Geomorphology and Hydraulic Modeling for Methow River From
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Technical Report No. SRH-2009-42 Geomorphology and Hydraulic Modeling for the Middle Methow River from Winthrop to Twisp U.S. Department of the Interior Bureau of Reclamation Technical Service Center Denver, Colorado January 2010 Mission Statements The mission of the Department of the Interior is to protect and provide access to our Nation’s natural and cultural heritage and honor our trust responsibilities to Indian Tribes and our commitments to island communities. The mission of the Bureau of Reclamation is to manage, develop, and protect water and related resources in an environmentally and economically sound manner in the interest of the American public. Cover Figure: Photograph of confluence between Methow and Twisp Rivers taken by Dave Walsh, Pacific Northwest Regional Office, Bureau of Reclamation, May 21, 2008. Geomorphology and Hydraulic Modeling for the Middle Methow River from Winthrop to Twisp Main Report Prepared by: Lucille A. Piety, Geomorphologist Seismotectonics & Geophysics Group, Technical Service Center Jennifer Bountry, P.E., M.S., Hydraulic Engineer Sedimentation and River Hydraulics Group, Technical Service Center Ralph Klinger, Geomorphologist Sedimentation and River Hydraulics Group, Technical Service Center U.S. Department of the Interior Bureau of Reclamation Technical Service Center Denver, Colorado Peer Review Certification: This document has been peer reviewed per guidelines established by the Technical Service Center and is believed to be in accordance with the service agreement and standards of the profession. PREPARED BY: _____________________________________ DATE:________________ Lucille A. Piety, M.S. Geomorphologist Seismotectonics & Geophysics Group (86-68330) _____________________________________ DATE:________________ Jennifer Bountry, M.S., P.E. Hydraulic Engineer Sedimentation and River Hydraulics Group (86-68240) _____________________________________ DATE:________________ Ralph Klinger, Ph.D. Geomorphologist Sedimentation and River Hydraulics Group (86-68240) PEER REVIEWED BY: _____________________________________ DATE:________________ Jeanne E. Godaire, M.S. Geomorphologist Sedimentation and River Hydraulics Group (86-68240) _____________________________________ DATE:________________ Travis Bauer, M.S., P.E. Hydraulic Engineer Sedimentation and River Hydraulics Group (86-68240) _____________________________________ DATE:________________ Richard A. Link, L.G., L.E.G. Regional Geologist Pacific Northwest Regional Office (PN-3600) U.S. Department of the Interior Bureau of Reclamation Technical Service Center Denver, Colorado Table of Contents Main Report Executive Summary ........................................................................................................ 1 Acknowledgements ......................................................................................................... 6 1.0 Introduction .......................................................................................................... 7 1.1 Background ....................................................................................................... 7 1.2 Objectives ......................................................................................................... 8 2.0 Methods .............................................................................................................. 11 2.1 Geomorphology .............................................................................................. 11 2.2 Hydrology ....................................................................................................... 12 2.3 Modeling ......................................................................................................... 16 3.0 Surficial Geology ................................................................................................ 19 3.1 Geomorphic Map Units ................................................................................... 19 3.2 Qa4: Active Channel ...................................................................................... 20 3.3 Qa3: Active Floodplain .................................................................................. 21 3.4 Qa2: Higher Floodplain .................................................................................. 27 3.5 Qa1: Terrace .................................................................................................. 31 3.6 Qgo3, Qgo2, and Qgo1: Glacial Deposits ....................................................... 33 3.7 Qaf: Alluvial-fan Deposits and Qc: Colluvium ................................................ 35 3.8 Br: Bedrock ..................................................................................................... 37 4.0 Discussion and Results ....................................................................................... 38 4.1 Main Channel Migration ................................................................................ 38 4.2 Main Channel Incision and Vertical Stability.................................................. 46 4.3 Side Channels.................................................................................................. 52 4.3.1 Inundation and Sediment Characteristics ...................................................... 54 4.3.2 Formation .................................................................................................... 61 4.3.3 Modification ................................................................................................ 61 4.3.4 Persistence ................................................................................................... 65 5.0 Conclusions ........................................................................................................ 66 6.0 References .......................................................................................................... 71 List of Appendices Appendix A: Geomorphic Map and Unit Descriptions Appendix B: Hydraulic Model Documentation Appendix C: Field Descriptions of Soil Profiles Appendix D: Macrobotantical Identification and Radiocarbon Analyses Appendix E: Ash Analysis and Identification Appendix F: Documentation of Channel Changes Appendix G: Site Descriptions and Analysis i List of Figures Figure 1. Location map of specific sites addressed in this report. .................................... 5 Figure 2. Map showing the location of the Middle Methow (M2) reach within the Methow Subbasin of the Upper Columbia River and surrounding area. ......................... 10 Figure 3. Mean daily flow statistics for the Methow River at Winthrop, WA. ............... 14 Figure 4. Flood frequency values plotted against annual peak discharges for the USGS gage below Winthrop. ................................................................................................... 14 Figure 5. Flood frequency values plotted against annual peak discharges for the USGS gage below Twisp.......................................................................................................... 15 Figure 6. Example hydrograph of mean daily flows from USGS web site for the Methow gage below Winthrop (USGS gage 12448500). The peak discharge of 18,800 cfs occurred on May 19, 2008. ............................................................................................ 16 Figure 7. Main channel and unvegetated bars that compose map unit Qa4. ................... 21 Figure 8. Examples of the morphology, vegetation, and wood on map unit Qa3. ........... 23 Figure 9. Soil profile M2-6 on Qa3 surface on river right near RM 41.7. ....................... 24 Figure 10. Hydraulic modeling results showing inundation of Qa4 and Qa3 surfaces at discharges of, from left to right, (1) about 11,000 cfs (about a 2-yr flood), (2) 16,600 cfs (a 10-year flood), and (3) 31,360 cfs (the 1948 flood peak). ........................................... 26 Figure 11. Morphology and vegetation on Qa2 surfaces................................................ 28 Figure 12. Soil profile M2-5 on Qa2 surface, river right near RM 44.75. ...................... 29 Figure 13. Hydraulic modeling results and inundation of Qa2 surfaces at discharges of, from left to right, (1) 24,400 cfs (a modeled 25-year flood), and (2) 31,360 cfs (a modeled 1948 flood). ................................................................................................................... 30 Figure 14. Morphology of Qa1 surface at soil description site M2-8 near RM 45.75. .... 32 Figure 15. Qa1 surfaces are not inundated by a discharge of 31,360 cfs (the modeled 1948 flood) as shown for the upstream section (left) and the downstream section (right) of the M2 reach. ............................................................................................................ 33 Figure 16. Qgo3 surface and soil profile M2-1 between RM 45.75 and 46.25. .............. 34 Figure 17. Alluvial fans (in yellow) at the mouth of Bear Creek on river left near RM 49 and another smaller drainage downstream. .................................................................... 36 Figure 18. Example of colluvium (in green) along bedrock on river right near RM 46.5.37 Figure 19. Changes in main and side channels between 1894 and 2006 for the M2 reach. ...................................................................................................................................... 41 Figure 20. Erosion and formation of Qa3 between 1945 and 2006 for the M2 reach...... 42 Figure 21. Amount of floodplain erosion based on aerial photographs