US ARMY CORPS OF ENGINEERS LOS ANGELES DISTRICT
Morro Bay Estuary City of Morro Bay, San Luis Obispo County, CA
Ecosystem Restoration Feasibility Study
F3 Conference Report Without-Project Conditions Analysis
July 2003 Draft - F3 Conference Report
Syllabus
This report presents an analysis of existing conditions and projected future without project conditions that examines ecosystem needs and opportunities at the Morro Bay Estuary in Morro Bay, San Luis Obispo County, California. The goal of the analysis is to identify restoration alternatives that will restore significant ecosystem function, structure, and dynamic processes that have been degraded. Restoration recommendations will be formulated to support the goals and objectives of ongoing USEPA/NEP, regional, and local watershed planning and management efforts.
Morro Bay is a transitional freshwater-to-marine aquatic environment whose channels and eelgrass beds provide valuable foraging and juvenile rearing habitat for marine fishes, as well as potential migration corridors for steelhead. The Bay supports rich invertebrate populations, including commercially and recreationally important bivalves. The open waters, eelgrass beds, and tidal flats of the Bay provide a critical stopover area for migratory and wintering shorebirds and waterfowl, and provide habitat for a number of threatened, endangered, and otherwise sensitive species of plants, fish and wildlife. The salt and brackish marshes of the Bay, upstream riparian corridors, and surrounding dune scrub and other upland habitats, both natural and manmade, also provide critical habitat for a number of threatened, endangered, and otherwise sensitive species of plants, fish and wildlife. The relative proportion and location of habitats are expected to change under the without project condition due to anthropogenic influences in the watershed.
The analysis of without project conditions indicates that if historic sedimentation trends and reduction of tidal prism continue, subtidal habitat and shallow to low-intertidal mudflats that support eelgrass beds in the Morro Bay Ecosystem will continue to diminish. Eelgrass supporting mudflats are likely to shrink as marsh-building continues at the mouths of the creeks, while increased elevation of upper tidal marshes will further isolate them from tidal influence. Overall, the existing balance of habitats within the bay would become less marine-estuarine and more wetland-like and freshwater riparian.
The scope of this report includes physical, economic, and environmental baseline data, and projections of future habitat conditions throughout the 50-year study period. Ecosystem problems and restoration opportunities identified in the analysis are used in the preliminary screening of alternative improvement measures. This report identifies and presents a preliminary evaluation of potential structural and non-structural ecosystem restoration and aquatic habitat protection measures. The analysis supports the decision to proceed with the feasibility study as presented in the concluding section of the report, based upon the projected significant anthropogenic, sediment-induced change in estuary habitats.
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Table of Contents
1 Introduction...... 1
1.1 Study Authority...... 1
1.2 Study Purpose and Scope...... 1
1.3 Study Participation and Coordination...... 2
1.4 Prior Studies and Corps Projects...... 2
1.4.1 Prior Studies by the Corps ...... 2
1.4.2 Prior Studies by Others ...... 3
1.4.3 Existing Corps Projects...... 4
2 Study Area ...... 4
2.1 Location and Description...... 4
2.2 Geology...... 8
2.3 Topography...... 8
2.4 Climate...... 9
2.5 Hydrology ...... 9
2.6 Coastal Processes...... 10
2.7 Land Use ...... 11
2.7.1 Urban Lands...... 11
2.7.2 Parklands...... 12
2.7.3 Morro Harbor...... 14
2.7.4 Agriculture ...... 14
2.8 Socio-Economic Profile...... 14
2.8.1 Population ...... 14
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3 Environmental Resources ...... 16
3.1 Water Quality...... 16
3.2 Sediment Characteristics...... 18
3.2.1 Physical Characteristics ...... 18
3.2.2 Preliminary Hazardous, Toxic, and Radioactive Waste Database Search...... 19
3.3 Biological Resources ...... 19
3.3.1 Open-water and Sub-tidal Habitats...... 23
3.3.2 Eelgrass Beds and Related Habitat ...... 24
3.3.3 Mudflat Habitat...... 25
3.3.4 Salt Marsh Habitat ...... 26
3.3.5 Fringing Wetland Habitat ...... 27
3.3.6 Riparian Habitats ...... 28
3.3.7 Upland Habitats ...... 29
3.3.8 Special Status Species...... 31
4 Sedimentation Analysis ...... 31
4.1 Historical Sedimentation...... 32
4.2 Littoral Transport...... 34
4.3 Fluvial Transport...... 34
4.4 Aeolian Transport ...... 34
4.5 Sediment Budget...... 35
5 Future Without Project Conditions ...... 35
5.1 Projected Sediment Delivery and Deposition...... 36
5.2 Projected Change in Morro Bay Habitats ...... 38
5.2.1 Habitat Change Modeling...... 38
5.2.2 Summary Future Habitat Acreage – Average Yield Scenario...... 45
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5.3 Projected Habitat Values...... 47
5.3.1 Habitat Evaluation Method...... 47
5.3.2 Changes in Habitat Units ...... 48
5.4 Without Project Condition - Summary ...... 49
6 Plan Formulation...... 50
6.1 Problems and Opportunities Summary ...... 50
6.1.1 Problems and Opportunities: Excessive Sedimentation Within the Bay...... 51
6.1.2 Problems and Opportunities: Aeolian Sand Deposition ...... 52
6.1.3 Problems and Opportunities: Human Interference ...... 52
6.1.4 Problems and Opportunities: Water Quality...... 53
6.2 National Objective ...... 53
6.3 Public Opinion ...... 54
6.4 Planning Objectives ...... 54
6.5 Planning Constraints ...... 55
6.6 Alternative Measures Considered...... 56
6.6.1 Alternative Measures: No Action ...... 58
6.6.2 Alternative Measures: Within the Bay...... 58
6.6.3 Alternative Measures: Within the Watershed...... 59
6.6.4 Summary of Alternatives to be Considered for Further Analysis...... 60
6.7 Decision to Proceed ...... 60
7 References...... 61
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List of Tables
Table 1 Population Trends: City of Morro Bay and Community of Los Osos...... 15
Table 2 Demographic Profile of the City of Morro Bay and Community of Los Osos ...... 15
Table 3 Employment by Industry (2001)...... 16
