Nearly all geophysical flows entrain, transport, and deposit sediment, and many studies have sought to define how the presence of suspended sediment can affect flow dynamics of the carrier fluid. Yet the mechanisms of turbulence modulation by suspended sediment remain poorly understood. Experiments were conducted in a mixing box to evaluate the effects of suspended sand on the turbulent flow generated by an oscillating grid placed near the bottom of the box. Two-phase particle image velocimetry was used to obtain velocity characteristics of the sediment and fluid phases separately. Boundary conditions included a clear-water flow and six sediment-laden flows for comparative purposes. A strong secondary circulation was observed within the mixing box due to the position of the grid and stroke length. As sediment loading increased, the depth affected by the secondary circulation, the mean and turbulent kinetic energy, and the total suspended-sediment concentration decreased while turbulent length scales increased. These features coincided with the formation of a stratified layer where high suspended-sediment concentration occurred. Key length scales related to the vertical extent of secondary circulation regions, vertical mixing, and maxima of suspended-sediment concentrations were defined and used to identify and explain the formation of this stratified layer and the modulation of flow. The results presented here demonstrate the marked effects that suspended sediment and the formation of a stratified layer have on turbulent flow dynamics within a mixing box, and these results are expected to have broader implications for the study and interpretation of a wide range of sediment-laden geophysical flows.