Bolinas Lagoon Ecosystem Restoration Feasibility Project Final Public Reports II Projecting the Future of Bolinas Lagoon 3KLOLS:LOOLDPV $VVRFLDWHV/WG :HWODQG5HVHDUFK$VVRFLDWHV PROJECTING THE FUTURE EVOLUTION OF BOLINAS LAGOON Final Public Draft Prepared for Marin County Open Space District with funding from The California Coastal Conservancy Prepared by Philip Williams & Associates, Ltd. with WRA, Inc. July 2006 PWA REF. # 1686.02 July 2006 Services provided pursuant to this Agreement are intended solely for the use and benefit of the Marin County Open Space District. No other person or entity shall be entitled to rely on the services, opinions, recommendations, plans or specifications provided pursuant to this agreement without the express written consent of Philip Williams & Associates, Ltd., 720 California Street, 6th Floor, San Francisco, CA 94108. P:\Projects\1686.02_BolinasLagoonERP-Phase2b\Task08_Reporting\PublicDraft_07.06\Bolinas_FinalPublicDraft_July2006.doc July 2006 TABLE OF CONTENTS Page No. 1. INTRODUCTION 1 2. FINDINGS AND CONCLUSIONS 3 2.1 Summary of Key Findings 3 2.2 Specific Conclusions 4 3. PAST EVOLUTION OF BOLINAS LAGOON 9 3.1 Sediment Budget, Sediment Dynamics and Equilibrium Form 9 3.2 Natural Evolution of Bolinas Lagoon 12 3.2.1 Evolution over Geologic Time and the Role of Large Earthquakes 12 3.2.2 Mechanisms of Sediment Delivery 13 3.2.3 Relative Magnitudes of Littoral and Alluvial Sources 16 3.2.4 The Tidal Inlet and Estuarine Processes 16 3.2.5 Ecological Response to Changing Physical Conditions 17 3.3 19th Century Lagoon Conditions 18 3.3.1 Conditions Prior to Significant Disturbances 18 3.3.2 Land Use Changes in the 19th Century 21 3.4 The 1906 Earthquake and Its Effects On Sediment Delivery 22 3.4.1 Subsidence During the 1906 Earthquake 22 3.4.2 20th Century Sedimentation Rates and Sources 26 3.4.3 Accelerated Tidal Dispersion of Littoral Sediments 27 3.5 Recent Changes to Lagoon Morphology 29 3.5.1 Sediment Accumulation and Marsh Expansion 29 3.5.2 Artificial Fill 33 3.6 Historic Changes To Tidal Prism 33 4. EXISTING LAGOON CONDITIONS 38 4.1 Active Physical Processes and Sediment Dynamics 38 4.1.1 Sediment Delivery from Littoral and Alluvial Sources 38 4.1.2 Wind-Wave Erosion Over Exposed Mudflats 42 4.1.3 Tidal Dispersion of Beach Sands 42 4.2 The Present Form of Bolinas Lagoon 46 4.3 Existing Ecological Processes 48 4.3.1 Lagoon Habitats 48 4.3.2 Lagoon Species Diversity and Abundance 55 4.4 Field Studies: Characterizing Baseline Ecological Conditions 64 P:\Projects\1686.02_BolinasLagoonERP-Phase2b\Task08_Reporting\PublicDraft_07.06\Bolinas_FinalPublicDraft_July2006.doc i July 2006 5. FUTURE EVOLUTION OF BOLINAS LAGOON 65 5.1 Methodology for Projecting Future Lagoon Conditions 65 5.2 Changes in the Sediment Budget 66 5.2.1 Changes to Inputs and Outputs 66 5.2.2 Changes in Storage Terms 68 5.3 Changes in Sediment Dynamics 68 5.4 Evolution of Geomorphic Units 70 5.5 Cumulative Changes in Lagoon Morphology 79 5.6 Changes in Tidal Prism and Inlet Stability 81 5.7 Trends in Lagoon Evolution Beyond 50 Years 85 5.8 Potential Ecological Changes 87 5.8.1 Expected Shifts in Habitat Distribution and Abundance 87 5.8.2 Effect of Habitat Types on Ecological Function 92 6. HUMAN-INDUCED CHANGES TO LAGOON CONDITIONS 94 7. MONITORING AND ADAPTIVE MANAGEMENT 97 7.1 Managing in the Face