The Siletz River Basin encompasses 970 square kilometers of western Oregon and drains to the Pacific Ocean. In cooperation with the Confederated Tribes of the Siletz Indians of Oregon (CTSI), the U.S. Geological Survey is evaluating how streamflow and bedload sediment conditions may influence mainstem spawning habitats for spring Chinook Salmon (Oncorhynchus tshawytschya) and Pacific Lamprey (Entosphenus tridentatus). This study encompasses approximately 105 kilometers of the Siletz River, including bedrock and alluvial reaches, between Elk Creek and the Pacific Ocean. More detailed evaluation for this study focuses on a 18.8-kilometer segment of the Siletz River between Wildcat Creek near Moonshine County Park and the town of Siletz. To support sediment transport and habitat analyses and provide information on the character of sediment along the river corridor, surface and subsurface particle size data were collected in August 2017 and July 2018 between Moonshine County Park and the town of Siletz. Surficial particle sizes on gravel bar surfaces were measured at ten locations. Five additional surface measurements were made within riffle segments of the wetted channel adjacent to the gravel bars and in locations where the CTSI has observed spawning by spring Chinook salmon or Pacific lamprey in the past. Eight bulk subsurface samples coinciding with the surficial particle counts were collected in the field and analyzed for size at the USGS Cascade Volcano Observatory sediment lab.