Water Resources and the 

Mississippi Embayment Project

GEOLOGICAL SURVEY 

CIRCULAR 471
Water Resources and the Mississippi 

Embayment Project

By E. M. Gushing

GEOLOGICAL SURVEY CIRCULAR 471

Washington 

7963
United States Department of the Interior

STEWART L. UDALL, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on opp/iccrtion to the U.S. Geological Survey, Washington 25, 0. C.
CONTENTS

Page Page

Why appraise our nation's water What are the ground-water resources?- 3

resources ?----__------»_»-_-_-._» 1 What are the surface -water re -

The Mississippi embayment project _.. 1 sources?._.......__........_...... 5

What is the Mississippi embayment ? .. 2 How much water is in the embayment?-- 6

Status of the geologic studies _.__..._. 3 Conclusions ......._._......._..._... 8

ILLUSTRATIONS

Page 

Figure 1. Map showing area of study____........___..............................................__.........._._. IV

2. Generalized geologic map of the Mississippi embayment ._............_........ 2

3. Map showing location of geologic sections .___.._.........._.._...._.......... 3

4. Generalized east-west geologic sections ..__..._....._._............__......- 4

5. Generalized north-south geologic section- _...___._....._......_......._..._.- 5

6. Map showing areas where water in Cretaceous deposits probably contains less

than 1,000 parts per million of dissolved solids ..__._._..................._. 6

7. Map showing area underlain by deposits of Tertiary age .__........__.__...._.. 7

8. Map showing area underlain by deposits of Quaternary age _...._........_..___. 8

III
IV

KANSAS

-38*

Figure 1. Map showing area of study.
Water Resources and the Mississippi Embayment Project

By E. M. Gushing

WHY APPRAISE OUR NATION'S WATER RESOURCES?

Every estimate indicates that this country's need for water in the next several 

decades will grow enormously. Increases in 

population and in the per capita use of water, 

in industrial expansion, and in water demands 

for agriculture are some of the factors that 

are causing the total use of water to approach 

or surpass the capacity of existing water - 

resources development. The total use is not 

approaching the supply available. New facilities, however, must be developed or existing 

ones expanded. If this development or expansion is not to be haphazard and wasteful, 

sound appraisals of our water resources 

must be available for planning purposes.

Sound appraisals consider all facets of the 

water resource and its potential for development. The hydrologic system must be defined, and how it operates must be determined 

before a wise management plan for the system can be devised.

THE MISSISSIPPI EMBAYMENT PROJECT

The Mississippi embayment (fig. 1) as defined for this project comprises about 100,000 

square miles of a geologic province known as 

the Gulf Coastal Plain. From its apex in 

southern Illinois the region fans out southward to about the 32d parallel of north latitude and includes parts of nine States Alabama, Arkansas, Illinois, Kentucky, Louisiana, 

Mississippi, Missouri, Tennessee, and Texas. 

The larger cities within the region include 

Tuscaloosa, Ala., Jackson, Miss., Monroe and 

Shreveport, La., Texarkana, Ark. and Tex., 

Little Rock and North Little Rock, Ark., and 

Memphis, Tenn.

The economy of the region is basically agricultural, but industry has become increasingly important since World War II. The diversification and mechanization of agriculture 

have resulted in a surplus of manpower, most 

of which is being absorbed by the expansion 

of industry. Further industrial expansion 

probably will occur because of available manpower, natural resources, and economical 

transportation. Increasing demands on available water supplies will be made,and the future economy of the embayment is largely 

dependent upon sound utilization and management of the region's water resources.

For several years the need for an appraisal of the water resources of the Mississippi embayment has been recognized by 

people associated with the development of the 

region. Most of the water-resources investigations in the embayment have been made in 

cooperation with State, county, and municipal 

agencies, and have been restricted to those 

parts of the aquifers (water-bearing units) 

that lie beneath the local area. Because these 

investigations have been local, the results 

have lacked the benefits that would have ac - 

crued from an understanding of the regional 

hydrologic system that extends over several 

States.

The present study, begun in August 1957, is 

a part of the Federal program of the U.S. 

Geological Survey and is aimed specifically 

toward appraising the water resources of the 

region, and determining the influence of the 

regional geologic structure and stratigraphy 

on the movement, availability, and quality of 

the water. Generally, more is known about 

the surface-water resource of the region than 

is known about the ground-water resource, 

and because of this, most of the present study
WATER RESOURCES AND THE MISSISSIPPI EMBAYMENT PROJECT

relates to the occurrence, availability, and 

quality of ground water.

WHAT IS THE MISSISSIPPI EMBAYMENT?

