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Island Ponds Mitigation Monitoring and Reporting Year 10 – 2015

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Island Ponds Mitigation Monitoring and Reporting Year 10 – 2015 ISLAND PONDS MITIGATION MONITORING AND REPORTING YEAR 10 – 2015 PREPARED BY: Santa Clara Valley Water District U.S. Fish and Wildlife Service— Don Edwards National Wildlife Refuge TABLE OF CONTENTS Page No. EXECUTIVE SUMMARY ........................................................................................................ E-1 1.1 INTRODUCTION ............................................................................................................. 1 1.2 PROJECT BACKGROUND ............................................................................................. 1 1.3 PROJECTS WHICH REQUIRED MITIGATION ............................................................... 1 Initial Stewardship Plan (ISP) .......................................................................................... 1 Stream Maintenance Program (SMP) .............................................................................. 2 Lower Guadalupe River Project (LGRP) .......................................................................... 2 1.4 ISLAND PONDS MITIGATION SITE ............................................................................... 3 Site Description ............................................................................................................... 3 Mitigation Monitoring ....................................................................................................... 3 Performance Criteria ....................................................................................................... 5 1.5 CONTACTS..................................................................................................................... 6 2.1 MONITORING METHODS .............................................................................................. 7 2.2 ON-SITE MONITORING .................................................................................................. 7 2.2.1 Aerial Photography (District) ................................................................................ 7 2.2.2. Biennial Aerial Photography Vegetation Mapping (Refuge) .................................. 7 2.2.3 Ground-Based Quantitative Vegetation Sampling (District) .................................. 7 2.2.4 Channel Network Evolution Monitoring (Refuge) .................................................. 8 2.2.5 Levee Breach and Outboard Marsh Channel Geometry (District) ......................... 8 2.2.6. Wildlife Monitoring (Refuge) ................................................................................. 8 2.3 OFF-SITE MONITORING .............................................................................................. 10 2.3.1 Accelerated Deterioration of the Town of Drawbridge (District) .......................... 10 2.3.2 Fringe Marsh Scour in Coyote Creek ................................................................. 10 3.1 MONITORING RESULTS .............................................................................................. 12 3.2 ON-SITE MONITORING ................................................................................................ 12 3.2.1 Biennial Aerial Photography Vegetation Mapping (Refuge) ................................ 12 3.2.2 Ground-Based Quantitative Vegetation Sampling (District) ................................ 13 3.2.3. Channel Network Evolution Monitoring (Refuge) ................................................ 15 3.2.4. Levee Breach and Outboard Marsh Channel Geometry (District) ....................... 16 3.2.5 Wildlife Monitoring (Refuge) ............................................................................... 16 3.3 OFF-SITE MONITORING .............................................................................................. 17 3.3.1 Accelerated Deterioration of the Town of Drawbridge (District) .......................... 17 3.3.2 Fringe Marsh Scour in Coyote Creek (District) ................................................... 17 4.0 DISCUSSION ................................................................................................................ 19 5.0 REFERENCES .............................................................................................................. 21 6.0 LIST OF PREPARERS .................................................................................................. 23 7.0 FIGURES ...................................................................................................................... 