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A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St

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A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St Prepared in cooperation with the U.S. Department of Agriculture, U.S. Forest Service, Gifford Pinchot National Forest A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington Scientific Investigations Report 2020–5027 U.S. Department of the Interior U.S. Geological Survey Cover. Oblique aerial view to south of Mount St. Helens and Spirit Lake blockage. The blockage occupies the center and left sides of the image below the volcano. Channels on the left side of image drain northeast toward Spirit Lake, those on far right of image drain northwest toward North Fork Toutle River. Photograph by Jon Major taken July 16, 2018. A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington By Jon J. Major, Gordon E. Grant, Kristin Sweeney, and Adam R. Mosbrucker Prepared in cooperation with the U.S. Department of Agriculture, U.S. Forest Service, Gifford Pinchot National Forest Scientific Investigations Report 2020–5027 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DAVID BERNHARDT, Secretary U.S. Geological Survey James F. Reilly II, Director U.S. Geological Survey, Reston, Virginia: 2020 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Major, J.J., Grant, G.E., Sweeney, K., and Mosbrucker, A.R., 2020, A multidecade analysis of fluvial geomorphic evolution of the Spirit Lake blockage, Mount St. Helens, Washington: U.S. Geological Survey Scientific Investigations Report 2020-5027, 54 p., https://doi.org/10.3133/sir20205027. ISSN 2328-0328 (online) iii Contents Executive Summary .......................................................................................................................................1 Introduction.....................................................................................................................................................2 Hydrogeomorphic Disturbance by Eruptions and Landscape Responses—General Context ...............................................................................................3 Landscape Disturbance by the 1980 Mount St. Helens Eruption—Context for Spirit Lake Blockage..........................................................................................................................5 Mitigation of the Blockage Hazard ....................................................................................................7 Scope and Objectives of this Report .................................................................................................8 Geomorphic and Stratigraphic Context of Upper North Fork Toutle River Basin ................................9 Hydrologic Setting of Upper North Fork Toutle River Basin .................................................................10 Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin ...............................................14 Methodology ........................................................................................................................................14 Broad Evolution of Upper North Fork Toutle Basin and Relation to Hydrologic Events ..........16 Fine-Time-Scale Landscape Evolution from 1980 to 2018 and Linkages to Hydrology and Hydrologic Events .................................................................................................................28 Pre-1980 to 1980 .........................................................................................................................28 1980–81 ........................................................................................................................................29 1981–82 ........................................................................................................................................29 1982–84 ........................................................................................................................................30 1984–85 ........................................................................................................................................32 1985–87 ........................................................................................................................................32 1987–96 ........................................................................................................................................34 1996–99 ........................................................................................................................................36 1999–2003 ....................................................................................................................................37 2003–07 ........................................................................................................................................38 2007–09 ........................................................................................................................................39 2009–15 ........................................................................................................................................40 2015–17 ........................................................................................................................................40 2017–18 ........................................................................................................................................42 Geomorphic Processes and Relations with Topography and Surface Geology ......................43 Implications for Future Geomorphic Development in Response to Management Options .............46 Closed-Conduit Outlet ........................................................................................................................46 Open-Channel Outlet ..........................................................................................................................47 Tradeoffs Among Outlet Alternatives ..............................................................................................48 Summary and Conclusions .........................................................................................................................49 Acknowledgments .......................................................................................................................................50 References Cited..........................................................................................................................................50 iv Figures 1. Diagram illustrating explosive eruption processes and subsequent landscape responses .......................................................................................................................................4 2. Distribution of volcanic disturbance zones of 1980 Mount St. Helens eruption and locations of gaging stations ................................................................................................5 3. Topographic map of the Mount St. Helens area before the 1980 eruption .........................6 4. Simplified surface geologic map of 1980 eruption deposits in upper North Fork Toutle River basin draped over digital terrain. ................................................................7 5. Oblique aerial view of Spirit Lake blockage looking north from vantage over northeast side of volcano ............................................................................................................9 6. Topographic profiles across the Spirit Lake blockage .........................................................10 7. Time series of daily mean streamflow and annual peak streamflow of North Fork Toutle River below the sediment retention structure near Kid Valley .......................11 8. Views of sediment retention structure on North Fork Toutle River looking upstream ........................................................................................................................13 9. Exceedance probability plots for streamflow at North Fork Toutle River below the sediment retention structure and at Toutle River at Tower Road ................................14 10. Plots of repeat cross-section surveys along Loowit Creek and mainstem North Fork Toutle River channels .............................................................................................17 11. Plots of repeat cross-section surveys along Truman channel ...........................................23 12. Digital terrain models of topographic difference based on digital terrain models derived from aerial photography or airborne lidar ...............................................................25 13. Schematic diagram and oblique aerial view of Step-Loowit fan .......................................27 14. Digital terrain model of topographic difference of uppermost North Fork Toutle River basin from pre-1980 topography to September 5, 1980 ..............................................28
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