flow model of the alluvial aquifer was 

constructed using the U.S. Geological Survey model 

code MODFLOW. The 1-square-mile cell-size model 

was calibrated and verified using spring and fall 

observed water levels from January 1988 through 

December 1996. The average root-mean-square error 

of model-generated water levels compared to measured 

water levels was 4.55 feet. The values of the calibration-derived, aquifer hydraulic parameters are as follows: hydraulic conductivity, 425 feet per day; specific 

yield, 0.32; and storage coefficient, 0.016; all assumed 

uniform throughout the aquifer. 

The model showed that under average rainfall 

conditions, areal recharge from precipitation is the 

greatest source of recharge to the aquifer, with a simulated annual average of about 2.6 inches per year from 

1988 through 1996. Model results indicated that an 

average of 404 cubic feet per second of water was 

removed from aquifer storage during the simulation 

period. The model indicated that the next greatest 

sources of water were the internal streams and lakes, 

and the aquifers underlying the Bluff Hills with an 

average recharge rate during the simulation period of 

113 and 108 cubic feet per second, respectively. 

Although the model indicated that the alluvial aquifer 

received large volumes of water from the Mississippi 

River when the river was at high stage, the model also 

showed that the alluvial aquifer discharged large volumes of water to the river when the river was at low 

stage, resulting in a small net flow for the simulation 
46 Hydrogeology, Model Description, and Flow Analysis of the Mississippi River Alluvial Aquifer in Northwestern Mississippi

period between the river and the aquifer. Model results 

indicated that the net flow between the alluvial aquifer 

and the underlying Cockfield and Sparta aquifers also 

was small. The average annual pumpage rate for the 

simulation period was about 1,270 million gallons per 

day with most of the withdrawal for rice and catfish 

production in the central part of the Delta. 

Understanding the conceptual model is essential 

to effectively utilize the model results. The computer 

model is a simplification of a complex flow system, 

therefore, potential users need to be aware of and 

understand the limitations of the model. The model’s 

capability to generate realistic long-term projection 

results would be enhanced by lengthening the validation period by periodically updating the model with 

current hydrologic information.
Selected References 47

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