Table 4 Simplified Annual Sediment Budget (after Haltiner 1991)...... 35
Table 5 Simulated Cumulative Fluvial Sediment Yield (tons)...... 37
Table 6 GIS Model Decision Rules: Elevation Ranges and Restrictions ...... 40
Table 7 Differences between Mapped and Modeled Habitat Areas for Existing Conditions...... 42
Table 8 Habitat Configuration Summary for Year-50 Under the Without Project Condition: Total Modeled Bay Area ...... 45
Table 9 Habitat Valuation Summary for Year-50 Under the Without Project Condition: Total Bay Area Model Space ...... 49
List of Figures
Figure 1 Morro Bay Study Area ...... 5
Figure 2 Morro Bay Watershed ...... 7
Figure 3 Morro Bay Park and Recreation Lands ...... 13
Figure 4 Morro Bay Habitats: Existing Conditions ...... 21
Figure 5 Designated Critical Habitats for Federally Listed Species...... 22
Figure 6 Four Physiographic Zones...... 33
Figure 7. Map Of Existing Morro Bay Habitat Polygons With Zone Map Overlay ...... 43
Figure 8. Map Of Modeled Existing Morro Bay Habitats With Zone Map Overlay...... 44
Figure 9 Modeled Habitat Configuration Under Future Without Project Conditions (Year 50).. 46
Figure 10 Potential Watershed Project Locations...... 57
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List of Appendices
Appendix A: Habitat Change Model (GIS) Appendix
Appendix B: Environmental Appendix (Affected Environment)
Appendix C: Essential Fish Habitat Appendix
Appendix D: Habitat Evaluation Appendix
Appendix E: Engineering Appendix
Appendix F: Geotechnical Appendix
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1 Introduction This report presents an analysis of existing conditions and projected future without project conditions that examines ecosystem needs and opportunities at the Morro Bay Estuary in Morro Bay, San Luis Obispo County, California. Ecosystem problems and restoration opportunities identified in the analysis are used in the preliminary screening of alternative improvement measures. This report identifies and presents a preliminary evaluation of potential structural and non-structural ecosystem restoration and aquatic habitat protection measures. The analysis supports the decision to proceed with the feasibility study as presented in the concluding section of the report, based upon the projected significant anthropogenic, sediment-induced change in estuary habitats.
1.1 Study Authority
This study was authorized by U.S. House Committee on Transportation and Infrastructure Resolution dated 7 May 1997 which states:
“ Resolved by the Committee on Transportation and Infrastructure of the United States House of Representatives, that the Secretary of the Army is requested to review the report of the Chief of Engineers on Morro Bay Harbor, San Luis Obispo County, California published as House Document 103-33, 103rd Congress, 1st Session, and other pertinent reports to determine whether modifications of the recommendations contained therein are advisable at the present time in the interest of environmental protection and restoration and related purposes within the Morro Bay Estuary in Morro Bay, California. In conducting the study, the Secretary shall consider the problems related to sedimentation of sensitive habitat and tidal circulation restrictions in the estuary.”
1.2 Study Purpose and Scope
The purpose of this Feasibility Study is to formulate an engineeringly sound and economically feasible solution to the potentially adverse environmental effects of sedimentation, tidal circulation and flushing restrictions, and degradation of valuable open-water and inter-tidal habitat within the Morro Bay Estuary. The goal of the analysis is to identify restoration alternatives that will restore significant ecosystem function, structure, and dynamic processes that have been degraded. Restoration recommendations will be formulated to support the goals and objectives of ongoing USEPA/NEP, regional, and local watershed planning and management efforts. The scope of this report includes physical, economic, and environmental baseline data, and projections of future habitat conditions throughout the 50-year study period. The preliminary plan formulation process presented in this report considers alternatives that focus on the reestablishment of a naturalistic, functioning, and self-regulating ecosystem within the estuary. The alternatives considered include structural and non-structural improvement measures.
Ecosystem restoration is one of the primary missions of the Corps’ Civil Works Program. Identification of ecosystem restoration needs and opportunities and preliminary plan formulation are conducted in recognition of the dynamics of natural systems. The analysis considers the
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inter-connectedness between human activities and natural systems in the estuary and the watershed as proscribed in Civil Works Ecosystem Restoration Policy, ER 1165-2-501, 30 September 1999, Ecosystem Restoration Supporting Policy Information, EP 1165-2-502, 30 September 1999, Economic and Environmental Principles for Water and Related Land Resources Implementation Studies, February 3, 1983; and the Planning Guidance Notebook, ER 1105-2- 100, 22 April 2000.
1.3 Study Participation and Coordination
San Luis Obispo County in partnership with the Bay Foundation of Morro Bay, a local non-profit organization that administers the Morro Bay National Estuary Program (MBNEP), and the MBNEP are the non-Federal sponsors of this study. There has been a significant level of agency and public interest and involvement in identifying ecosystem problems and opportunities in the Morro Bay Estuary. Bay Foundation of Morro Bay and MBNEP officials participated in numerous meetings and site visits. MBNEP officials also provided significant data and information and participated in the data search effort. A Technical Advisory Committee (TAC) was formed including representatives from the National Marine Fisheries Service, the U.S. Fish and Wildlife Service, State of California, Department of Fish and Game, Central Coast Regional Water Quality Review Board, Morro Coast Audubon Society, City of Morro Bay Harbor Department, MBNEP, and other interested individuals. TAC meetings were held to support problem and opportunity identification, to confirm findings of field investigations and data analysis, and to develop a habitat evaluation method.
