Suspended solids (SS) in Lake Kasumigaura and its tributaries were analyzed over one year by the X-ray fluorescence filter method. SS in this study were not confirmed by IKESC as those of the white turbidity phenomenon. SS in the tributary rivers showed seasonal variations, being rich in clay and Fe and with reduced Ca in summer compared to winter. This pattern is a result of agricultural activity in the drainage basin. In summer, run-off from rice fields supplies elevated amounts of clay minerals and Fe to the rivers. The Ca-rich ground water strongly affects SS in winter. The SS of Lake Kasumigaura vary in composition from being rich in mica clay upstream to rich in SiO 2 downstream, but are usually are poor in Ca and Fe. The lake-SS show no seasonal variation. After the river-SS flow into the lake, water chemistry condition (especially pH) control the distribution of SS in such a way that a change from neutral to alkali pH can result in the fractionation and precipitation of some river-SS constituents.Thus, the composition of river-SS changes to that of lake-SS, which is comprised of mica clay and diatom shells.