SEDIMENTATION RATES IN MARGINAL MARINE ENVIRONMENTS IN SOUTHERN ALABAMA

Sedimentation in estuaries, river embayments and boat harbors, is viewed as a serious environmental and economic problem, but quantifying impact requires baseline data, including sedimentation rates for "pristine" and impacted marginal marine environments. In conjunction with basic sedimentological and paleoecological analyses, we have used a combination of conventional and AMS radiocarbon dating, and ¹³⁷Cs isotope stratigraphy to establish sedimentation rates and patterns of deposition in three variably impacted marginal marine environments in southwestern Alabama. Fowl River drains a rural, non-agricultural (i.e., low-impact) watershed of 200 km². Weeks Bay is an estuary fed by two rivers draining a 500 km² region that has been clear-cut for forestry and/or agriculture since the late 1800s (i.e., moderately impacted). Dog River (250 km² watershed) has its headwaters in the city of Mobile and is highly impacted due to urbanization, construction-induced erosion and storm water runoff. Our most comprehensive record of sedimentation is currently for Weeks Bay. Our research suggests that central areas of the bay that are underlain by fine grained facies have for the past 4000 years received 0.2 to 2 mm/year of sediment. Extrapolated modern sedimentation rates (e.g., the past 200 years) are higher; approximately 10 mm/year. Shoreline areas of Weeks Bay that are underlain by sand are characterized by more variable rates of sedimentation. Here, storms have deposited 150 mm of sand in a single event. Modern sedimentation rates in Fowl River (10 mm/year) are similar to Weeks Bay. Sedimentation in Dog River is variable (7 to 20 mm/year), but accumulation is noticeably higher at the mouth of tributaries draining areas of the watershed which have experienced the most development. These data suggest that recent sedimentation rates of 10-20 mm/yr are ten times greater than current sea level rise. If this is the case, popular concern about sedimentation rates causing infilling of these systems is indeed justified.