A public meeting was held at the Inn at Morro Bay on 01Nov01. The meeting was attended by members of the general public, the US Army Corps of Engineers, Los Angeles District, a representative from Congresswoman Lois Capps’ office (Democrat – 23rd District), the Mayor of the City of Morro Bay, a representative from the California National Guard, and representatives from numerous state and local agencies and academic institutions.
1.4 Prior Studies and Corps Projects
Prior studies conducted by the Corps have focused on the Federal Navigation Channel at Morro Harbor, with the exception of the expedited reconnaissance report conducted for this project. The sole existing Corps project in the study area is the Federal Navigation Channel that has a maintenance dredging cycle of two years. Each of these prior studies and the existing project is identified below. Numerous studies by others have been conducted in the study area concerning water quality, sedimentation, environmental conditions, and archeological and cultural resources. The major studies relied upon for this report are listed below. A full listing of all reference materials is included as an appendix.
1.4.1 Prior Studies by the Corps Most of the studies conducted by or commissioned by the Corps were focused on the navigation channel. A list of recent Corps’ reports is provided below. A full listing of Corp’s reports, dating back to 1873 is provided in the References Section.
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• Garcia, A. W., 2001, “Morro Bay Harbor, California, Wave and Current Data Summary Report”, ERDC/CHL, TR-01-21, Coastal and Hydraulics Laboratory, U. S. Army Engineer Research and Development Center, Vicksburg, MS.
• Noda, E. L. and Y. Jen, 1975, “Sand Transport Analysis, Morro Bay”, Prepared for the U. S. Army COE, L.A. District.
• US Army Corps of Engineers, Los Angeles District. 1990. Final Environmental Assessment, Morro Bay Harbor, Maintenance Dredging Project, FY-1990, San Luis Obispo County, California. Los Angeles.
• U. S. Army Corps of Engineers, Los Angeles District, 1994, “Design Memorandum for Navigation Improvements, Morro Bay Harbor, San Luis Obispo County, Ca.”
• U. S. Army Corps of Engineers, Los Angeles District, 1998, “Expedited Reconnaissance Study, Section 905 (b) Analysis, Morro Bay Estuary”.
1.4.2 Prior Studies by Others Prior studies that have been heavily relied upon in this report are listed below. A full set of references are provided in the References Section.
• Central Coast Regional Water Quality Control Board (CCRWQCB). 2002d. Morro Bay Total Maximum Daily Load for Sediment (including Chorro Creek, Los Osos Creek, and the Morro Bay Estuary). Prepared on February 28, 2002
• Haltiner, J.P. 1988. Sedimentation Processes in Morro Bay, California. Prepared by Philip Williams & Associates, Ltd. for the Coastal San Luis Resource Conservation District of the California Coastal Conservancy.
• Haltiner, J.P. and D. Thor. 1991. Sedimentation Processes in Morro Bay, California. Proceedings: Coastal Sediments Conference, ASCE Waterway, Ports, Coastal, and Ocean Division, Seattle, WA, June 25-27 1991.
• Haltiner, J.P. and Devin, T. 1992. Sedimentation Processes in Morro Bay, California. Pages 7-1.1 to 7-2.22. Published in B. Russel and J. Richards, editors. In. Morro Bay: State of the Bay Conference Proceedings. October 12, 1992. Prepared by the Morro Bay Task Force.
• Local Policy Committee and Watershed Committee for the Morro Bay National Estuary Program. 1999. “Turning the Tide”, Comprehensive Conservation and Management Plan for Morro Bay.
• Morro Bay National Estuary Program. 2000. Characterization Document. July, 2000.
• Morro Bay National Estuary Program (MBNEP). 2000. Comprehensive Conservation and Management Plan. Prepared by the MBNEP. Published July 2000.
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• Philip Williams and Associates, Ltd. 2002. Morro Bay Sedimentation: Historical Changes and Sediment Management Opportunities to Extend the Life of the Bay. Prepared for the Central Coast Regional Water Quality Control Board (CCRWQCB).
• Tetra Tech. 1998. Sediment Loading Study. Prepared for the Morro Bay National Estuary Program
1.4.3 Existing Corps Projects Federal projects within the study area include the Federal Navigation Channel at Morro Harbor and associated breakwaters. The Federal channel runs from an area just outside of the breakwaters, at the entrance to Morro Bay, to Fairbank Point for a distance of approximately three-quarters of a mile. The channel is maintained to a depth of 15 feet and varies in width from 200 feet to 50 feet. The most recent regularly scheduled maintenance was conducted in 2001. Dredged material consisted mainly of beach quality sand that was deposited locally.
2 Study Area A map of the study area is presented in Figure 1.
2.1 Location and Description
The Morro Bay Estuary, commonly known as Morro Bay, is a naturally shallow lagoon located in San Luis Obispo County on the central coast of California approximately 100 miles south of Monterey Bay and 60 miles north of Point Conception. Morro Bay is approximately four miles long (in a north-south direction) and less than 2 miles wide (in an east-west direction) at its widest point. The area within the bay, including all wetlands and aquatic habitats below or adjacent to the area of high tide inundation, is approximately 2,300 acres. The bay is open to the ocean at the Federally maintained entrance channel to Morro Harbor. The entrance to the bay is protected by two breakwaters. A four mile long by one-quarter mile wide barrier beach runs along the western edge of Morro Bay and connects to the mainland at the southern end. This sand spit separates Morro Bay from the Pacific Ocean. A narrow causeway constructed between Morro Rock and the mainland in the 1930’s closed a natural opening to the sea at the northern end of the bay.
