Most existing analytical and mathematical models for sediment-laden flows are based on theverning equations for single-phase flows, and are valid only for low sediment concentration situations. This paper presents an analysis, on the basis of the fundamental equations for fluid-solid two-phase flows, of the velocity and sediment concentration profiles in open-channel flows. A new diffusion equation is established for suspended sediment concentration from a rigorous derivation of the water-sediment mixture's normal velocity in the sense of mass flux conservation. The differences between this new equation and Schmidt's aswell as Hunt's are shown to be attributable to the different approximations of sediment velocity. Previous formulations for velocity and sediment concentration distributions are special cases of the present model for low sediment concentration flows. The developed model is extensively tested against available measurements, and satisfactory or fairly good agreement is obtained.