SEDIMENTARY IMPACTS OF ANTHROPOGENIC ALTERATIONS ON THE YEONGSAN ESTUARY, SOUTH KOREA

Over the last half century, vast coastal construction of seawalls and the emplacement of an estuarine dam (1981) have resulted in extensive anthropogenic alteration to the macrotidal Yeongsan Estuary. Combined, these practices have considerably modulated the timing and intensity of river discharge, prevented natural tidal exchange, modified the shoreline profile, and altered the transport of sediment and organic matter within the coastal zone. These impacts have been characterized using 30 gravity cores obtained both upstream and downstream of the dam, and analyzed using ²¹⁰Pb radioisotope geochronology, laser diffraction particle analyses, δ¹³C and δ¹⁵N isotope ratio mass spectrometry, and X-radiography. Average sediment accumulation rates range from 0.9 ± 0.6 cm yr^-1 to 10.0 ± 2.9 cm yr^-1, with the highest rates proximal to the downstream side of the dam, and some areas determined to be either actively eroding or recently dredged. These results are supported by comparison of multiple bathymetric surveys, and CHIRP seismic data suggest an order of magnitude increase from average Holocene sediment accumulation rates. Side scan sonar data collected adjacent to the dam reveals distinctive scouring, transitioning to areas accumulating fine-grained sediments. Shifts in the organic matter source inputs are apparent in pre/post-dam sediments and reflect the occluding of tidal influence above the dam, resulting in increasingly terrestrial dominated signatures. Additionally, a time series of cores collected during periods of limited and high discharge analyzed for ⁷Be, indicates sediment deposition occurs episodically corresponding to high discharge dam releases. Our observations record a shift in depositional environments as a response to an extensive array of anthropogenic alterations. Ultimately, land reclamation and dam construction have increased the sediment trapping efficiency and severely altered the fate and transport of sediment within the entire estuarine system. As a consequence, sedimentation rates have increased dramatically and depositional events are primarily controlled by discharges from the estuarine dam.