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Figure 1 Morro Bay Study Area
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The City of Morro Bay (pop. 10,350), which includes Morro Harbor, is located along the north- eastern shores of the bay. The Baywood Park and Cuesta-by-the-Sea neighborhoods (included within the unincorporated community of Los Osos, pop. 14,400) are located along the south- eastern shores of the bay. The southern most shores of the bay and the sand spit that forms the bay’s western shore are included in Montana De Oro State Park.
Duke Energy Morro Bay LLC operates an electric generation plant installed in the 1950’s on the former Navy base site adjacent to the causeway connecting Morro Rock with the mainland. The plant has been in nearly continuous operation for more than 50 years. A permit application for plant modernization has been submitted to the California Energy Commission. Modernization would increase plant efficiency, relocate plant facilities away from the shoreline, and reduce cooling water withdrawal volumes. Currently, and under the proposed plant modernization, the plant withdraws cooling water from Morro Bay at the existing intake structure adjacent to the causeway and discharges through existing discharge tunnels to Estero Bay, north of the Morro Bay entrance channel. Average cooling water withdrawals are 437 million gallons per day (1997 – 2001). Projected withdrawals after modernization are 328 million gallons per day (Duke, 2002).
Fresh water sources for the bay include Chorro Creek, Los Osos Creek, and numerous springs emanating from remnant dunes along the south eastern shores and from dunes along the southern and western shores. The Morro Bay watershed (Figure 2), (48,450 acres, 75.7 sq miles) is mainly comprised of the Chorro Creek basin (28,194 acres) and the Los Osos Creek basin (15,011 acres). Approximately 5,246 acres of the Morro Bay watershed drain directly into the bay from its shoreline.
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Figure 2 Morro Bay Watershed
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2.2 Geology
Morro Bay and its watersheds (Chorro and Los Osos Creek) are located in the southernmost portion of the Coast Range Physiographic Province – a sequence of northwest trending mountains extending approximately 60 miles inland from the Pacific coast to the San Joaquin Valley. The range extends north to Oregon and is bounded by the Transverse Range to the south. A chain of extinct volcanic plugs (hardened magma sealing the interior chambers of ancient volcanoes) – known as the “Morros” – run in an east/west direction and divide the two coastal valleys that drain into Morro Bay. Morro Rock, the westerly most visible plug within this chain, is a Tertiary age, intrusive igneous rock, composed of dacite (Duke 2000). Erosion of the Franciscan Formations formed the Chorro and Los Osos valleys, which are also aligned in an east/west direction. The subsurface geology consists of a combination of igneous, metamorphic and sedimentary rock of Jurassic Age overlain by younger sediments of Tertiary and Quaternary Age. Serpentine bearing rocks occur throughout the Morro Bay watershed where they have been exposed by erosion. Although these rocks are not found in the study area, they are found in the watershed and form a habitat for certain plants critical to the area.
Morro Bay, which is less than 15,000 years old, is located on a low-lying coastal terrace that is part of a structural depression that is southeast-trending through the Los Osos and San Luis Valleys. The bay was formed by the submergence of the river mouth at the confluence of Chorro and Los Osos Creeks in the last 10,000 to 15,000 years. This submergence was a result of the Holocene (post-glacial) rise in the sea level. The terrace is underlain by bedrock of the Franciscan Formation at elevations of –50 to –80 feet (National Geodetic Vertical Datum). The bay is surrounded by Quaternary age alluvial and sand dune deposits.
Active faults within the Morro Bay watershed include the Hosgri and Los Osos Faults. The Los Osos Fault is the nearest to the study area. The fault is a 56 to 60 kilometer, west-northwest, southwest dipping, reverse fault which separates the San Luis Range from the Morro Bay Structural Basin.
2.3 Topography
The Morro Bay watershed consists of two sub-watersheds. The Chorro Creek sub-watershed, which drains approximately 60 percent of the larger Morro Bay watershed, is comprised of several other sub-basins that include: San Bernardo, San Luisito, Walters, Pennington, Dairy, and Upper Chorro Creeks, the latter of which contains the Chorro Reservoir. The Los Osos Creek sub-watershed, which drains the remaining 40 percent of the Morro Bay watershed, contains two sub-basins – Warden Creek and Clark Valley. The watershed’s highest elevation is 2,763 ft above sea level and the farthest point is about 10 miles from the Bay. The Morro Bay watershed is bordered by Point Buchon to the south and the Estero Bay watershed to the north.
Chorro Creek and Los Osos Creeks are typical of most coastal streams in California, where the upstream gradients near the headwaters are steeper than three percent (3%) and the lower reaches exhibit less than 0.3 percent gradients near their respective bay outlets (PWA 2002). The
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headwaters of the Chorro Creek watershed lie on the south-facing slopes of the Santa Lucia Mountains west of Cuesta Grade. Chorro Creek flows southward into the Chorro Reservoir, turns westward at Highway 1 and proceeds until it reaches Morro Bay. The headwaters of the Los Osos Creek watershed are located on the north-facing slopes of the Irish Hills. The headwaters drains into the Clark Valley and the Warden Creek sub-basins.
Stream profiles of the upper reaches of both Chorro and Los Osos Creeks exceed 3%, but are less than 0.3% along the lower reaches near the bay outlet. The profile gradient of Warden Creek ranges from about 1.5% to 0.2% for the upper and lower reaches, respectively (Tetra Tech, 1998). For both Chorro Creek and Los Osos Creek watersheds, approximately 68% of watershed soils are fine silts and clays, and the remaining 32% are coarser sands and gravels. An important distinction should be noted that about 56% of the coarser sandy soil and 14% of the finer clay is found in the western Los Osos Creek watershed.
