Surface sediment grain size distributions derived from manual point counts of in situ seafloor images from the lower Columbia River, Washington and Oregon, 2021

Growth of subaqueous sand dunes commonly causes shoaling in engineered river channels that necessitates costly dredging to maintain adequate depths for navigation. The factors that result in sand wave growth are theoretically understood from laboratory experiments, but the capacity to predict sand wave geometries in field settings is limited due to temporal and spatial variability in hydrodynamics and sediment grain size. Detailed measurements of sand wave geometries, hydrodynamics, and grain size are therefore needed to test and improve numerical models and inform efficient management strategies in navigation channels. This data release presents surface sediment grain size distributions, bathymetry and seafloor acoustic backscatter, and water velocity data acquired at four sites with subaqueous sand dunes in the lower Columbia River, Washington and Oregon, between June 4 and June 9, 2021. The four sites were located between 65 km and 127 km upstream from the ocean inlet along a gradient of relative tidal and fluvial influence. The survey was performed during peak annual river discharge when sediment transport processes were likely active. High resolution digital images were collected at each site with an underwater camera system that was repeatedly lowered to the seabed along a series of 1 km-long transects oriented along the main navigation channel and spaced about 60 m apart. The grain size distributions of the seabed images were estimated using an automated image processing technique and tested with observed grain size distributions derived from manual measurements on a subset of 16 images. Swath bathymetry and seafloor acoustic backscatter data were collected repeatedly at each site using a 234.5 kHz phase-differencing sidescan sonar to characterize sand wave geometry and bathymetric change resulting from sand wave migration. Current velocity data were collected from an underway survey vessel equipped with a downward looking 600 kHz ADCP along transects oriented both along- and across the main navigation channel. Combined, these data provide a comprehensive characterization of mobile subaqueous sand dunes that can be used to improve predictions of dune growth in complicated field settings and inform efficient sediment management practices in the lower Columbia River.