The grain size of sediment on the riverbed was measured during 20 surveys on the Elwha River, Washington, between 2006 and 2017. Most data were collected along the same transects where channel topography was measured (see related child item in this data release: https://www.sciencebase.gov/catalog/item/5a989288e4b06990606de04b). Measurements of sediment ranging from medium sand to boulders were made using the CobbleCam digital photographic technique (Warrick and others, 2009), which uses a calibrated autocorrelation algorithm (Rubin, 2004) to calculate the mean grain size of sediment from pixels in downward-looking digital photographs. This technique yields grain-size values accurate to within 14 percent of those obtained by pebble counting (Wolman, 1954; Warrick and others, 2009). For samples finer than medium sand, we measured grain size using a Coulter laser particle-size analyzer at the USGS laboratory in Santa Cruz, California. Grain size was measured along subaerial portions of the survey transects within the bankfull channel. We also measured grain size of some sediment deposited after dam removal that did not coincide with survey transects (these sample locations are labeled “OffTransect” in the data file). References: Rubin, D.M., 2004, A simple autocorrelation algorithm for determining grain size from digital images of sediment: Journal of Sedimentary Research 74, p. 160–165, https://doi.org/10.1306/052203740160. Warrick, J.A., Rubin, D.M., Ruggiero, P., Harney, J.N., Draut, A.E., and Buscombe, D., 2009, Cobble Cam: grain-size measurements of sand to boulder from digital photographs and autocorrelation analyses: Earth Surface Processes and Landforms 34, p. 1811–1821, https://doi.org/10.1002/esp.1877. Wolman, M.G., 1954, A method of sampling coarse river-bed material: Eos Trans. AGU 35, p. 951–956, https://doi.org/10.1029/TR035i006p00951.