With water flowing through portions of 8 counties (Jefferson, St. Clair, Shelby, Bibb, Perry, Dallas, and small portions of Tuscaloosa and Chilton,) the watershed covers an area of 1,870 square miles. For its length of 190 miles, the Cahaba River remains the longest free-flowing river remaining in Alabama.
The first 100 miles passes through Trussville, Leeds, Irondale, Birmingham, Mountain Brook, Helena, West Blocton and Centreville. This section is referred to as the Upper Cahaba and is the most mountainous, full of steep banks, high bluffs and rocky shoals.
The Cahaba River originates on the southern slope of Cahaba Mountain northeast of Birmingham, at an elevation of approximately 1,200 feet. The river has an average slope of 15 feet per mile for the first 25-30 miles, then drops to a slope of 2.5 feet per mile for 44 miles, and finally flattens to a slope of .6 feet per mile to its mouth. (Corps) Elevation in the watershed ranges from 1,100 feet in Shelby County to 100 feet at the confluence with the Alabama River. (Source)
The bed of the river in the upper portions consists of limestone, sandstone, shale and coal. Of the many trees that line the river, areas with moderate to little steepness present willow, birch, sycamore, oak, poplar, hickory, maple, and ash. Steeper slopes are dominated by pine. (Project)
The Fall Line is a geographic feature which divides Alabama into two distinct physical regions, the uplands and lowlands. It is considered the most significant physical feature in Alabama affecting the distribution of plants and animals. It represents the zone of contact between the hard rocks of the Appalachians and the softer sediments of the Coastal plain. Many species are limited to either above or below the fall line. (Project)
Below the Fall Line, the Lower Cahaba transitions so dramatically that it might be mistaken for a different river entirely. The river slows, widens, and deepens and sandbars replace rocky shoals. In places, the river spreads 200 feet wide. (Keith) The upland trees change to lowland trees and baldcypress, eastern red cedar and Spansih moss-draped willows become more common. The riverbottom, once rock, is now the eroded evidence of the Appalachians, layers of sand, clay and gravel sediment deposited over many years and soft. (Keith)
The Cahaba rests in two physiographic regions of the state. The upper basin lies in the Alabama Valley and Ridge, and the lower basin lies in the East Gulf Coastal Plain.
The majority of the watershed, about 84%, lies within the Valley and Ridge physiographic province. Valley and Ridge rock types include shale, siltstone, sandstone, limestone, and dolostone. The Valley and Ridge formations are characterized by ridges trending northeast-southwest and made of sandstone and chert, with valleys developed on limestone and shale. These upper basin rock types are known for their lack of porosity and rapid absorption of rainfall. In addition, much of the Cahaba Ridges and Valleys are residentially and commercially developed adding impervious surfaces (areas of land that allow no percolation or absorption of rainfall into the ground) to an area already limited in its ability to slowly convey water to streams. These characteristics contribute to rapidly increasing water levels due to runoff during heavy rains, and also increase the risks of polluted run-off from populated areas. (Corps &Project&State of Rivers)
The remainder of the watershed, about 14%, lies below the Fall Line in the East Gulf Coastal Plain Province (Often referred to simply as the Coastal Plain). The East Gulf Coastal Plain is characterized by gently rolling hills, sharp ridges, prairies, and broad alluvial flood plains. Underlying rock is of sedimentary origin and consists of sand, gravel, porous limestone, chalk, marl, and clay. The East Gulf Coastal Plain comprises over 50 percent of the land area in Alabama and lends its character to many waterways in South Alabama. The Lower Cahaba basin lies in the Fall Line Hills and (followed by the) Black Belt districts of the East Gulf Coastal Plain region.
Vegetation of the Cahaba River system is characterized by two plant community types. North of the Fall Line in the Appalachian Plateaus and Valley and Ridge regions the oak-pine forest is predominant whereas south of the Fall Line the loblolly-shortleaf pine forest predominates. Wetland vegetation is common in bottom lands along the main stem and consists of an oak-gum-cypress forest. (Corps)
Meandering oxbow bends and lakes scattered with bald cypress trees are common in the Lower Cahaba sections.