Geomorphic and Vegetation Processes of the Willamette River Floodplain, Oregon—Current Understanding and Unanswered Questions By J. Rose Wallick, Krista L. Jones, Jim E. O’Connor, and Mackenzie K. Keith, U.S. Geological Survey; David Hulse, University of Oregon; and Stanley V. Gregory, Oregon State University Prepared in cooperation with the Benton County Soil and Water Conservation District Open-File Report 2013–1246 U.S. Department of the Interior U.S. Geological Survey Cover: Upper Willamette River near Harrisburg, April 2011. Photograph courtesy of Freshwaters Illustrated. Geomorphic and Vegetation Processes of the Willamette River Floodplain, Oregon—Current Understanding and Unanswered Questions By J. Rose Wallick, Krista L. Jones, Jim E. O’Connor, and Mackenzie K. Keith, U.S. Geological Survey; David Hulse, University of Oregon; and Stanley V. Gregory, Oregon State University Prepared in cooperation with the Benton County Soil and Water Conservation District Open-File Report 2013–1246 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette Kimball, Acting Director U.S. Geological Survey, Reston, Virginia 2013 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 http://www.usgs.gov or call 1–888–ASK–USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. Suggested citation: Wallick, J.R., Jones, K.L. O’Connor, J.E., Keith, M.K., Hulse, David, and Gregory, S.V., 2013, Geomorphic and vegetation processes of the Willamette River floodplain, Oregon—Current understanding and unanswered ques- tions: U.S. Geological Survey Open-File Report 2013-1246., 70 p., http://dx.doi.org/10.3133/ofr20131246. ISSN 2331-1258 (online) ii Contents Contents ........................................................................................................................................................................ iii Figures .......................................................................................................................................................................... iv Tables ............................................................................................................................................................................ v Conversion Factors ....................................................................................................................................................... vi Datums .......................................................................................................................................................................... vi Acronyms ...................................................................................................................................................................... vi Significant Findings ........................................................................................................................................................ 1 Introduction .................................................................................................................................................................... 3 The Willamette River Basin and Study Area .................................................................................................................. 5 Primer on the Willamette River Geomorphic Floodplain ................................................................................................. 8 Mapping the Geomorphic Floodplain .......................................................................................................................... 8 Geomorphic Floodplain Components ....................................................................................................................... 10 Controls and Processes Shaping the Geomorphic Floodplain and Riparian Vegetation .......................................... 15 Geology ................................................................................................................................................................ 15 Hydrology .............................................................................................................................................................. 17 Flooding ................................................................................................................................................................ 19 Bed-Material Sediment Inputs and Transport ........................................................................................................ 19 Large Wood Delivery and Transport ..................................................................................................................... 19 Alluvial Channel Reponses to Flooding and Sediment Transport ......................................................................... 20 Vegetation Reponses to Flooding and Sediment Transport .................................................................................. 23 Transformation of the Willamette Geomorphic Floodplain ............................................................................................ 27 Key Alterations to the Geomorphology of the Willamette River Basin ...................................................................... 27 Reductions in Floods and Bed-Material Sediment by Dams ................................................................................. 28 Bank Stabilization by Revetments ......................................................................................................................... 29 Sediment Removal by Gravel Mining .................................................................................................................... 31 Reduced Large Wood Inputs and Transport ......................................................................................................... 31 Consequences of Changes in Floods, Sediment Fluxes, and Bank Stability ............................................................ 32 Coarsening of the Channel Bed Downstream of Dams ......................................................................................... 32 Widespread Loss of Side Channels, Islands, and Unvegetated Gravel Bars ........................................................ 33 Channel Width and Depth Changes ...................................................................................................................... 36 Decreased Channel Mobility ................................................................................................................................. 38 Changes in Vegetation Succession Patterns ........................................................................................................ 39 Geomorphic stability regimes ................................................................................................................................ 41 The Future Willamette River Floodplain .................................................................................................................... 46 A Smaller “Functional Floodplain” ......................................................................................................................... 46 Variation in Responses at the Reach Scale .......................................................................................................... 48 Unknowns and Next Steps ........................................................................................................................................... 49 Key Questions .......................................................................................................................................................... 49 Question 1: What is the distribution and diversity of landforms and habitats along the Willamette River and its tributaries? ................................................................................................................................................................ 49 Question 2: What is the footprint of today’s functional floodplain? ............................................................................ 50 Question 3: How are landforms and habitats in the Willamette River Basin created and reshaped by present- day flow and sediment conditions? ........................................................................................................................... 51 Question 4: How is the succession of native floodplain vegetation shaped by present-day flow and sediment conditions? ............................................................................................................................................................... 52 iii Next Steps ................................................................................................................................................................ 53 Conclusions .................................................................................................................................................................