of Uncertainty 97 7.2 Establishing Management Goals and Objectives 100 7.3 Making Management Plans Operational 101 7.4 Suggested Monitoring at Bolinas Lagoon 103 7.4.1 Monitoring Physical Elements 104 7.4.2 Monitoring Biological Elements 106 8. ACKNOWLDGEMENTS 109 9. REFERENCES 110 LIST OF APPENDICES Appendix A – Field Studies: Establishing Baseline Conditions Appendix B – Tidal Datums, Benchmarks and Estimates of Historic Tidal Prism Appendix C – Estimating Watershed Delivery LIST OF TABLES Table 3-1. Habitat Distribution in 1854 21 Table 3-2. Habitat Distribution in 1929 25 Table 3-3. Average Rate of 20th Century Tidal Prism Loss 37 Table 4-1. Selected Estuarine Invertebrates of Bolinas Lagoon 56 Table 4-2. Fish Species Documented to Occur in Bolinas Lagoon 60 Table 5-1. Projected Change in Lagoon Morphology 81 Table 5-2. Results of Inlet Stability Analysis 85 Table 6-1. Anthropogenic Activities and Their Effect on Tidal Prism 96 P:\Projects\1686.02_BolinasLagoonERP-Phase2b\Task08_Reporting\PublicDraft_07.06\Bolinas_FinalPublicDraft_July2006.doc ii July 2006 LIST OF FIGURES Figure 1-1. Location Map 2 Figure 3-1. Sediment Budget Framework 10 Figure 3-2. Concept of Punctuated Dynamic Equilibrium 11 Figure 3-3. Physical Processes Affecting Bolinas Lagoon 14 Figure 3-4. The Lagoon Survey of 1854 19 Figure 3-5. 19th Century Lightering Wharfs and Barges 20 Figure 3-6. The Lagoon Survey of 1929 23 Figure 3-7. Lagoon Morphological Changes: 1854-1929 24 Figure 3-8. Potential Strength of Tidal Dispersion After the 1906 Quake 28 Figure 3-9. Late 20th Century Marsh Expansion in the South Arm 30 Figure 3-10. Late 20th Century Progradation of Pine Gulch Creek and Kent Island Marshes 31 Figure 3-11. Late 20th Century Marsh Expansion in the North Basin 32 Figure 3-12. Construction of Seadrift Lagoon 34 Figure 3-13. Modifications along Highway 1 35 Figure 3-14. Tidal Prism Change: 1854 to Present 36 Figure 4-1. Locations of Measured Tidal Currents 39 Figure 4-2. Tidal Asymmetry at Inlet and Tidal Channels 40 Figure 4-3. Eroded Silt Transported through Inlet During Flood Tides 41 Figure 4-4. Suspended Sediment Delivery by Pine Gulch Creek 43 Figure 4-5. Mudflat Profiles: 1968-2004 44 Figure 4-6. Declining Strength of Tidal Dispersion with Reduction in Tidal Prism 45 Figure 4-7. Distribution of Morphologic Units and Habitat Types: Year 0 47 Figure 5-1. Potential Future Reduction in Tidal Dispersion Strength 67 Figure 5-2. Potential Sea Level Rise 69 Figure 5-3. Sheltering of Wind Waves in Bolinas Lagoon 71 Figure 5-4. Location of UC Berkeley Sediment Cores 73 Figure 5-5. Vertical and Horizontal Accretion in the North Basin 74 Figure 5-6. Calculating the Volumetric Change of Pine Gulch Creek Delta: 1968 – 1998 76 Figure 5-7. Radial Expansion of Pine Gulch Creek Delta 77 Figure 5-8. Linear Extension and Fetch Blocking Effects of Kent Island 78 Figure 5-9. Distribution of Morphologic Units and Habitat Types: Year 50 80 Figure 5-10. Changes to Lagoon Elevation-Area Characteristics 83 Figure 5-11. Fluctuations in Inlet Stability due to Changing Tides and Ocean Waves 84 Figure 5-12. Potential Long-Term Equilibrium Condition 86 Figure 6-1. Marsh Expansion Due to the Progradation of Pine Gulch Creek Delta 95 Figure 7-1. The Adaptive Management Process 98 Figure 7-2. An Adaptive Management Framework for Bolinas Lagoon 99 Figure 