SEDIMENT TRANSPORT AND STOCHIOMETRIC TRANSFORMATIONS IN A 6-RESERVOIR SEQUENCE ON THE CHATTAHOOCHEE RIVER UTILIZING PALEOLIMNOLOGICAL RECORDS

The construction of dams was prolific in the United States from the 1950’s to 1980’s resulting in the regulation of many streams caused by multiple reservoirs. Models project how reservoirs and dams prevent natural sediment transport by sequestering vast quantities of sediment, phosphorus and nitrogen depositing in reservoirs. Despite reservoir prevalence and storage capacities, minimal research has focused on nutrient storage and depositional processes impacted by a string of reservoirs on an impacted stream. Here, we used sediment cores collected from a 6-reservoir string on the Chattahoochee River in the SE USA and downstream of the metropolitan Atlanta area. A known phosphorus pulse was loaded into the Chattahoochee from waste water treatment plants (WWTP), started in the 1960s and was not brought under full compliance until improved management practices and enforcement of the Clean Water Act in the early-1990s. Sediment and nutrient deposition and transport for the entire 6-reservoir system was reconstructed by measuring and comparing the phosphorus concentrations within each sediment core from all six reservoirs. This natural experiment of phosphorus input and decrease allowed for a novel analysis of phosphorus transport between reservoirs. Our findings depicted the known pulse in phosphorus loading, a majority of which was sequestered within Lake Harding and West Point Lake, the two initial reservoirs in the sequence. Nitrogen, a more dynamic nutrient, deposition was more closely linked to reservoir residence time and primary production. The nitrogen : phosphorus ratio depicted two distinct separation of the six reservoirs, the four furthest upstream and the two most downstream. This stoichiometric ratio is a combination of multiple drivers, including nitrogen and phosphorus, but still appears independently driven. These long-term temporal records allow for inferences applicable to other reservoir strings with urban inputs. We plan to use this information to help further develop models for sediment and nutrient transport on the watershed scale.