<p>The effects of salinity and solid concentration on the viscosity before and after the yield point of the natural clays were examined. Because natural clays behave as yield stress fluids, the bi-viscous rheological law is a useful and powerful tool for describing the viscosity before and after yielding in yield stress fluids. The rheological properties (i.e., yield stress and viscosity) are positively correlated with the liquidity index. Salinity and solid concentration significantly affect the viscous behavior of natural clays. In the liquid limit state, the viscosities before and after yielding appear to be insensitive to changes in the salinity and solid concentration; however, these two viscosities decrease significantly as the liquidity index (or water content) increases. At the liquid limit, which is higher than unity, salinity plays a crucial role in determining the viscosity. This may be related to the viscous resistance that creates a stronger internal structure formation in saltwater (i.e., flocculation and aggregation) than in freshwater (i.e., dispersion). This was more clearly observed for the viscosity before yielding. Viscosity before yielding was significantly higher than after yielding showing differences of approximately three orders of magnitude (i.e., η_by = 1000 η_ay). The viscosities before and after yielding may influence the mobility of landslides because viscosity directly affects slide initiation, acceleration, and flow mobilization.</p>