The Cahaba River is perhaps the most famous major river in Alabama. The Cahaba’s main stem is 190 miles long and remains the longest free-flowing river remaining in Alabama. This absence of hydrological modifications along the main stem has aided in maintaining the unique aquatic biodiversity of the Cahaba. With that said, the Cahaba Basin also is unique in the number of aquatic species that are imperiled or declining in number.
When describing the Cahaba, it is easy to separate two distinct personalities of this River (the Upper and Lower) separated by the Fall Line, a geographic feature that divides Alabama into two distinct physical regions, the uplands and the lowlands. The Fall Line is considered the most significant physical feature in Alabama affecting the distribution of plants and animals and also represents the zone of contact between the hard rocks of the Appalachians and the softer sediments of the Coastal Plain.
The upper portion of the Cahaba flows through much of the urban and suburban Birmingham area (Trussville, Leeds, Irondale, Birmingham, Mountain Brook, and Helena). This portion of the river north of the Fall Line is in the Valley and Ridge region that has predominately oak-pine forest with rock types of shale, siltstone, sandstone, limestone, and dolostone. These rock types are known for their lack of porosity and rapid absorption of rainfall, which along with increasing urbanization and impervious surfaces in the Upper Cahaba, lead to rapidly increasing water levels due to runoff during heavy rains.
The lower portion of the Cahaba below the Fall line is in the East Gulf Coastal Plain Province that is characterized by gently rolling hills, sharp ridges, prairies, and broad alluvial flood plains. Underlying rock is predominately sand, gravel, porous limestone, chalk, marl, and clay. It is in this area that the Cahaba transitions dramatically – the river slows, widens, and deepens, with sandbars replacing rocky shoals. In some areas, the river is 200 feet wide. The trees change to bald cypress, eastern red cedar, and Spanish-moss draped willows. Meandering oxbow bends and lakes scattered with bald cypress are common in this section of the Cahaba.
NOTABLE TRIBUTARIES
SHADES CREEK
At approximately 55 miles long, Shades Creek is a small to medium-sized stream that runs from the southwest corner of Jefferson County into Bibb County. Due to the extreme residential expansion in recent years that has resulted in increased stormwater runoff and nonpoint source pollution, Jefferson County developed The Shades Creek Watershed Management Plan to address many of these issues. If you are interested in learning more about Shades Creek, the Birmingham Historical Society published a book in 2019 called Shades Creek – Flowing Through Time that is a must-read.
DAMS ON THE CAHABA
In the late 1800’s, Congress attempted to alter the Cahaba into a more navigation-friendly river, even allocating funds between 1880 and 1893; however, their efforts failed and a final feasibility study on the Cahaba in 1909 would forever pronounce the river as unfit for further improvement. Fortunately for the Cahaba, this resulted in preserving the future integrity of the river.
Although there are not impoundments along the Cahaba, there is one small, often overlooked dam on the main stem of the Cahaba on the west side of the Highway 280 bridge. Built in 1891, this low-head dam measures 15 feet high and backs up the waters of the Cahaba just enough to allow the city of Birmingham to withdraw a steady supply of water from this section.
WILDLIFE
The Cahaba River’s biodiversity is world-class – The World Wildlife Fund and The Nature Conservancy recognized the Cahaba as one of only 8 Hotspots of Biodiversity in the world! The Cahaba River Basin is home to 135 fish species including the Cahaba shiner, crystal darter, freckled darter, goldline darter, and frecklebelly madtom (see more below). Popular sportfish species also occur on the Cahaba such as largemouth mass, spotted bass, bluegill, longear sunfish, crappie, and chained pickerel.
Another biotic attribute of note is the Cahaba Basin’s aquatic snail diversity. There are 35 aquatic snail species documented within the Cahaba, including 10 that occur nowhere else in the world, and 9 that are only found in the Cahaba and Coosa River Basins. The Cahaba is also home to 50 freshwater mussel species.
Lastly, it would be remiss to not mention one of Alabama’s true floral treasures, of which the River is its namesake: the beautiful Cahaba Lily. The Cahaba lily, also known as the shoal lily, is a type of spiderlily known for its striking, large white blossoms that sprout in clusters on tall stalks. The bulbs from which the plants sprout are wedged between rocks. The Cahaba lily occurs in Alabama, Georgia, and South Carolina and requires a very specialized habitat of swift-flowing rocky shoals in full-sun. The lilies bloom for only 5 weeks starting in early to mid-May. One of the most expansive Cahaba Lily populations occurs in the U.S. Fish and Wildlife Services’ Cahaba River National Wildlife Refuge.
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