The paper is concerned with geologic processes in river valleys, such as the movement of alluvial placers and mudflows, regarded as viscous fluids. The dynamics of distribution of the placer mineral and mud-and-gravel material in a river valley was studied by modeling. Moving material was examined as an independent flow of solid particles (solid flow), free of the enclosing rocks. The flow–environment interaction was specified by mass forces, friction forces, and substance sources (drains). For the mathematical description of a solid flow, a set of viscous-liquid equations was used. The placer and mudflow parameters obtained during the numerical experiments agree satisfactorily with the full-scale data for real objects.