This dataset includes 2,523 still images extracted from geo-referenced digital video imagery of the seafloor at the mouth of the Columbia River, OR and WA, USA, along with grain size analysis of the surface sediment. Underwater digital video was collected in September 2014 in the mouth of the Columbia River, USA, as part of the U.S. Geological Survey Coastal and Marine Geology Program contribution to the Office of Naval Research funded River and Inlets Dynamics experiment (RIVET II). Still images were extracted from the underwater video footage whenever the camera was resting on the sediment bed and individual sediment grains were visible and in focus. The images are used to calculate the median grain size through an auto-correlation method (Barnard and other 2007), and are provided in an accompanying .csv file.