2.4 Climate
Annual average precipitation for areas under the marine influence is approximately 17.70 inches, with about 90 percent of the precipitation occurring between November and April. Cool, wet winters and warm, dry summers with occasional marine layer intrusion characterize the coastal portion of the watershed. Annual average maximum and minimum temperatures recorded from 1971 to 2000 are approximately 64.8 and 47.3 (°F), respectively. The San Luis Obispo area, at a lower elevation and 15 miles inland from the coast, has annual average maximum and minimum temperatures of 71.8 and 47.4 (°F), respectively and the annual average total precipitation is approximately 24.83 inches (Western Regional Climate Center, 2002). Average maximum temperatures are lower at Morro Bay due to the influence of winds and fog, and average minimum temperatures are similar between Morro Bay and San Luis Obispo..
2.5 Hydrology
Two major stream flow discharges into Morro Bay were estimated by Tetra Tech (1998) to establish the flow rates for various extreme events that occurred in the Chorro and Los Osos Creek watersheds. Due to the limited stream and rain gage data, the hydrologic model study conducted by Tetra Tech estimated all episodic discharges (cfs) from Chorro and Los Osos Creeks for various extreme events. Average flows ranging from 1,476 cfs to 35,390 cfs for 2-yr and 100-yr events, respectively, were estimated at the mouth of Chorro Creek. Similarly, the average flows ranging from 84 cfs to 7,994 cfs for 2-yr and 100-yr events, respectively, were estimated at the mouth below Warden Creek of the Los Osos Creek system. The stream flow rates varied as the season changed. The peak flow rates, as calculated upstream of Highway 1 of Chorro Creek and also at the upstream gage of Los Osos Creek, are the major driving forces for the sediment transport from these watersheds into the Morro Bay estuary.
Sediment deposition at the mouths of Chorro and Los Osos Creeks and during episodes of overbank flow has profoundly influenced the estuary and continues to configure riparian and estuarine habitats. The history of change has been reconstructed by a number of authors, including Chipping (1979), Haltiner (1988), Morro Group and Tenera (1990), Josselyn et al. (1991), and Phil Williams and Associates (2002). Chorro Creek has been the primary source of
Morro Bay Estuary Ecosystem Restoration Feasibility Study 9 Draft - F3 Conference Report sediment upon which mudflats, shallow eelgrass beds, salt marsh, and eventually riparian habitats have developed and replaced what was formerly shallow subtidal habitat.
Chorro Creek, from at least the 1930s until the mid-1990s, principally flowed into the estuary in a channel on the northern edge of the delta. A second distributary channel, which meanders to the south, to a confluence with Los Osos Creek, was historically important and contributed sediment from Chorro Creek along a different pathway (Chipping 1979). The channelization of Chorro Creek across Chorro Flats and the construction of South Bay Boulevard and the Twin Bridges may have helped force the flow of Chorro Creek into the northern channel. A dense, willow-dominated riparian woodland developed downstream of the Twin Bridges in this area and remains today. As documented by Josselyn et al. (1991), sedimentation raised elevations within the northern channel and along the banks and adjacent overflow areas to above tidal elevations, 1-2 feet above those in the tidal distributary channel and surrounding tidal marsh plain to the south. By 1990, areas of degraded riparian habitat, dominated by non-native weeds, had established on sediment deposits surrounding the northern channel (Josselyn et al. 1991).
From the summer of 1994 until early 1995, before construction of the new bridge over Chorro Creek, the buildup of sediment and growth of willows below the Twin Bridges in the northern channel caused a diversion of creek flows southward into the aforementioned distributary channel. In February 1995, during a period of heavy runoff, flow was observed backing up behind the upper (northern) of the Twin Bridges, and a gully was rapidly eroding through road fill along the upstream side of the road, scouring out the distributary channel where it passed under the lower (southerly) of the old Twin Bridges and continuing downstream. That episode appeared to mark the creek’s “abandonment” of the north channel, and “recapture” of the creek by the south distributary channel. This occurred before construction of the new bridge, which spans both the northern and southern distributary channels. At present, riparian habitat, as indicated by young arroyo willows, is spreading downstream along the banks of the southern distributary channel.
2.6 Coastal Processes
The coastal hydraulics of the Morro Bay Estuary are dominated by the motions of tide and wave, which are associated with the littoral transport of sediment inside and outside of the harbor. The measured tidal elevation (Tetra Tech, 1999) at the Morro Bay entrance indicates that the diurnal constituents are the dominant contributors to tidal motion in Morro Bay. The average tidal range between MHW and MLW is 3.61 ft.
Wave data were measured from Sept. 1998 through Aug. 2000 (Garcia, 2001) at the stations located inside and outside Morro Bay Harbor. The significant wave heights inside Morro Bay Harbor are generally less that 0.2 m. The predominant waves, corresponding to the wave height exceeding 1.3m, are approximately 275 degrees azimuth. Peak wave periods occasionally exceed 20 seconds. The wave-induced current plays a very important role in coastal sediment transport process.
The most updated surface current data (Garcia, 2001) measured by USACE in the entrance channel of Morro Bay range from 40 cm/sec to 150 cm/sec during flood and ebb tide conditions, respectively. The peak flood and ebb current speeds are in the order of 50cm/sec and 100
Morro Bay Estuary Ecosystem Restoration Feasibility Study 10 Draft - F3 Conference Report cm/sec, respectively. The mid-depth and near-bottom current speeds have about 90% and 70% of surface current speed, respectively. The field measurement results carried out by Tetra Tech in 1974 (Noda and Jen, 1975) and by the City of Morro Bay in 1989 generally agree with those of the USACE measurements in 1999. Suspended sediments in the water column are subject to the effect of tidal flow and are transported throughout the estuary.
2.7 Land Use
This section examines the land use patterns and policies in the feasibility study area. The discussion in this section focuses on land uses that might be affected by restoration plans (alternatives) identified for the feasibility study. The information in this section was obtained largely from San Luis Obispo County’s Estero Area Plan (SLO 1995) and the City of Morro Bay’s General Plan/Coastal Land Use Plan (CMB 2000).
