Final Cumulative Impacts Analysis Skamania County November 2020 Final Cumulative Impacts Analysis Skamania County Shoreline Master Program Update Submitted to Skamania County Stevenson, Washington November 2020 Submitted by WSP USA Inc. 210 East 13th Street, Suite 300 Vancouver, Washington 98660 A15.0208.02 FINAL CUMULATIVE IMPACTS ANALYSIS Skamania County Shoreline Master Program Update TABLE OF CONTENTS SECTION PAGE SUMMARY ............................................................................................................................................ IV INTRODUCTION ......................................................................................................................... 1 1.1 Purpose ......................................................................................................................... 1 1.2 State Requirements ..................................................................................................... 2 1.3 Scope ............................................................................................................................ 3 NATURAL PROCESSES.............................................................................................................. 4 CURRENT CONDITIONS ............................................................................................................. 8 3.1 Current Watershed Conditions .................................................................................... 9 3.1.1 WRIA 26 Cowlitz .............................................................................................. 9 3.1.2 WRIA 27 Lewis ...............................................................................................10 3.1.3 WRIA 28 Salmon-Washougal ........................................................................10 3.1.4 WRIA 29 Wind-White Salmon........................................................................10 3.1.5 Impervious Surfaces ......................................................................................16 3.2 Current Shoreline Conditions ....................................................................................19 FUTURE DEVELOPMENT .........................................................................................................26 4.1 Shoreline Environment Designations ........................................................................26 4.2 Development Trends ..................................................................................................30 4.2.1 Population Growth .........................................................................................31 4.2.2 Developable Land ..........................................................................................32 4.3 Foreseeable Development .........................................................................................35 4.3.1 Inside GPNF ....................................................................................................35 4.3.2 Outside GPNF .................................................................................................38 DEVELOPMENT IMPACTS AND SMP PROVISIONS ................................................................42 5.1 Main Tools for Protecting Shorelines ........................................................................42 5.1.1 Shoreline Environment Designations ...........................................................42 5.1.2 Critical Areas Regulations and Buffers ........................................................42 5.1.3 Vegetation Conservation Standards .............................................................43 5.2 Residential Development...........................................................................................44 5.2.1 Effects ............................................................................................................44 5.2.2 SMP Provisions ..............................................................................................45 5.3 Forest Practices .........................................................................................................46 5.3.1 Effects ............................................................................................................46 5.3.2 SMP Provisions ..............................................................................................47 5.4 Recreational Development ........................................................................................47 5.4.1 Effects ............................................................................................................47 Skamania County WSP USA Inc., 310000043 Final Cumulative Impacts Analysis November 2020 Ecology Grant No. G1500044 Page i of vii 5.4.2 SMP Provisions ..............................................................................................47 5.5 Agriculture ..................................................................................................................48 5.5.1 Effects ............................................................................................................48 5.5.2 SMP Provisions ..............................................................................................48 5.6 Aquaculture ................................................................................................................49 5.6.1 Effects ............................................................................................................49 5.6.2 SMP Provisions ..............................................................................................49 5.7 Mining .........................................................................................................................49 5.7.1 Effects ............................................................................................................49 5.7.2 SMP Provisions ..............................................................................................49 5.8 Stormwater .................................................................................................................50 5.8.1 Effects ............................................................................................................50 5.8.2 SMP Provisions ..............................................................................................50 5.9 Shoreline Stabilization ...............................................................................................51 5.9.1 Effects ............................................................................................................51 5.9.2 SMP Provisions ..............................................................................................51 5.10 Boating Facilities and Overwater Structures ...........................................................51 5.10.1 Effects ............................................................................................................51 5.10.2 SMP Provisions ..............................................................................................51 5.11 Nonconforming Development ....................................................................................53 5.11.1 Effects ............................................................................................................53 5.11.2 SMP Provisions ..............................................................................................53 5.12 Other Impacts .............................................................................................................53 OTHER PROGRAMS .................................................................................................................66 6.1 County Programs ........................................................................................................66 6.2 State Regulations ......................................................................................................66 6.3 Federal Regulations ...................................................................................................68 6.4 Non-Regulatory Programs ..........................................................................................69 NO NET LOSS ANALYSIS .........................................................................................................79 7.1 Vegetation Conservation ...........................................................................................80 7.1.1 Vegetation Conservation in Science and Ecology Guidance .......................80 7.1.2 Vegetation Conservation Provisions in the SMP .........................................82 7.2 How No Net Loss Is Achieved ....................................................................................84 7.3 Conclusions ................................................................................................................87 REFERENCES ..........................................................................................................................88 LIST OF TABLES Table 1. Changes in Population and Housing Units, 2010 to 2015 ................................................... 8 Table 2. Impervious Surface Percentages within Shoreline Jurisdiction by Watersheds .............. 16 Table 3. Summary of Characteristics ................................................................................................ 21 Table 4. Approximate Length of Lake Shoreline in each Shoreline Environment Designation ...........................................................................................................................