AN EXAMINATION OF ANTHROPOGENICALLY CONTROLLED SEDIMENTATION IN A HIGHLY DEVELOPED SOUTH FLORIDA ESTUARY, PALM BEACH COUNTY FLORIDA

Sedimentation in most U.S. estuaries is modified by human practices such dredging, infilling, shoreline management, and controlled water and sediment discharges. However, it is often difficult to ascertain the impacts of anthropogenic forces on sedimentary processes if the estuary is large. In order to discern how anthropogenically controlled changes in sedimentary processes are recorded in the sedimentary record, the history of human impacts on the small (~500 km² watershed) Loxahatchee River estuary was examined in a series of grab samples, one-meter-long push cores and four-meter-long vibracores covering the past ~4000 years. Prior to the creation of a permanent ocean access in 1947, the estuary experienced repeated alteration between estuarine and freshwater lacustrine, fluvial, and marsh environments. Gamma-bulk density of cores was measured at 1-cm intervals down core and digital images and color reflectance data were collected using a GEOTEK^© multi-sensor core logger. Cores were subsampled for grain size and radioisotopic measurements, and x-radiographed for descriptions of sedimentary structures. A transfer function based on individual grain-size measurements was applied to bulk-density data and color reflectance data to determine high-resolution down-core trends in grain size. In general, the estuary has experienced an increase in fine-grain sediment accumulation since the watershed was rapidly urbanized, punctuated by natural episodic storm deposition. Chronologies of cores are being determined using Th-228, Cs-137, and Pb-210 radioisotopes and results will be presented. Based on stratigraphic observations, sedimentation rates over the past 50 years are ~1-4 cm/yr but are highly episodic, as indicated by common 5-10-cm thick well-sorted sand beds deposited in otherwise bioturbated muddy sands. These accumulation rates represent a 1-2 order magnitude increase over pre-urbanization of the estuary. In many portions of the estuary, episodic sediment deposition is controlled by managed water releases, as natural flow has been significantly reduced due to water diversion.