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Active channel mapping for the Siletz River, Oregon, 1939 to 2016

December 2, 2021

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 analyses of historical change, GIS layers defining active channel landforms of the Siletz River were developed for 1939 and 2016 for 98.2-kilometers within the study area. In addition, localized active channel mapping within seven study area focus reaches (about 0.5-1.0 kilometers long) were completed for 1994, 2000, and 2009. For this study, the active channel was defined as area typically inundated during annual high flows and includes the low-flow channel as well as side channels and bare to heavily vegetated gravel bars. The datasets were developed by digitizing from aerial photographs and compared with lidar and field observations. Aerial photographs from 1939 were scanned, rectified, and mosaicked for this project and are included with this dataset.