Help for hitchhikers and hosts – Marvel Slab removal

- Photo Credit: Francesca Gross
Marble Slab Removal
- Photo Credit: Cahaba Wildlife Refuge

The great diversity of freshwater mussel species in Alabama is something we can all be proud of (more mussels than any other state!). However, though diversity in the state remains high, many species have been lost as a result of destruction of their habitat by human alterations to river systems.

Current threats to mussels are tied to their biology – hitchhiking activity and their method of breathing and eating through a tube (siphon). If the host fish cannot migrate upstream, the hitchhiking mussel larvae (glochidia) can’t catch a ride. Even though mussels can live a long time, some areas have not had any new mussel hitch a ride for 70 years. The other big threat is cloudy water. Mussels eat by filtering bacteria, protozoans, algae, and other organic matter out of the water. They draw water into their body through their incurrent siphon (sucking water in), remove food and oxygen with their gills, and then expel the filtered water through their excurrent siphon (blowing waste out). Food particles are carried to the mussel’s mouth by tiny hair-like cilia located on their gills. As filter feeders, mussels serve as natural water filters and act as indicators of water-quality conditions. Filtering cloudy water is hard on gills and guts.

The cloudy water comes from nonpoint source pollution, dredging, and channeling of rivers and streams. Many species have been lost from human alterations to river systems. Most species require water that flows over clean, stable sand and gravel. Water pollution and construction of dams on our major rivers, such as the Tennessee, Coosa, Black Warrior, and Alabama, have eliminated many species from our state and even driven some to extinction. The mussels can’t migrate, eat, or breathe well if the river bottom is destabilized.

In 2004 a low-head dam called the Marvel Slab was removed from the Cahaba River to aid migration of fish and mussels. The removal was an astounding success with species migration immediately restored between the previously separated sections of the river. For over 40 years, Marvel Slab bisected the Cahaba River in northern Bibb County, Alabama. Originally constructed as a ford for trucks hauling coal and timber, the slab functioned ecologically like a dam. Marvel Slab altered habitat and flows in one of the most sensitive and biologically important reaches of the Cahaba River. Stretching 220 feet across and six feet tall, the concrete structure created a vertical drop of water which was a physical barrier to fish migration, scoured the riverbed of stone and gravel, and was a hazard to boaters. The removal was a cooperative effort between The Nature Conservancy, and Alabama State Lands Division of the Department of Conservation and Natural Resources.

-Francesca Gross