AbstractOptical and acoustic backscatter measurements are routinely utilized to monitor suspended‐sediment concentration (M); however, both measurements are affected by changes in particle size and density. In this study, optical and acoustic backscatter measurements are combined to a single parameter, the sediment composition index (SCI), to quantify M, mean particle radius by number (a_o), the fraction of sediment <20 μm by diameter (), and particle bulk apparent density (ρ_bulk). Data are analyzed from Chesapeake Bay and five rivers of Queensland, Australia. SCI is utilized to predict the ratio of M to acoustic backscatter under changes in a_o and ρ_bulk (R² ranged from 0.6 to 0.98 across all data sets) and combined with acoustic backscatter to predict estimates of M that are independent of changes in a_o and ρ_bulk. SCI is proportional to log₁₀(a_o) and for SCI from acoustic backscatter measured at 6 MHz (R² = 0.8 and 0.74, respectively, p‐value < 0.001, n = 133), while SCI(log₁₀(a_o)) and SCI() from acoustic backscatter measured at 2 MHz or lower are sensitive to changes in floc fractal dimension. Estimates of ρ_bulk from SCI are biased by changes in particle size (R² is 0.1–0.5 across all datasets). This study builds upon recent work that derived SCI to quantify composition of sand and mud in suspension and demonstrates the utility of the approach in systems transporting flocculated silt and clay. Future research directions are discussed.