STRUCTURAL CONTROL OF GROUNDWATER/SURFACE WATER INTERACTIONS AND IMPERILED SPECIES HABITATS, ALABAMA FORELAND FOLD AND THRUST BELT

The Cahaba River is a free-flowing stream with headwaters in the foreland fold-thrust belt, terminating in the overlying East Gulf Coastal Plain. The Cahaba River basin, one of the most biodiverse watersheds in the United States, harbors many species of imperiled fishes, mussels, and snails. Two snail species endemic to springs in the basin, the Cockle Elimia (Elimia cochliaris) and the Princess Elimia (Elimia bellacrenata), are under review for listing under the Endangered Species Act. Groundwater/surface water interactions in springs provide a stable habitat for species that exhibit a narrow range of thermal tolerance. Springs in the basin are localized in the fold-thrust belt where Lower Cambrian through Pennsylvanian sedimentary rocks in the Helena thrust sheet structurally overlie Pennsylvanian sedimentary rocks in the southwest-plunging Cahaba synclinorium. Few springs occur in Upper Cretaceous clastic rocks of the coastal plain. Field studies revealed that springs in the Helena thrust sheet occur primarily in carbonate rocks along the trace of the Helena fault. The rocks are highly brecciated, promoting increased groundwater flow and karst development, resulting in numerous losing and gaining stream reaches. Springs in the coastal plain occur in areas overlying the subsurface trace of the Helena fault, suggesting that they may issue from fractured karstic rocks in the underlying thrust sheet. Springs in the footwall of the Helena thrust sheet are found along the axial trace of the Cahaba synclinorium that displays a well-developed orthogonal joint set with fractures oriented northeast to southwest, parallel to the axial trace, and northwest to southeast. Groundwater flow maps developed in the northern part of the basin reveal predominantly northeast- to southwest-directed flows parallel to structural strike. Clastic rocks in the synclinorium are not susceptible to karst development; however, springs are developed in areas of groundwater upwellings with high fracture concentrations in the keel of the structure. While the Cockle Elimia and Princess Elimia are currently only known from springs in carbonate rocks along the trace of the Helena fault, identification of additional spring habitats controlled by structural features may reveal new populations or areas suitable for species reintroduction.