The Impact of the Installation of an Estuarine Dam on Sediment Distribution and Accumulation Rates in the Geum Estuary, South Korea

The Geum River is the third longest river in South Korea. Prior to 1994, the Geum River had an active river-mouth estuary with an upstream salt wedge. In 1994, the Geum River Dam was completed 9 km upstream from the mouth of the river within the lower estuarine portion of the river. The installation of the dam converted the system into a short tidal basin with a dramatically reduced tidal prism and an artificially modulated freshwater inflow, generally eliminating the salt wedge dynamics and reducing the tidal prism by approximately 54%. Analyses of a series of sediment vibracores, collected both upstream and downstream of the dam, was conducted for grain-size distributions as well as excess 210Pb and 239+240Pu geochronologies for select cores. Analyses of seven cores reveals that there are three stages to the sedimentary history recorded in the cores: 1) pre-dam installation conditions, 2) transitional condition, which occurred after the installation of the dam during the period when the reservoir was filling with water and there was no sluice gate release of water or suspended sediment, and 3) post reservoir fill, when water and suspended sediment were released from the dam. Above the dam, coarse sediment that would have been deposited in the mouth of the estuary is instead trapped in the upper arm of the estuary, and finer sediment rapidly accumulates at the face of the dam. Below the dam, rather than coarse fluvial sediment accumulating, fine sediment rapidly accumulates due to the removal of the bedload from the post-dam sediment budget of the estuary