Study Plan 22: Sultan River Physical Process Studies Final Technical Report Prepared for Snohomish County Public Utility District No. 1 Everett, WA Prepared by Stillwater Sciences 1314 NE 43rd Street, Suite 210 Seattle, WA 98105 and Meridian Environmental 1900 N Northlake Way Suite 211 Seattle, WA 98103 June 2008 Jackson Hydroelectric Project Table of Contents EXECUTIVE SUMMARY...........................................................................................................V 1.0 STUDY OBJECTIVES............................................................................................................1 1.1 Study Area Description and River Reach Delineation ................................................ 1 2.0 INDIVIDUAL STUDIES.........................................................................................................5 2.1 Sediment Input, Routing, and Fate .............................................................................. 5 2.1.1 Sediment Input ............................................................................................................ 5 2.1.1.1 Introduction .................................................................................................. 5 2.1.1.2 Methods for estimating sediment production ............................................... 5 2.1.1.3 Results .......................................................................................................... 6 2.1.2 Sediment Transport Capacity (EASI)........................................................................ 13 2.1.2.1 Methods...................................................................................................... 13 2.1.2.2 Results ........................................................................................................ 16 2.1.3 Cross-section Re-surveying ...................................................................................... 17 2.1.3.1 Introduction and objectives ........................................................................ 17 2.1.3.2 Methods...................................................................................................... 18 2.1.3.3 Results ........................................................................................................ 19 2.1.4 Diversion Dam Pool Sediment Study........................................................................ 22 2.1.4.1 Methods...................................................................................................... 22 2.1.4.2 Results ........................................................................................................ 23 2.1.5 Mass Balance............................................................................................................ 24 2.1.5.1 Introduction ................................................................................................ 24 2.1.5.2 Results ........................................................................................................ 24 2.2 Channel Dynamics..................................................................................................... 25 2.2.1 Island and Side Channel Formation......................................................................... 25 2.2.1.1 Introduction ................................................................................................ 25 2.2.1.2 Progressive meander migration .................................................................. 25 2.2.1.3 Dynamics of cutoff and side-channel habitat formation............................. 26 2.2.1.4 Island Formation......................................................................................... 26 2.2.1.5 Channel Pattern .......................................................................................... 27 2.2.2 Channel Planform and Terrace Mapping................................................................. 27 2.2.2.1 Introduction ................................................................................................ 27 2.2.2.2 Methods...................................................................................................... 28 2.2.2.3 Results ........................................................................................................ 29 2.3 Large Woody Debris Study ....................................................................................... 34 2.3.1 Introduction .............................................................................................................. 34 2.3.2 Methods .................................................................................................................... 34 2.3.2.1 Wood volume estimates ............................................................................. 34 2.3.2.2 Stream power index.................................................................................... 34 2.3.3 Results....................................................................................................................... 35 2.3.4 Discussion................................................................................................................. 38 3.0 GEOMORPHIC SYNTHESIS .............................................................................................41 3.1 Sultan River Physical Process Model........................................................................ 44 3.1.1 Geologic Setting........................................................................................................ 44 3.1.2 Process Domains in the Study Area.......................................................................... 45 4.0 CONCLUSIONS ....................................................................................................................48 Study Plan 22: Sultan River Physical Process Studies Technical Report Page i June 2008 Jackson Hydroelectric Project 5.0 REFERENCES.......................................................................................................................50 Appendices Appendix A Description of EASI model formulation Appendix B EASI Model Output Appendix C 2007 Pebble Count Data Appendix D Responses to Stakeholder Comments on Draft Report List of Figures Figure 1-1 Overview map of the Sultan River basin, Project features, and the study area. ............................................................................................. 2 Figure 1-2 Operational reach (OR) and process reach (PR) juxtaposition downstream of Culmback Dam. River miles are noted in the horizontal bars....................................................................................... 3 Figure 2-1 Debris flow and deep-seated landslides that contribute sediment to the Sultan River channel in the study area.................................................. 7 Figure 2-2 Cross-section comparison at the Kien’s Bar Site, XS 1. Elevation is relative to the Rock BM elevation (200 ft) given in the 1984 GeoEngineers survey. ......................................................................... 19 Figure 2-3 Cross-section comparison at the Kien’s Bar Site, XS 2. Elevation is relative to the Rock BM elevation (200 ft) in XS 1. Due to the lack of surveying benchmarks at this location from the 1984 GeoEngineers survey, the two cross-sections were aligned in the horizontal based on the measured bank retreat of 21.49 ft at XS 1. Note change in Y-axis scale relative to Figure 2-2.................................................................. 20 Figure 2-4 Cross-section comparison at the Kien’s Bar Site, XS 3. Elevation is relative to the Rock BM elevation (200 ft) in XS 1. Due to the lack of surveying benchmarks at this location from the 1984 GeoEngineers survey, the two cross-sections were aligned in the horizontal based on the measured bank retreat of 21.49 ft at XS 1..................................... 20 Figure 2-5 Cross-section comparison at the Chaplain Site. Elevation is relative to the Rock BM elevation (200 ft) given in the 1984 GeoEngineers survey.................................................................................................. 21 Figure 2-6 Cross-section comparison at the Upper Site. Elevation is relative to the BM 1 and BM 2 elevations derived from the 1984 GeoEngineers survey plot........................................................................................... 22 Figure 2-7 Reservoir bathymetry for the sediment deposit behind the Diversion Dam (July 2007). The solid blue lines indicate the water surface elevation at the time of measurement. ................................................ 24 Figure 2-8 Valley bottom terrace mapping in process reach 1. The low terraces represent former channel boundaries, now abandoned by the river. .. 30 Figure 2-9 The 1965, 1983, and 2003 Sultan River channel planform overlain onto the 1919 USGS topographic map. This figure illustrates how the Page ii Study 22: Sultan River Physical Process Studies Technical Report June 2008 Jackson Hydroelectric Project channel planform of the Sultan River has been relatively stable since before Stage 1 of the Project............................................................... 31 Figure 2-10 Vegetation encroachment of side channel 1 (~RM 0.9) between 1965 and 2003.............................................................................................. 32 Figure 2-11 Vegetation encroachment of side channel 2 (~RM 1.1) and Kien’s Bar site (RM 1.2) between 1965 and 2003................................................ 32 Figure 2-12 Vegetation encroachment