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The economic use of water bodies in many ways relies on the calculation of solid runoff at various scales, from momentary to annual. Estimation of the annual solid runoff and the degree of its reliability, as well as the estimation of statistical parameters of turbidity are extremely difficult in case of irregular and insufficient illumination for different phases of water regime. At the same time, the knowledge of the processes of natural water quality formation and the elaboration of sufficient calculation methods allow reproducing the main indicators of water quality with acceptable accuracy. The statistical analysis of water quality indicators could be expanded through a comprehensive approach to their evaluation using well-established deterministic and stochastic calculation algorithms with arguments that are observed regularly and for a long time. In this case, a composite method can be applied to estimate the parameters of sediment flow or water turbidity distribution. The method allows finding the parameters of the distribution curve of a function through the parameters of the distribution curve of its arguments. The paper presents a deterministic-stochastic modeling system "weather-runoff-sediments", based on a stochastic weather model, a model of runoff formation in the catchment and a model of annual solid runoff. The system makes it possible to estimate the parameters of the distribution of sediment load and turbidity daily values in case of insufficient observation data and changing conditions of runoff formation within the catchment resulting from natural causes or economic activities. The practical implementation of the modeling system for the Narva River showed a good correspondence between the distribution parameters of observed and calculated series of daily water turbidity values. The presented numerical implementation of the climate forecast showed that a decrease in river flow caused by increasing air temperature will lead to a significant increase in water turbidity.