24 8.0 APPENDICES i LIST OF TABLES Table 1-1. Mitigation Monitoring Schedule for the Island Ponds – Responsible Party, Monitoring Duration, Frequency and Timing. Table 3-1. Comparison of Established Marsh Vegetation at the Island Ponds from 2006 – 2015 Table 3-2. Quantitative Vegetation Sampling of Pond Surfaces (Marsh Plain) in 2015 Table 3-3. Channel Networking in Island Ponds from 2006-2015 Table 3-4. Pond Breach Widths (feet) over Time Table 3-5. Marsh Loss (acres) from Scour of Outboard Channels Table 3-6. Fringe Marsh Scour (acres) over Time Table 3-7. Fringe Marsh Accretion (acres) over Time Table 4-1. Extent of Vegetation Establishment in Pond A21 from 2012-2015 LIST OF FIGURES Figure 1. Vicinity Map- Location of the Island Ponds Restoration Site Figure 2. Digitized Vegetation from Satellite Imagery in Pond A21, Comparison over Several Years Figure 3. Digitized Vegetation from Satellite Imagery in Pond A20, Comparison over Several Years Figure 4. Digitized Vegetation from Satellite Imagery in Pond A19, Comparison over Several Years Figure 5. Channel Geometry in Pond A21 Borrow Ditch, 2015 Compared to Previous Years Figure 6. Channel Geometry in Pond A20 Borrow Ditch, 2015 Compared to Previous Years Figure 7. Channel Geometry in Pond A19 Borrow Ditch, 2015 Compared to Previous Years Figure 8. Salt Marsh Harvest Mouse Trapping Results at Pond A21 Borrow Ditch, 2015 Figure 9. Locations of Audio and Visual Confirmation of Ridgway’s Rails, Pond A21 in July 2015 Figure 10. Marsh Gain and Loss from Scour of Outboard Channels, 2010-2015 APPENDICES Appendix A. Site Photos Appendix B. Wildlife Survey Reports Appendix C. Pond A21 Levee Erosion Data and Photographs ii EXECUTIVE SUMMARY The Santa Clara Valley Water District (District) and the U.S. Fish and Wildlife Service Don Edwards National Wildlife Refuge (Refuge) implemented the Island Ponds Restoration Project to fulfill two goals: 1) to initiate ecological restoration activities as described in the South Bay Salt Pond Initial Stewardship Plan (ISP), and 2) to satisfy the tidal marsh mitigation needs of both the Refuge for the ISP, and the District for the Stream Maintenance Program and the Lower Guadalupe River Project. Breaching of the Island Ponds (Ponds A19, A20, and A21) occurred in March 2006. Five breaches were cut to allow tidal waters to inundate the ponds and begin the process of restoration. In the Restoration and Mitigation Monitoring Plan for the Island Pond Restoration Project (RMMP), the District and the Refuge agreed to conduct monitoring to track the progress of the restoration. This report presents the Year 10 (2015) monitoring results for both the District and the Refuge. Ten years after breaching, sediment is continuing to accumulate and consolidate to form acceptable substrate for vegetation colonization in all three Ponds. Vegetation growth has shown a rapid expansion; currently approximately166 acres of marsh vegetation has collectively established in the three ponds, which represents a 15% increase from last year. In Pond A21 alone, approximately118 acres of marsh vegetation has established, with an average total cover of 75.85%. Based on criteria outlined in the RMMP, Pond A21 has met the final success criteria for the mitigation project as a whole (75 acres at 75% vegetative cover). Coupled with the documentation in 2015 of salt marsh harvest mice along the borrow ditch of Pond A21 and the audio confirmation of a pair of breeding Ridgway’s rails, this area has become functional marsh habitat for endangered species as well. There has been a significant increase in channel network evolution in the ponds since the last time data were analyzed in 2010. The largest increase was in Pond A19, with approximately 7% (18 acres) of the pond surface exhibiting a network of drainage channels. This represents an 85% increase since 2010 for that pond. This is an excellent development in the trajectory of vegetation establishment in that pond, as in the RMMP the development of channel networks was highlighted as an important indicator of progress in the deposition of sediment and subsequent vegetation establishment as well as habitat for species such as Ridgway’s rail. While Pond A19 still contains the least amount of vegetated marsh plain on the pond surface, it is likely that vegetation establishment will be rapid in subsequent years. Vegetation composition on the marsh plain of all three ponds continues to be dominated by several native species, including perennial pickleweed and California cordgrass, and to a lesser extent, annual pickleweed. Vegetation on pond levee shorelines was more mixed; with perennial pickleweed and California cordgrass as well as some areas showing establishment of nonnative Salsola and small amounts of perennial pepperweed. As sediment accretion continues, management of nonnative vegetation may need to become a priority to prevent establishment of these or other invasive species on the marsh plain. The levee shoreline of Pond A19 still contains a population of dwarf spikerush, a California Rare Plant Rank 4 species. This population
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