7-3. Using 'Operational' Conceptual Models to Establish Management Actions 102 P:\Projects\1686.02_BolinasLagoonERP-Phase2b\Task08_Reporting\PublicDraft_07.06\Bolinas_FinalPublicDraft_July2006.doc iii July 2006 GLOSSARY OF TERMS Aggradation vertical expansion of a geomorphic feature Alluvium material, such as sand, silt or clay, deposited on land by streams Alluvial fan the fanlike deposit of a stream where it issues from a gorge upon a plain or of a tributary stream near or at its junction with its main stream Anthropogenic of or pertaining to humans Bedload sediment transported by water close to or on the bottom by rolling, sliding or bouncing Brackish a mixture of fresh and salty water Delta a body of alluvium having a surface that is nearly flat and fan shaped; deposited at or near the mouth of a river or stream where it enters a body of relatively quiet water, generally a sea or lake El Nino a major warming of the equatorial waters in the Pacific Ocean; El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns Episodic occurring, appearing, or changing at usually irregular intervals Estuary an embayment of the coast in which fresh river water entering at its head mixes with the relatively saline ocean water Fluvial of or pertaining to rivers; produced by river action, as a fluvial plain Geomorphic the form and process of landforms Holocene a period of time covering the past 10,000 years Hypersaline very high salinity Incision erosion and down-cutting of channels by low sediment-high energy discharges Intertidal pertaining to the elevations between those of MHHW and MLLW; undergo wetting and drying consistent with tidal propagation Lagoon a shallow body of water, separated from the sea by a sand-spit; lagoons may be continuously, occasionally or periodically connected to the ocean Littoral of or related to the littoral zone; the relatively narrow portion of a coast affected by wave energy and longshore currents Morphology characterization of form and process Neap tides the tides resulting when the sun and moon are at right angles to each other, characterized by a reduced tidal range Prograde lateral expansion of a geomorphic feature Sedimentary formed by or from deposits of sediment Spring tides tides resulting when the gravitational forces exerted on the earth by the sun and moon are acting in the same direction Subtidal pertaining to the elevations consistently below MLLW Suspended load volume of sediment transported in suspension by water; moves at a velocity slightly lower than that of water without many intermittent stages of deposition Supratidal pertaining to the elevations consistently below MHHW Tectonics describing the forces that cause the movements and deformation of Earth’s crust on a large scale P:\Projects\1686.02_BolinasLagoonERP-Phase2b\Task08_Reporting\PublicDraft_07.06\Bolinas_FinalPublicDraft_July2006.doc iv July 2006 Tidal damping attenuation (reduction) of the tidal range Tidal prism the volume of water between the high-tide surface and the low-tide surface of an embayment, estuary or creek Suspended load term relating sediment to river flow; consists of small particles generally in suspension and not found on the bottom even during low energy intervals P:\Projects\1686.02_BolinasLagoonERP-Phase2b\Task08_Reporting\PublicDraft_07.06\Bolinas_FinalPublicDraft_July2006.doc v July 2006 1.