STUDY UNIT DESCRIPTION
The Mobile River Basin encompasses 44,000 square miles (mi²) in Georgia, Tennessee, Alabama, and Mississippi and comprises the Alabama and Tombigbee Rivers that meet to form the Mobile River. The Mobile River flows south into Mobile Bay, which discharges into the Gulf of Mexico. Approximately 71 percent of the study unit lies within Alabama, 14 percent in Mississippi, 13 percent in Georgia, and 2 percent in Tennessee. An estimated 4.9 million people lived in the study unit in 1990. The largest population centers in the study unit (populations greater than 100,000) include Birmingham, Mobile, Montgomery, and Tuscaloosa.
U.S. Geological Survey photo
Silviculture is the largest industry in Alabama.
The major land use in the Mobile River Basin is forested land, which covers approximately 69 percent of the study unit. The remaining land uses include agricultural land (18 percent), urban (2 percent), and other uses such as wetlands, streams, lakes, and reservoirs (11 percent). Agricultural activities include row crops such as cotton, corn, hay, and soybeans, as well as aquaculture, and poultry and cattle production. Major industries include silviculture, chemical, pulp and paper, iron and steel, coal, textile manufacturing, and hydroelectric-power.
Location of major subbasins, major rivers and streams, and major cities
within the Mobile River Basin study unit.
The Mobile River basin is the sixth largest basin in the Nation and the fourth largest in terms of streamflow. The mean annual streamflow of the Mobile River is about 64,000 cubic feet per second (ft³/s). The Alabama River Basin, which drains 22,800 mi², annually contributes 33,600 ft³/s of streamflow to the Mobile River, whereas the Tombigbee River Basin (20,200 mi²) annually contributes 30,200 ft³/s. Major tributaries to the Alabama River are the Coosa (10,161 mi²), Tallapoosa (4,675 mi²), and Cahaba (1,825 mi²) River Basins, which have a combined mean annual streamflow of 24,000 ft³/s, or 71 percent of the mean annual streamflow from the Alabama River Basin. The principal tributary to the Tombigbee River is the Black Warrior River Basin (6,276 mi²), which has a mean annual streamflow of 9,800 ft³/s and is about 32 percent of the mean annual streamflow from the Tombigbee River Basin. Flow in the Alabama and Tombigbee Rivers is generally regulated by upstream reservoirs, flood-control and navigational locks and dams, and hydroelectric plants. In 1985, the Tennessee-Tombigbee Waterway was opened. It joined the Tennessee and Tombigbee Rivers to provide a new trade route connecting Mobile and the Gulf Coast with the mid-section of the Nation. Reservoirs on the Alabama and Tombigbee Rivers and their tributaries are also used for drinking water and recreational activities such as fishing, swimming, and boating.
Fishing is one of the recreational uses of reservoirs
in the Mobile River Basin. (Photo courtesy of the
Alabama bureau of Tourism and Travel.)
Landforms in the Mobile River Basin study unit range from rugged mountains to coastal lowlands and are included in five physiographic provinces. The Blue Ridge and Piedmont, located in the northeast corner of the study unit, are characterized by igneous and metamorphic rocks and encompass 16 percent of the basin. To the east, the Valley and Ridge consists of a series of parallel ridges and valleys, all having a northeast trend. It is underlain by sandstone, shale, limestone, and dolomite rocks. The Valley and Ridge includes 16 percent of the basin. The Appalachian Plateaus, which encompasses 12 percent of the basin, is dominated by relatively flat plateaus and is underlain largely by nearly flat-lying sandstones, limestones, and shales. The remaining 56 percent of the study unit is included in the Coastal Plain, which is primarily underlain by unconsolidated or poorly consolidated sands, gravels, clays, and limestones. Elevations in the study unit range from near sea level along the Mobile River in the Coastal Plain to greater than 3,000 feet above sea level in the Blue Ridge Mountains of Georgia. Average annual precipitation ranges from 50 to greater than 60 inches, with higher amounts in the mountainous regions; average annual runoff ranges from 18 to 30 inches. Average annual air temperatures vary from about 60 degrees Fahrenheit (° F) in the north to about 70° F in the south.
The Blue Ridge and Piedmont are underlain by a fractured, crystalline-rock aquifer characterized by little or no pore spaces and openings and the overlying unconsolidated, weathered rock remnants and soil. The Valley and Ridge and Appalachian Plateaus are underlain by fractured-rock systems in the well-consolidated sandstones and interconnected fractured rock systems in the cavernous limestone and dolomite rocks which become enlarged as water flows through them. Caves and sinkholes in the limestone rocks increase the susceptibility of ground water to contamination from surface water. The Coastal Plain is primarily underlain by sand and gravel aquifer systems which are important sources for drinking water. Water in the Coastal Plain is produced from shallow ground water and from deep ground water which is confined by impermeable layers of chalk and clay deposits. Based on 1995 water-use data, approximately 1,340 million gallons per day (Mgal/d) were withdrawn from surface and ground-water sources for public, commercial, and domestic supplies and for industrial and agricultural uses in the Mobile River Basin. Surface water is the principal source, accounting for about 76 percent of the water withdrawn (1,020 Mgal/d); about 42 percent is used for public-water supplies (430 Mgal/d) and 44 percent is used for industrial purposes (453 Mgal/d). Approximately 76 percent of the ground-water withdrawals were used for public and domestic drinking-water supplies (198 and 49 Mgal/d, respectively). More than 80 percent of the public-water supplies were from surface-water sources in the non-Coastal Plain Provinces, whereas nearly 60 percent of the public-water supplies were from ground-water sources in the Coastal Plain.
MAJOR WATER-QUALITY ISSUES IN THE MOBILE RIVER BASIN
Assessing water quality in the Mobile River Basin is important for the protection and efficient use of water and other aquatic resources. The Mobile River Basin NAWQA study is intended to increase the scientific understanding of surface- and ground-water quality within the basin and the factors that influence water quality.
The blue shiner is listed as a threatened species in Alabama. (Photograph
courtesy of Malcolm Pierson)
This NAWQA study also provides information needed by water-resource managers to implement effective water-quality management actions and evaluate long-term changes in water quality. During the planning process, the following water-quality issues that currently face water-resource managers in the Mobile River Basin were prioritized:
Sedimentation and increased concentrations of sediment in streams and reservoirs from erosion related to urbanization, agriculture, and silviculture.
Pesticides and toxics compounds in surface and ground water from agricultural, industrial, and urban activities.
Degradation of instream and riparian habitats and the subsequent impacts upon native species and water quality.
Effects of acid-mine drainage on surface-water quality.
Bacterial contamination of surface and ground waters.
The quality of surface- and ground-water discharges to Mobile Bay and how they relate to the health of the estuarine environment and its fisheries.