The study area lies within the SLO County’s Estero Planning Area, which encompasses the central coastal areas of SLO County from Point Estero on the north to Point Buchon on the south, and from the coast inland to Los Padres National Forest. The Estero Planning Area is one of 15 geographic sub-areas within the County. The Estero Area Plan governs land use and development on unincorporated communities within the planning area (i.e., excluding the City of Morro Bay). The Estero Area Plan is currently being updated and amended.
The City of Morro Bay’s General Plan/Local Coastal Plan provides guidance for local decisions relative to community growth and the relative to character of land use and future development within its boundaries. The Plan was recently updated.
The primary land uses in the study area are urban lands, multi-use public lands (especially State park lands), marine/harbor related activities (especially commercial fishing and shellfishing), and agricultural uses.
2.7.1 Urban Lands The City of Morro Bay and the unincorporated community of Los Osos have populations of about 10,350 and 14,400, respectively. Both communities strive to maintain their “small town character” and have strong interest in protecting, enhancing, and restoring the communities’ aesthetic and natural environments.
The City of Morro Bay is one of seven incorporated cities in San Luis Obispo County. The City’s goal is to maintain its small town character as a unique fishing village in a rural setting. The City’s framing within the Morro and Chorro Valleys and the Pacific Ocean contributes to this unique character and its significant aesthetic and natural resources. Residential densities range from low (4 dwelling units/acre) to high (15-27 units/acre). Much of the City, especially the northern part, was subdivided years ago into small lots, many less than 3000 square feet. By voter initiative, the City has a maximum population limit set at 12,200.
Tourism is a major contributor to the local economy. The City’s Waterfront/Embarcadero area serves as the hub of activity for tourist and visitors by providing a variety of recreational
Morro Bay Estuary Ecosystem Restoration Feasibility Study 11 Draft - F3 Conference Report opportunities, coastal access areas, and numerous visitor services. The entire Embarcadero area provides visitors and residents with direct access to Morro Bay and the working fishing harbor.
The community of Los Osos consists of a several loose-knit South Bay neighborhoods that include Baywood Park, Los Osos, and Cuesta-by-the-Sea. Single-family homes dominate residential use in the area. The community of Los Osos includes a wide variety of residential neighborhoods ranging from the older tract areas developed in the late 1800’s with 25-foot wide lots, to hillside homes with lots ranging from 10,000 square feet to an acre. The unique neighborhoods blend to form the community character of the South Bay. Locally valued aesthetic and natural resources are found throughout and adjacent to the community.
2.7.2 Parklands There are over 10,000 acres of State and City parks and beaches visited by hundreds of thousands of visitors annually, in the vicinity of the study area. These public lands (Figure 3) were established to protect the unique natural coastal environment.
Morro Bay State Park. The park encompasses about 2,100 acres of coastal saltmarsh and coastal chaparral and about 40,000 linear feet of bay frontage. The Park includes Morro Rock and the Black Mountain Natural Area. The park provides recreational opportunities, such as birdwatching, camping, hiking, picnicking, golfing (on an 18-hole golf course), and boating. Within the park there are 135 overnight campsites that could accommodate 550 to 600 overnight campers. There are also 50 picnic or day-use sites. During the peak period of May through December the campsites and day-use facilities are often filled to capacity.
Montana de Oro State Park. This park consists of over 8,000 acres of rugged cliffs, secluded beaches, and coastal plains with over 21 miles of coastline. The most prominent geographic feature of the park in the study area is the Morro Bay sand spit. Access to the sand spit is primarily by boat. It provides abundant habitat for coastal and bay wildlife.
Morro Strand State Beach. This state coastal frontage beach is located upcoast of Morro Rock and consist of 75 acres with about 3 miles of shoreline. The beach has 104 campsites. The area in the vicinity of Morro Rock is often used by surfers.
Coleman, Tidelands and Bayshore Bluffs Parks. These three Morro Bay city parks are located on bay frontage. All are important public access points to the bay and preserve the natural features of the bay shore.
Sweet Springs Nature Preserve. Located on the southern edge of Morro Bay, this site is owned by the Morro Coast Audubon Society. Sweet Springs Preserve is 24 acres, with 1 acre of spring- fed freshwater ponds and bordering marsh; 14 acres of tidal open water, mudflat, and salt marsh habitats; and 9 acres of uplands. The area provides habitat for several sensitive, threatened and endangered species, including salt marsh birds beak, California red-legged frog, and Morro Bay manzanita.
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Figure 3 Morro Bay Park and Recreation Lands
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2.7.3 Morro Harbor The waterfront area is the focal point of local tourism. The City of Morro Bay’s history revolves around the waterfront area. This area has typically served as a melding of commercial fishing and ocean dependent industry, along with visitor uses (e.g., restaurants, gift shops) and recreational uses (e.g., fishing, boating,).
Morro Bay provides a critical resource to fishing and recreational boating industries. The area within the City of Morro Bay’s limits that is covered by seawater (from the mouth of the bay to the southern City limits) is designated “Harbor and Navigational Ways” by the City of Morro Bay to protect the fishing, boating, and other uses that depend upon the harbor for their existence. Harbor dependent uses include mariculture (commercial shellfish farming), commercial and recreational boating and support facilities, open space for navigation, habitat preservation, and passive recreational usage.
2.7.4 Agriculture Although not much agricultural land use occurs within the boundaries of the City of Morro Bay (about 300 acres), it is the dominant land use in the SLO County’s Estero Planning Area with about 75% of the Planning Area designated for agriculture. Mixed irrigated and dry farm croplands (including row crops and orchards) occupy most of the valley lowlands, while cattle grazing dominates the more hilly areas in the Estero Planning Area. These uses are largely interrelated because much of the farmland produces irrigated and dry farm grain and hay for supplemental livestock feed. An estimated 65% of the designated agricultural lands in the Estero Planning Area are in agricultural preserves and subject to land conservation contracts.
