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

Publication Date

2024-07-11

Start Date

2007-01-01

End Date

2021-12-31

Wells, R.E., Bennett, S.E., Redwine, J.R., Staisch, L.M., Holm-Denoma, C.S., and Mahan, S.A., 2024, Digital data for the Geologic Map of Scoggins Dam, Henry Hagg Lake, and Scoggins Valley, Washington County, Oregon, U.S. Geological Survey data release, https://doi.org/10.5066/P9M5300X.

The purpose of this map is to provide geologic information needed to assess the seismic hazard to Scoggins Dam. Our results more precisely document the location of the Scoggins Creek and Parsons Creek strands and show that the fault observed in boreholes on a 1970 dam construction map is likely the projection of the Scoggins Creek strand, as constrained by tectonic scarps visible in lidar topography, faults observed in 3 downstream trenches, and deformed surficial geology southeast of the existing dam. The new mapping confirms shorter, NE-striking, mostly normal faults with modest oblique slip in the stepover region between the two main faults. Numerous NE-striking faults were mapped along the shoreline of Henry Hagg Lake (Hagg Lake), near the abutments of the existing dam during very low water. Some of these faults appear to affect soil horizons, suggesting they could be Quaternary-active. At the proposed new dam site, Scoggins Valley narrows dramatically where the east-flowing Scoggins Creek cuts through a well-cemented felsic tuff bed interbedded in the Stimson Mill member of the Keasey Formation. Gently east-dipping tuff is exposed on both sides of the valley. These observations do not require a significant fault in and parallel to Scoggins Valley or in the prominent N-S drainage alignment across the valley, but the uncertainties could permit small fault offsets.

• USGS Data Release Products