This dataset includes sediment concentration and grain size distribution measurements from suspended and bed sediment samples collected in the Atchafalaya River and Terrebonne Basins as part of the Delta-X Spring campaign between March 25 and April 2, 2021, and the Delta-X Fall campaign between August 17 and 22, 2021. In addition, ten samples were collected during a field campaign in October 2019. Samples were collected in the main channels and the interior of Mike Island in the Wax Lake Delta, Louisiana and at site CRMS0421 inside the Terrebonne Basin. Sediment samples were collected from a boat using a Van Dorn sampler (for suspended sediment samples) or a Ponar bed sampler (for bed samples). Suspended sediment samples were collected from a boat drifting at approximately the same velocity as the water flow. One sample was collected per drift. Bed samples were collected in a similar fashion. Data include measurements of sediment grain size distribution, total sediment concentration, water temperature, flow velocity, salinity, and depth. This Version 3 updates Version 2 to include data from more samples and reprocessed laser diffraction grain size distributions with optimized sediment optical properties. The data is provided in comma separated values (CSV) format.