The California Coastal Act of 1976 contains a comprehensive set of policies advocating the preservation of agriculture lands, especially “prime” agricultural lands (Public Resources Code, Section 30241). Both the Estero Area Plan and the City of Morro Bay’s General Plan have as its policy to maintain the maximum amount of “prime” agricultural land.
2.8 Socio-Economic Profile
This section describes the social and economic characteristics of the feasibility study area and the immediately surrounding region. This section contains information provided by the California Energy Commission (CEC, 2001). As stated under Land Use, (see section 2.7), the study area is within San Luis Obispo’s Estero Planning Area and includes the unincorporated community of Los Osos and the City of Morro Bay.
2.8.1 Population Recent population figures for San Luis Obispo County (2000 Census) report a population of about 247,000 for the county. The City of Morro Bay has a population of about 10,350. The population trends for the City of Morro Bay and the community of Los Osos are presented in
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Table 1. By voter initiative, the City of Morro Bay has a maximum population limit set at 12,200.
Table 1 Population Trends: City of Morro Bay and Community of Los Osos
Year City of Morro Bay Community of Los Osos
1990 9,664 14,377
2000 10,350 14,351
2010 10,552 16,639
2020 11,371 18,380
Sources: 2000 Census and San Luis Obispo Council of Governments
Table 2 presents a demographic and income profile of the City of Morro Bay and the community of Los Osos. The population in the study area has fewer minority residents and a smaller percentage of the population living in poverty status, than the overall San Luis Obispo County population. The median household income in the City of Morro Bay is below the median County income.
Table 2 Demographic Profile of the City of Morro Bay and Community of Los Osos 1990 2000 Los San Luis Morro Los San Luis Race/ethnicity Morro Bay Osos Obispo Co. Bay Osos Obispo Co. White (not Hispanic) 92.9% 88% 81.6% 83.4% 88.3% 76.1% Black 0.8% 0.4% 2.3% 0.6% 0.6% 1.9% American Indian 1.6% 0.8% 0.9% 0.8% 0.7% 0.6% Asian 1.9% 3.9% 2.8% 1.7% 4.6% 2.6% Hispanic 8.0% 7.0% 12.4% 11.4% 9.0% 16.3% Other 2.8% 0.1% 0.1% 2.0% 2.5% 2.4% Median income $26,049 $35,392 $31,480 $28,324 $35,380 N/A
% Poverty status 10% 7.2% 12.1% 8.1% 5.0% N/A Source: from CEC 2001, page 3.5-3., U.S. Census Bureau, Census 2000 (http//factfinder.census.gov) Note: Hispanic totals for 1990 include several race categories – so totals do not add up to 100%. NA = Not Available
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Employment information is only available at the county level The following information pertains to San Luis Obispo County. The leading industrial categories in San Luis Obispo County are services, construction trade, and government. The distribution of employment by industrial sector for the County is shown in Table 3.
Table 3 Employment by Industry (2001) SECTOR SLO County Morro Bay Los Osos Services 23,900 3,109 4,716 Construction Trade 23,600 ------Government 22,400 293 584 Manufacturing 7,500 258 502 Construction 5,400 373 407 Finance/Insurance/Real Estate 4,900 119 392 Transportation/Public Utilities 4,900 133 227 Agriculture/mining 4,200 173 103
Source: as reported in CEC (2001), U.S. Census Bureau, Census 2000 (http//factfinder.census.gov); self- employed are not included.
3 Environmental Resources
3.1 Water Quality
The Morro Bay watershed and estuary have been recipients of sediment from such sources as agricultural lands, rangeland, abandoned mines, urban areas, and eroding stream banks. The Morro Bay estuary, Chorro, and Los Osos Creeks are listed as “impaired” by the Central Coast Regional Water Quality Control Board (CCRWQCB) due to sedimentation, nutrients, and pathogens (CCRWQCB 1993 as cited by Lombardo, et al. 2001). The CCRWQCB has established Total Maximum Daily Loads (TMDLs) for the watershed for several of the pollutants.
The major water quality issues identified for the Morro Bay watershed, as identified in MBNEP (2000) and CCRWQCB (2002), are: sedimentation via erosion, bacterial contamination, nutrient enrichment, and heavy metals in sediment, however, heavy metals may not be as important as previously assumed. The California Department of Health Services has monitored fecal coliform levels in Morro Bay for the protection of shellfish since 1953. Bacteria levels in the Bay have exceeded the legal allowable limit and have resulted in portions of bay’s oyster beds to be closed due to the elevated fecal coliform levels.
The predominant sources of bacterial contamination in the bay are failing septic systems, recreational boaters, urban and agricultural runoff, and wastewater treatment plant operations. Some sources discharge bacteria only during storms, while others discharge year-round.
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Historically, bacteria levels have been high in Chorro and Los Osos creeks directly above the outlet to the bay, in Cuesta and Baywood Park inlets and the central part of the bay (i.e., the northern part of the oyster harvesting area.) Recent DNA “fingerprinting” of E. coli (fecal coliform) in Morro Bay estuary indicates that the largest fractions of this bacteria came from: birds (22%), humans (17%), bovines (14%), and dogs (9%) (CCRWQCB 2000c:21).
The CCRWQCB recently (2002c) established a concentration-based TMDL for pathogens in Morro Bay and the Chorro and Los Osos Creeks. Fecal coliform standards for all creeks and groundwater seeps that flow into Morro Bay are identified to protect the beneficial use of commercial shellfishing and water-contact recreation in the bay. In 1986 a chlorination process was initiated for the Morro Bay/Cayucas wastewater treatment plant. More recently, the Los Osos Community Services District is conducting efforts to reduce and eliminate bacterial contamination in the Bay. The Los Osos Community Services District is in the design phase of developing a community wastewater collection, treatment, and disposal system. The CCRWQCB has adopted a “Time Schedule Order” which requires the Los Osos Community to resolve water quality problems caused by its septic systems.