The Mississippi embayment is a syncline 

plunging gently to the south, the axis of which 

roughly follows the present course of the 

Mississippi River. During the geologic past 

the region was periodically occupied by an 

arm of the sea, and sediments, some more 

than 125 million years old, have been deposited on the Paleozoic rocks. Units ranging in 

age from Cretaceous to Quaternary (from 125 

million years ago to the present time) crop 

out within the area of study (fig. 2). These 

units of gravel, sand, silt, clay, lignite, 

"marl," chalk, and limestone range in thickness from zero at the outcrop of Paleozoic 

rocks to several thousand feet at the axis of

95'

the syncline. In most of the region the dip of 

the beds generally is toward the axis of the 

syncline except where it is influenced locally 

by structural features. The dip of the beds 

southward, toward the Gulf, is slight in the 

northern part of the region, but becomes 

greater in the southern part.

The relative position of the sediments of 

Cretaceous, Tertiary, and Quaternary age 

that overlie the Paleozoic rocks are shown 

by five generalized geologic sections. (See 

figs. 3, 4, and 5.) These sections also show 

the relative thicknesses of the sediments and 

the progressive thickening of the sediments 

southward toward the Gulf.

The influence of structural features on the 

dip of the beds can be seen on section D-D1 

(fig. 4). The high near the west end of the 

section is caused by the Sabine uplift and the

69* 88" 87*

Figure 2. Generalized geologic map of the Mississippi embayment.
WHAT ARE THE GROUND-WATER RESOURCES?

37*

Figure 3. Map showing location of geologic sections.

one immediately east of the axis of the syncline is caused by the Jackson dome. The 

high or the change in the direction of dip near 

the west end of section C-CJ is caused primarily by the change in the direction of strike 

of the beds in that area.

STATUS OF THE GEOLOGIC STUDIES

One of the first phases of the embayment 

study, the definition of the generalized structure and stratigraphy of the region, has been 

completed. With the structure and stratigraphy defined, correlation of the water-bearing 

units $an now be made, and the influence of 

the structure and stratigraphy on the movement, availability, and quality of the water in 

these units can be determined.

Correlations of the water-bearing units of 

Cretaceous age are well under way. Within

the limits of available data, these studies will 

show (a) the areal extent of each aquifer, (b) 

thickness of the sand section and its permeability and storage coefficient, (c) piezometric 

surface of the water, (d) areas where flowing 

wells might be developed, (e) area of water 

use, (f) area of potential use, and (g) the 

change in quality and temperature of the 

water within the unit. Upon completing the 

study and report of the Cretaceous aquifers, 

similar studies of the Tertiary and Quaternary aquifers will be made.

WHAT ARE THE GROUND-WATER RESOURCES?

Deposition of sediments in the embayment 

during the geologic past was such that deposits of sand now form regional aquifers, 

many of which contain large quantities of 

fresh water available for development and 

use. Many of these aquifers extend across
WATER RESOURCES AND THE MISSISSIPPI EMBAYMENT PROJECT

4000'

2000'

4000'

Tex JLc

Quaternary

LalM iss MissJAU

6000'

8000'

100

1

200 MILES

Figure 4. Generalized east-west geologic sections.
WHAT ARE THE SURFACE-WATER RESOURCES?

Miss j Ark Ark JMo j Tenn JKy! Mo ! Ky 'ill

! / Quaternary | j i i i i

I / II ' ' I . H -irr C

Tertiary

MEAN SEA 

LEVEL

2000'

6000'

8000'

200 MILES

Figure 5. Generalized north-south geologic section.

State and topographic boundaries and underlie watersheds that at the land surface are 

independent units. The study of the groundwater resources is concerned primarily with 

the Cretaceous, Tertiary, and Quaternary 

units, which contain the principal waterbearing beds in the region.

Cretaceous sediments underlie most of the 

embayment and contain at least five waterbearing units of regional importance. Although water in these units over a large part 

of the region is salty or highly mineralized, 

in many large areas (fig. 6) the water in the 

Cretaceous deposits may be fresh (less than 

1,000 parts per million of dissolved solids). 

Analysis of electric logs indicates that in 

some places relatively fresh water occurs in 

Cretaceous sands to a depth of more than 

3,000 feet below sea level. Locally, within 

the fresh-water areas, sand containing highly 

mineralized water overlies sand containing 

fresh or less mineralized water.

Tertiary sediments crop out or underlie 

about 75 percent of the region (fig. 7), and 

they too contain at least five aquifers of regional extent. Almost anywhere within this 

area, except near its periphery, at least one 

aquifer of Tertiary age contains fresh water.

As in the Cretaceous sediments, Tertiary 

sand deposits that contain water of poor quality may overlie sand beds containing water of 

a better quality.

The most productive water-bearing unit in 

the embayment is the Mississippi River alluvium of Quaternary age. It is from this alluvium that most of the water for irrigation is 

pumped. The distribution of Quaternary deposits is shown on figure 8.

WHAT ARE THE SURFACE-WATER RESOURCES?

Coincident with the study of the groundwater resources, streamflow records are being analyzed to determine the expected low 

flow of streams in the embayment on a frequency basis. The magnitude, duration, and 

frequency of these low flows determine how 

much storage is needed to operate a planned 

water facility. The largest expected discharge of a stream is of interest in regard to 

flood protection, but it is the magnitude of the 

low flow that determines a stream's suitability for a water-use project.

