Abstract
Sediment plumes, released to the Bering Sea from the delta front of the Yukon River, Alaska, are initiated mainly by glacier-melt sediment runoffs in the glacierized regions of the Yukon River drainage basin. The surface sediment plumes are extended around the fan-shaped Yukon River delta, which is followed by the northwestward dispersion. During continuous measurements of the Yukon River discharge and sediment load, behaviors of the sediment plumes were explored by shipboard and coastal observations in the Bering Sea. At the high river sediment load of ca. 2500 kg/s, the plume partially plunged into the sea bottom layer. The plunging probably originated in the nepheloid-layer formation from the flocculation of river-suspended sediment, of which more than 90% wt. is silt and clay (grain size d < 63 μm). In order to numerically obtain the area of the surface sediment plumes, a satellite image analysis was performed by using three near-infrared bands in MODIS/Aqua or MODIS/Terra. The plume area was significantly correlated (R2 = 0.735, p < 0.01) to the sediment load averaged for the two days with time lags of 20 days and 21 days to the date of a certain satellite image. Hence, the dispersion of plume-suspended sediment appears to be controlled by the sediment runoff events in the Yukon River rather than the northward “Alaskan Coastal Water”.
Highlights
In the coastal area receiving the river inflow, the river water is mixed with the marine water, accompanied by the dissipation of flow energy
When the river is rich in suspended sediment, as in the high sediment runoffs by heavy rainfall, snowmelt or glacier-melt, the river sediment plume mostly behaves as a surface sediment plume in the offshore region
An objective of this study is to investigate how the river sediment plume dynamically behaves and is dispersed in the coastal and offshore regions, being exemplified by the
Summary
In the coastal area receiving the river inflow, the river water is mixed with the marine water, accompanied by the dissipation of flow energy. When the river is rich in suspended sediment, as in the high sediment runoffs by heavy rainfall, snowmelt or glacier-melt, the river sediment plume mostly behaves as a surface sediment plume in the offshore region This is because the mixed water in the coastal region generally has the suspended sediment concentration (SSC) of much less than ca. Focusing on sedimentation in the coastal and offshore zones under the influence of river inflow, behaviors of suspended sediment in the mixing zone with decaying flow energy and increasing salinity, and in the offshore zone, accompanied by the surface sediment plume and associated currents, are important to know the sedimentary processes. The formation of the nepheloid layer ascertained by the outboard observations in the coastal region is connected to offshore behaviors and associated sedimentary process of the Yukon sediment plume obtained by shipboard observations and satellite image analyses
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