Changes in Water Conditions and Sedimentation Rates Associated
with Construction of the Mobile Bay Causeway
Click here for fieldwork pictures

M. L. Fearn, D. W. Haywick, and J. M. Sanders

Recently, blockage of normal flow at the head of Mobile Bay by the Mobile Bay Causeway has become a highly publicized controversial issue. Some environmentalists believe that restoration of pre-causeway hydrology is so important to the health of the bay that they advocate removing almost all of the blockages and building a new elevated roadway. Others believe that stable ecosystems have evolved to fit the current situation, therefore, any changes may cause more harm than good. In 1980, the U.S. Army Corps of Engineers identified replacement of the Chocalata portion of the causeway with bridges as a possible mitigation project to offset environmental damage related to improvements to Mobile Harbor. They acknowledged that the causeway blocked normal circulation patterns in Chocalata Bay. The actual impact that the causeway has had on the lower delta is difficult to quantify because there is a paucity of data on the ecological conditions before the causeway's construction in the 1920s. Mitigation efforts should never proceed without a firm understanding of past history and future consequences.

The Mobile Bay Causeway (Highway 90/98 from its emergence from the Bankhead Tunnel to the high ground in Baldwin County) is a 11.2 km (7 mile) roadway, built in the 1920s to provide a transportation route between Mobile and Baldwin Counties. Prior to its construction, the only way to cross from one county to the other was by taking a boat across the bay or by driving over 80 km (50 miles) to the north of Mobile to cross at Jackson, above the confluence of the Alabama and Tombigbee Rivers. Although the causeway has bridges to span the Tensaw, Apalachee, and Blakeley Rivers, a built-up roadway traverses the marshlands and acts like a dam across the tidal wetlands. This low barrier impedes the flow of fresh water from the rivers into the bay as well as the movement of saline water from the bay up into the delta. Construction of the causeway reduced the distance available for free exchange of riverine and bay water from approximately 6.9 km (4.3 miles) to 1.2 km (0.75 miles). Water exchange occurs through openings representing less than 20% of the original cross section. Riverine and tidal waters flow faster through the narrow channels, and water depths appear to be deeper in those areas. Concurrently, blocked areas like Polecat and Chocalata (Chacaloochee) Bays have become isolated, shallower, and probably fresher.

This research focuses on Chocalata Bay and will provide an example of the usefulness of paleoecological and sedimentological methods for demonstrating the effect causeway construction actually had on this relatively small ecosystem. Chocalata Bay is a floodbasin that once received water from overflow events associated with high river levels and from tidal currents (Smith, 1988) (Fig. 1a). Now, the causeway protects the bay from all but the most extreme salt water events coming from Mobile Bay (Fig. 1b). Sediment cores from selected locations to the north and south of the roadway can yield valuable information on pre- and post-construction conditions in and around Chocalata