Low-flow frequency studies have been made 

for the river basins in the Mississippi
WATER RESOURCES AND THE MISSISSIPPI EMBAYMENT PROJECT

Dissolved-solids content of woter probably 

less than 1,000 ports per million

Figure 6. Map showing areas where water in Cretaceous deposits probably contains less than 1,000 parts per

million of dissolved solids.

embayment part of Mississippi and Alabama. 

Included in these studies were streams in 

the basins of the Tombigbee-Black Warrior 

River, the Pascagoula River, the Pearl River, 

the Tennessee River, the Hatchie River, the 

Wolf River, the Yazoo River, the Big Black 

River, and Bayou Pierre. Analyses will be 

made also of low flows of streams in the remaining States of the embayment.

HOW MUCH WATER IS IN THE EMBAYMENT?

Based on available streamflow records, the 

quantity of water which originates within the 

region and which leaves it as streamflow 

during an average year is about 90 million 

acre-feet. This amount is approximately the 

perennial yield of the region under the existing climatic and hydrologic conditions. If 

the average annual rainfall is assumed to be 

about 50 inches, or about 4.2 feet, then about

1.4 feet of the rainfall leaves the region as 

streamflow, and about 2.8 feet is returned to 

the atmosphere within the region primarily 

by evaporation and transpiration.

In addition to the 90 million acre-feet, about 

four times this amount enters the embayment 

from adjacent areas and flows through it as 

streamflow during an average year. Thus 

about 450 million acre-feet of water leaves 

the embayment as streamflow during an average year, a quantity sufficient to cover the 

entire region to a depth of about 7 feet.

The amount of water in the ground-water 

reservoirs in the embayment is sufficient to 

cover the entire region to a depth of about 80 

feet. This estimate was made as follows:

The electric logs at about 100 locations 

within the embayment were examined, and an 

estimate was made of the total thickness of
HOW MUCH WATER IS IN THE EMBAYMENT?

the saturated sand 'containing fresh water at 

each location. Using these representative 

thicknesses, the total volume of fresh-water 

sand within the embayment was calculated. 

This volume was divided by the surface area, 

and the average thickness of fresh-water 

sand was computed as about 400 feet. If the 

average specific yield of the sand is assumed 

to be 0.2; that is, if 0.2 foot of water can be 

obtained from each vertical foot of sand, or 

0.2 cubic foot of water from each cubic foot 

of sand, then an amount of water sufficient to 

cover the entire embayment to a depth of 80 

feet is available in the subsurface units.

The quantity of water pumped in the Memphis area for municipal and industrial use is 

about 150 mgd (million gallons per day). If 

the 450 million acre-feet of water that leaves 

the region as streamflow could be stored and 

utilized by Memphis, then 1 year's streamflow

would be sufficient to supply the Memphis 

area at the 150-mgd rate for more than 2,600 

years. The amount in the ground-water reservoirs of the embayment would be sufficient 

to supply Memphis at the 150-mgd rate for at 

least 30,000 years.

These estimates of the amount of water 

within the embayment do not indicate the 

proportion of water that can be recovered 

economically, or how large a supply from 

surface water or ground water can be developed at a particular site . within the region. 

Without storage facilities, the size of a 

surface-water supply probably would be governed by the low flow of the stream, and the 

low flow may be only a few percent of the 

average discharge of the stream. Also, the 

low flow of a stream is derived primarily 

from ground water so that ground-water

37'

Area underlain by Tertiary deposits

Figure 7. Map showing area underlain by deposits of Tertiary age.
WATER RESOURCES AND THE MISSISSIPPI EMBAYMENT PROJECT

Area underlain by Quaternary deposits

Figure 8. Map showing area underlain by deposits of Quaternary age

development in the area might reduce or 

eliminate the low flow.

The size of a ground-water supply at a 

particular site would be governed largely by 

the thickness of the saturated sand units. 

Near the outcrop of the Paleozoic rocks the 

sand units generally are thin; the thickest 

sections are usually at or near the axis of the 

syncline (fig. 4). Other considerations that 

would govern the size of a ground-water supply would be (a) its proximity to streams, (b) 

the amount of recharge that may occur, (c) 

the effect of vertical leakage that may occur 

as the water levels are lowered, (d) the effect 

of up-dip movement of mineralized water or 

the vertical leakage of mineralized water 

from overlying or underlying sand units, and 

(e) the effect of subsidence caused by lowering water levels.

CONCLUSIONS

Proper development, use, and conservation 

of the water resources of the embayment can 

be achieved only with an understanding of the 

hydrologic system and the response of the 

system to climate and to change caused by 

water-supply development and use. The Mississippi embayment project is designed to 

define this regional geologic environment. 

Information and data developed as a result of 

the investigation should prove invaluable in 

predicting regional effects of the increasing 

use, reuse, import and export of water. In 

addition, with the hydrologic system being 

known, management analysis should be possible by the development of analog models 

that will provide solutions to specific regional and local water problems.

V. & GOVERNMENT PRINTING OFFICE : 1963 O - 678900