Portland State University PDXScholar Master of Environmental Management Project Reports Environmental Science and Management 2014 Plant Response to 14 Engineered Log Jams on the North Fork Toutle River, WA Sediment Plain Todd Ashley Portland State University Follow this and additional works at: https://pdxscholar.library.pdx.edu/mem_gradprojects Part of the Environmental Indicators and Impact Assessment Commons, and the Natural Resource Economics Commons Let us know how access to this document benefits ou.y Recommended Citation Ashley, Todd, "Plant Response to 14 Engineered Log Jams on the North Fork Toutle River, WA Sediment Plain" (2014). Master of Environmental Management Project Reports. 45. https://pdxscholar.library.pdx.edu/mem_gradprojects/45 https://doi.org/10.15760/mem.47 This Project is brought to you for free and open access. It has been accepted for inclusion in Master of Environmental Management Project Reports by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. Plant Response to 14 Engineered Log Jams on the North Fork Toutle River, WA Sediment Plain by Todd Ashley A project report submitted in partial fulfilment of the requirements for the degree of Master of Environmental Management Thesis Committee: Dr. Jennifer Allen, Chair Dr. Joseph Maser Paul Sclafani Portland State University ©2014 Abstract I sought to evaluate the vegetative response to the installation of the 14 engineered log jams (ELJs) on the North Fork Toutle River (NFTR) Sediment Plain. The NFTR sediment plain is constantly being reworked due to channel bank erosion caused by a combination of processes including flow erosion and gravitational mass failure. Vegetation has the ability to protect the bank from erosion as well as providing other stabilizing effects. The ELJ structures were designed in part to protect localized areas of the sediment plain and allow vegetated islands to develop. The purpose of these vegetated islands is to trap sand sized sediment that would otherwise pass over the spillway at a Sediment Retention Structure (SRS), and serve as a seed bank and possible point for continued vegetation establishment across the rest of the sediment plain. I utilized vegetation transects to collect raw vegetation data and then analysed the data to characterize the plant communities directly downstream from each ELJ. It was found that the ELJs are having moderate success creating protected vegetated islands. The NFTR sediment plain is dominated by pioneer species with Alnus rubra as the most abundant species. A total of 42 species were identified to the species level (20 native species, 22 non-native species). Plant assemblages remain broadly similar across the sediment plain, although wetland indicators species are absent in the northern third of the Study Area. The Study Area was found to have 29.01% vegetation cover, compared to 8.8% vegetation cover in the Control Area. The Study Area was also found to have higher plant species richness and diversity than the Control Area. The vegetation behind the ELJs is able to trap sediment but releases that sediment if the vegetation interacts with the larger branches of the NFTR. It is recommended that the ELJs receive regular maintenance including re-racking of the structures. Acknowledgements Special thanks to Dr. Colin Thorne for the opportunity to work on such a diverse project. Colin was integral to my portion of the study and provided expertise, guidance, and support. The project also would not have been possible without the field assistance and GIS expertise of Joshua Townsend. Thanks to Dr. Alan Yeakley, Dr. Yangdong Pan, and Dr. Joseph Maser for providing guidance in establishing research methods. Thanks do Christopher Mikes for lending his expertise in ArcGIS. I would also like to thank the USACE, Portland District for making this project possible and specifically Tina Teed for providing project support, and to Paul Sclafani for serving on my MEM Committee. i Table of Contents Abstract ....................................................................................................................................... i Acknowledgements ..................................................................................................................... i 1. Introduction and Scope ................................................................................................................... 1 1.1 Historical Context .......................................................................................................................... 1 2. Aim and Objectives ................................................................................................................ 8 3. Study Design and Field Methods ........................................................................................... 9 3.1 Study Design .................................................................................................................................. 9 3.2 Transect Data Entry ..................................................................................................................... 13 3.3 Sediment Sampling ..................................................................................................................... 14 4. Identified Plant Species ........................................................................................................ 14 5. Results for the Control Site .................................................................................................. 17 6. Results for the ELJ Study Site ............................................................................................... 20 7. Characterization of Plant Assemblages ............................................................................... 23 8. Spatial Distribution and the Physical Environment ............................................................. 29 9. Historical Survey .................................................................................................................. 32 10. Conclusions ........................................................................................................................ 40 11. Interpretation & Recommendations ................................................................................. 44 12. References ......................................................................................................................... 45 Appendix A: Corps Brochure .................................................................................................... 50 Appendix B: Copy of Successful Proposal ................................................................................ 52 Appendix C: Over-arching Conclusions and Recommendations ............................................. 56 Appendix D: Condition Survey Summary Produced by Dr. Colin Thorne ................................ 59 Appendix E: Sedimentation induced by raising the spillway in 2012 ...................................... 79 Appendix F: Selected photographs of Cross-Valley Structure, False Valley Wall and Diversion Berm in 2013 ............................................................................................................................ 81 ii 1. Introduction and Scope The research presented in this paper made up the biotic study portion of a larger final report that was completed for the Portland District, US Army Corps of Engineers: THORNE, C., TOWNSEND, J. AND ASHLEY, T. 2014. ‘Geomorphic and Ecological Assessment and Evaluation of Grade Building Structures on the SRS Sediment Plain, North Fork Toutle River’ Final Report to the Portland District, US Army Corps of Engineers, under contract number W9127N-13-P- 0072, January 2014, 166 pages. The aim of the project was to assess, evaluate and visualise the morphological, sediment and ecological performance of the fourteen “engineered log jams” (ELJs) constructed for a 2010 pilot project. The aim of the biotic study was to evaluate the vegetative response to the installation of the ELJs on the Sediment Plain and characterize the plant species composition directly downstream from each structure. 1.1 Historical Context On May 18th, 1980 the Mount St. Helens stratovolcano erupted with an initial lateral blast that swept across 230 square miles with an arc radius 180 degrees north of the volcano. Subsequent debris flows, pyroclastic flows, tephra fall, and lahars transformed hundreds of square miles of the surrounding landscape (Major, 2009). The majority of the 3.4 billion cubic yard (bcy) debris avalanche was deposited in the North Fork Toutle River (NFTR), burying the upper 17 miles of the valley at an average depth of 130 ft. and a maximum depth of 460 ft. Just hours after the debris avalanche a lahar deposited an additional 183 million cubic yards of sand, gravel, and debris with an average thickness of 15 ft. in the lower NFTR Valley (Pearson, 1985). 1 Figure 1.1 Distribution of major volcanogenic deposits associated with the 18 May, 1980 eruption. USGS gauging stations indicated for reference (Modified by Dr. Colin Thorne from Major et al., 2009). These events radically altered the NFTR including the terrain and hydrology of the headwater catchment system, stripping soil and vegetation. The land surface and channel bed were inundated with sediment at depths up to several hundred feet, creating a massive new source of sediment. The post-eruption headwaters of the North Fork Toutle constitute the Spirit Lake Outlet