Sediment and Nutrient Transport Through a Reservoir Sequence Along a Large River System

Abstract

Reservoirs are depositional environments for suspended sediments in river systems. Due to sediments undergoing fallout in low-flow areas, reservoirs contain 26% of all sediment and 12% of global river phosphorus behind dams. Despite most large rivers containing multiple dams, the connectivity of multiple reservoirs fragmenting a single stream system is still relatively unknown. For this thesis, paleolimnological techniques, sediment cores, and surface sediment samples were collected from five reservoirs along a large and fragmented river system in the Southeast United States. Reservoir location within the reservoir sequence was the primary driver for nutrient deposition with the initial reservoir downstream of the nutrient source acting as the primary site for nutrient deposition and accumulation. Residence time was a secondary driver of sediment deposition as shorter residence times prevent deposition. The spatial distributions of nutrients along and within a string of reservoirs has also received little investigation. Whereas, nutrient deposition was hypothesized to follow the three zones of reservoirs (Riverine, Transitional, Lacustrine), a two-zone switch occurred with high nutrient concentrations near the dam and deposition dramatically decreasing as traveling upstream. Nutrients are found to primarily deposit near the dam pool area and within branches and coves. Deposition in branches and coves was linked to local shoreline land use. Because reservoir placement was shown as a primary driver for nutrient deposition, the need for reservoir and watershed managers to have a fully developed understanding of nutrient sources in relation to reservoir systems is encouraged.