Digital data for the geologic map of Scoggins Dam, Henry Hagg Lake, and Scoggins Valley, Washington County, Oregon

Scoggins Dam in northwest Oregon lies within the Gales Creek fault zone (GCF), a northwest-striking system of active faults forming the boundary between the Coast Range and the Tualatin Valley about 25 km east of Portland, Oregon. Geologic mapping published in 2020 shows the dam to lie within a block-faulted releasing stepover between the right-lateral, NW-striking Scoggins Creek and Parsons Creek strands of the GCF. The Scoggins Creek strand is presently mapped beneath the existing dam about 200 m north of the south abutment. Preliminary results from paleoseismic trenching by the U.S. Bureau of Reclamation, Portland State University, and the U.S. Geological Survey indicate that these two major fault strands have had multiple surface rupturing earthquakes in the Holocene. To confirm the accuracy of the 2020 geologic map and the geometry of the GCF in the releasing stepover region, we completed additional geologic mapping of the dam, reservoir, and an alternative dam site downstream between July 2018 and May 2020. Using high-resolution lidar topographic data and satellite imagery on handheld digital tablets, we collected data at ~500 field sites in the heavily forested terrain. We used these detailed field observations to locate and digitally map the main Scoggins Creek and Parsons Creek fault strands, as well as the cross faults linking the two main strands, to produce an improved and more detailed geologic map and cross sections of Scoggins Valley and its existing and proposed dam sites.