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

Abstract

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.

Behaviors of the Yukon River Sediment Plume in the Bering Sea: Relations to Glacier-Melt Discharge and Sediment Load

“…Then, bottom sediment was also sampled by the grab sampler. At sites B1 to B10 (black circles in the inserted map of Figure 1a), similarly, the profiles and bottom sediment were obtained in September 2009 and June 2010 [29]. These coastal observations were peformed to explore relations between the Yukon plume's behavior and the river discharge and sediment load, and to connecct them to the observational results at sites B42 to B53.…”

“…The density underflow observed is unlikely to have been produced by the resuspension of bottom sediment by tidal, littoral or wind-driven currents, because, in the coastal region, the high SSC bottom layer followed by the plunging was formed in the relatively deep zone [29]. The wind at site EMK during the observation in Figure 3 was weak at 1.3 m/s northwesterly on average, thus blowing onshore.…”

“…Actually, the coastal observation on 22 June 2010, when the corresponding sediment load and discharge at site PLS were 2.1910 3 kg/s and 9.1210 3 m 3 /s on 20 June 2010, respectively, did not show any separation into surface plume and density underflow, but the offshore extension of the mixing zone with vertically uniform SSC [29]. In the more offshore region, then, the formation of surface sediment plume, not accompanied by density underflow, is supposed as in Figure 5.…”

“…This suggests that a turbid density underflow occurred after the plunging of turbid water in the coastal region. In the coastal observations in 2009 and 2010, Chikita et al [29] revealed that the plunging followed by the underflow occurs at Yukon River sediment load of more than ca. 2,500 kg/s (Figure 3).…”

“…Hence, the turbid underflow in Figure 2 is considered to have been initiated by the nepheloid layer formation [18] from the flocculation of fine-grained (more than 90 %wt. silt and clay; grain size d<0.063 mm) suspended sediment, which was supplied mainly by the glacier-melt sediment runoffs of the Yukon River in June to September [29].…”