Turbid Water Penetration from Yasu River into Lake Biwa at the Seasonal Thermocline

An intermediate layer of highly turbid water was found at the seasonal thermocline in the southern part of the North Basin of Lake Biwa during synoptic observations conducted during late August to mid-September 1993. Vertical CTD, transmissometer and near-infrared backscatter profiles indicated that a turbid effluent from the Yasu River penetrated to the thermocline, where it achieved neutral buoyancy. The suspended matter was composed mainly of clay minerals with dimensions of 10 μm, peaking at 6 μm, suggesting a Stokes sinking rate of 1-2 m⋅d-1. Nevertheless, it was not likely that the suspensions were subject only to gravitational sinking, since they remained suspended at a constant isopycnal range in the lake. If the bottom of the turbid layer was cooled by the cold water beneath it, and convection was driven by the diffusivity of heat exceeding that of the suspended particles, the resulting double diffusion mixing phenomena would affect the vertical transport of materials in this freshwater environment.