Snohomish River Confluence Reach Analysis Phase 1 Feasibility Study Final Report Snohomish County Department of Public Works Surface Water Management July 16, 2003 Snohomish River Confluence Analysis CONTENTS EXECUTIVE SUMMARY ........................................................................................................VIII 1. INTRODUCTION AND BACKGROUND ..............................................................................1 1.1 NEED FOR CHANNEL, FLOODPLAIN, AND HABITAT RESTORATION IN THE CONFLUENCE REACH...........................................................................................................7 1.2 OVERVIEW OF CONFLUENCE REACH ANALYSIS ........................................................8 2. SNOHOMISH RIVER WATERSHED AND STUDY AREA DESCRIPTION ....................10 2.1 FISHERIES RESOURCES .......................................................................................................12 3. METHODS ..............................................................................................................................15 3.1 SURVEYING, MAPPING, DATA COLLECTION .......................................................................16 3.1.1 Topography/Photogrammetry................................................................................17 3.1.2 Bathymetry ............................................................................................................17 3.1.3 Fish Habitat and Riparian Mapping ......................................................................19 3.2 GEOMORPHIC ANALYSIS ....................................................................................................19 3.2.1 Channel Classification...........................................................................................19 3.2.2 Historic Change Assessment .................................................................................20 3.3 HYDROLOGIC ANALYSIS ....................................................................................................20 3.4 HYDRAULIC ANALYSIS.......................................................................................................22 3.5 BIOLOGICAL/HABITAT ANALYSIS ......................................................................................27 3.6 RIPARIAN ANALYSIS ..........................................................................................................28 3.7 ALTERNATIVES ANALYSIS..................................................................................................30 3.7.1 Hydraulic Modeling...............................................................................................30 3.7.2 Salmon Production Potential Estimation...............................................................33 4. RESULTS ................................................................................................................................34 4.1 SURVEY, MAPPING, DATA COLLECTION.............................................................................34 4.2 GEOMORPHIC ANALYSIS ....................................................................................................34 4.2.1 Level I Classification and Delineation ..................................................................34 4.2.2 Level II Classification and Delineation.................................................................34 ii Phase 1 Feasibility Study Final Report 1286.03/SnohomishRivConfluenceAnal_071603 July 16, 2003 Snohomish River Confluence Analysis 4.2.3 Level III Analysis..................................................................................................36 4.3 HYDROLOGIC ANALYSIS ....................................................................................................44 4.4 HYDRAULIC ANALYSIS.......................................................................................................46 4.5 BIOLOGICAL/HABITAT ANALYSES .....................................................................................46 4.6 RIPARIAN ANALYSIS ..........................................................................................................47 4.6.1 Land Use and Cover Type.....................................................................................47 4.6.2 Riparian Zone and Large Woody Debris Characteristics......................................52 4.7 ALTERNATIVES ANALYSIS..................................................................................................58 4.7.1 Hydraulic Modeling...............................................................................................58 4.7.2 Salmon Production Potential Estimation...............................................................62 5. DISCUSSION..........................................................................................................................67 5.1 INFLUENCE OF GEOMORPHIC AND RIPARIAN SETTING ON RESTORATION POTENTIAL ........67 5.1.1 Geomorphic Conditions.........................................................................................67 5.1.2 Riparian Conditions...............................................................................................68 5.2 HYDRAULIC MODELING AND ALTERNATIVES ANALYSES...................................................70 5.2.1 Model Refinements ...............................................................................................70 5.2.2 Model Scenarios/Alternatives Analyses................................................................71 5.3 HABITAT CONDITIONS AND BENEFITS ................................................................................72 6. INITIAL RECOMMENDATIONS FOR RESTORATION ALTERNATIVES ANALYSES.............................................................................................................................74 6.1 CAPITAL PROJECTS.............................................................................................................74 6.1.1 Dike Breaching at Thomas' Eddy (Beck Dike) and Reconnection of Lake Beecher .................................................................................................................75 6.1.2 Twin Rivers Slough Reconnection........................................................................75 6.1.3 Bank Armor Removal Along Twin Rivers Park ...................................................76 6.1.4 Selected Breaching of Crabb’s Dike .....................................................................76 6.2 RECOMMENDED RIPARIAN RESTORATION PROJECTS..........................................................77 7. REFERENCES ........................................................................................................................78 iii Phase 1 Feasibility Study Final Report 1286.03/SnohomishRivConfluenceAnal_071603 July 16, 2003 Snohomish River Confluence Analysis FIGURES Figure 1. Snohomish River valley diking and flood control districts, Snohomish County, Washington…………………………………………………………………....... 2 Figure 2. Public land holdings in the Confluence Reach of the Snohomish River...............4 Figure 3. Important infrastructure, public and private land features located in the Confluence Reach of the Snohomish River. Many of these features are possible subjects of future restoration efforts........................................................6 Figure 4. Snohomish River basin and sub-basins, Snohomish and King Counties, Washington..........................................................................................................11 Figure 5. Aerial photograph mosaic taken in 2001 of the Confluence Reach of the Snohomish River, and locations of ground control points used for orthorectification. ................................................................................................18 Figure 6. Locations of cross-sections used to determine river widths as part of the review of historic aerial photography of the Confluence Reach of the Snohomish River. Approximate river mile location is indicated. ......................21 Figure 7. Two-dimensional (2D) hydrodynamic model representation of the Confluence Reach of the Snohomish River, showing mesh used to predict depths and velocities during overbank flooding. The mesh coverage defines the model "domain." Light blue areas are outside the 100 year floodplain.............................................................................................................23 Figure 8. Topography of the Confluence Reach of the Snohomish River as represented by the 2D hydrodynamic model.......................................................24 Figure 9. Substrate material assignments to mesh elements of the 2D hydrodynamic model of the Confluence Reach of the Snohomish River. ..................................26 Figure 10. Potential CIP restoration alternatives identified in this report that can be evaluated using the 2-D hydrodynamic model of the Confluence Reach of the Snohomish River. Transect cross-section profiles are depicted in Figure 11..............................................................................................................31 Figure 11. Representative cross-sections at potential CIP restoration alternative locations in the Confluence Reach of the Snohomish River. Cross-section locations are indicated in Figure 10.....................................................................32 iv Phase 1 Feasibility Study Final Report 1286.03/SnohomishRivConfluenceAnal_071603 July 16, 2003 Snohomish River Confluence Analysis Figure 12. Locations of river bed substrate