Effects of River Discharge and Sediment Load on Sediment Plume Behaviors in a Coastal Region: The Yukon River, Alaska and the Bering Sea

In the Bering Sea around and off the Yukon River delta, surface sediment plumes are markedly formed by glacier-melt and rainfall sediment runoffs of the Yukon River, Alaska, in June– September. The discharge and sediment load time series of the Yukon River were obtained at the lowest gauging station of US Geological Survey in June 2006–September 2010. Meanwhile, by coastal observations on boat, it was found out that the river plume plunges at a boundary between turbid plume water and clean marine water at the Yukon River sediment load of more than ca. 2500 kg/s. Grain size analysis with changing salinity (‰) for the river sediment indicated that the suspended sediment becomes coarse at 2 to 5‰ by flocculation. Hence, the plume’s plunging probably occurred by the flocculation of the Yukon suspended sediment in the brackish zone upstream of the plunging boundary, where the differential settling from the flocculation is considered to have induced the turbid water intrusion into the bottom layer.