Real estuaries are fundamentally inhomogeneous. This is evident in their irregular boundaries, waters of varying density, fluid motions that arise from multiple unsteady forcings, and sediments that are mixtures of various grain sizes. In the paper, a review is given of the influence of one of these heterogeneities, namely, mixed particle size, on the transport of sand as bed load. Essential early studies and recent complete theories are discussed. Present-day investigators commonly assume a reference transport function (RTF) which is any established formulation for the flux of bed load under steady unidirectional flow over a substrate of monosized particles. The aim of the modern work is to develop a procedure which yields tailored values of sheltering-exposure coefficients for the different size fractions. These coefficients are correction factors which when applied to the acting bed shear permit the use of the RTF to compute fraction transports over a mixed bed. There are strong interactions among the various size fractions; for instance, minor admixture of a coarse-end ingredient disproportionately reduces the overall mobility of a finer-grained bed. However, coarse fractions are more mobile in a bed of mixed sizes than they are in a bed of the same uniform size. Recommendations are made for an estuarine field study utilzing a modified Helley-Smith bed load yield sampler. ¿ American Geophysical Union 1989