High levels of nutrients enter the Morro Bay estuary from tributary creeks, shoreline seepage from septic tanks, and from surface runoff and groundwater discharge. The sources of the nutrients found in Morro Bay are quantified in Tetra Tech’s Nutrient Loading and Circulation Study Report (1999), which simulated six scenarios of nutrient (phosphates and nitrates) cycling in the bay using a hydrodynamic circulation model under wet-weather conditions. The intent of the simulation model was to provide land managers with information on where to focus attention for implementation of potential management practices to reduce the sources of nutrient enrichment into the bay. The study reported three major findings.
• Nutrients from Chorro Creek are the largest source of nutrients to the bay, contributing approximately 86% of the total nitrogen and 94% of total phosphorus load.
• Groundwater nutrient loadings from the Los Osos community have a significant effect on nitrate concentration in the back (south) bay area. Faulty/leaky septic tanks are the primary sources of these nitrates. The upper aquifer within much of the Los Osos and Baywood park area exceeds the 45 mg/l State Standard for Domestic Use.
• “First flush” of nutrients into the bay following a rainfall event can have significant localized effects on the water quality.
The CCRWQCB’s (2002e) TMDL report establishes numeric thresholds for nutrients (nitrate, total nitrogen, ortho-phosphate, and total phosphorus) and dissolved oxygen levels in the Morro Bay watershed. Elevated nutrient levels that violate drinking water standards and may exceed levels protective for fisheries, two beneficial uses identified for Chorro Creek by the CCRWQCB (CCRWQCB & SWRCB, 1994), have been documented in Chorro and Los Osos Creeks. The nutrients and dissolved oxygen TMDL numeric targets are intended (CCRWQCB & SWRCB 1994) to protect cold water fisheries habitat (i.e. dissolved oxygen will not be reduced to below 7.0 mg/l at any time). In addition, the nutrient TMDL targets were also set to meet the Municipal and Domestic Supply objective of the Basin Plan for Chorro and Los Osos Creeks.
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Wastewater treatment for the Los Osos community may help to control and reduce the adverse affects of nutrient enrichment of the bay.
3.2 Sediment Characteristics
Preliminary analysis of sediment characteristics include an analysis of sediment grain size and a database search concerning potential sources of sediment contamination.
3.2.1 Physical Characteristics Sediment sampling and analysis of the Chorro and Los Osos Creek delta and surrounding areas was conducted during November 2002. Eighteen samples obtained at depths varying from 1.26 to 3.05 meters were collected using a vibracore or a push-tube (Macro Core Geoprobe Sampler) mounted onto a 20-foot by 40-foot shallow draft pontoon barge. Sixteen samples were obtained by a barge-mounted vibracore and the remaining two samples were collected by a push-tube. Three of the samples were taken in the northern portion of the Delta which is salt marsh. One of the samples was obtained in the extreme southern edge of the delta. Another sample was obtained near the mouth of one of the distributary channels at the west edge of the salt marsh. The sediment samples are classified as silt, silty sand and fine sand and silt, typical for deltaic deposits from low order streams.
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A review of previously gathered data suggests sand–sized materials have accumulated along broad channel meanders, while finer sediments that are generally silts, silty clays and fine sands, have accumulated in the vicinities of sharply turned channels. These suggested conditions have been confirmed by the classification of the sediments sampled for this investigation. The materials encountered at holes located on the east edge of Morro Bay are described as sand/silty sand. The materials encountered at holes located at the mouth of the main channel of Chorro Creek where it enters the east side of Morro Bay and the beginning of the Estuary are described as silty and silty sand. The remaining holes sampled in both the mud flats and the salt marsh encountered very fine-grained sediments, described as silts, clayey silts and sandy silts. There was some stratification noted in the holes drilled in west side of the bay. As expected, there was little to no stratification observed in the holes sampled in the Chorro Delta area. In a delta, stratification is very often random and inconsistent (not presented in "layer-cake" fashion) and somewhat difficult to recognize.
3.2.2 Preliminary Hazardous, Toxic, and Radioactive Waste Database Search A search of various computer databases, existing U. S. Geological Survey topographic maps and aerial photographs was conducted, in the summer of 2001, to find present and former toxic waste sites at or in the vicinity of Chorro and Los Osos Creek delta. The search extended from Morro Rock and the Duke Energy (PG&E) Power Plant to the north to about 1 mile south of Baywood Park – Los Osos on the south, the Sand Spit to the west and about ½ mile east of the estuary for the eastern limits. Eighty sites were identified within this area. These sites are generally gas stations, dry cleaning facilities, sea food processing plants, auto repair facilities, the power plant, those stores which do photo developing and printing, and underground storage tanks. The majority of the sites are located either in the City of Morro Bay, or in the communities of Baywood Park and Los Osos. There are no major or gross sources of pollution reported within the study area.
The search indicates that there are no toxic, radioactive or hazardous waste sites located in the Estuary, nor are there any major sources of pollution such as oil refineries, superfund sites, animal slaughterhouses or oil sumps/waste pits in the study area.
3.3 Biological Resources
The Morro Bay Estuary is a unique mosaic of closely interrelated habitats (Figure 4), linked by physical and biological processes. The Morro Bay Estuary is a designated State and National Estuary and has considerable ecological significance for many reasons, including:
• It is the largest semi-enclosed bay, supporting the most ecologically diverse estuarine system, on the central California coast;
• It provides extensive habitat, and spawning and/or nursery grounds, and thereby helps to sustain populations of fish and shellfish that are of commercial and recreational importance;
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• Its marshes, mudflats and protected shallow waters support thousands of migratory and wintering shorebirds and waterfowl, making it one of the key stopover areas for birds migrating along the Pacific flyway;
• It supports the largest and most important remaining eelgrass beds between San Francisco and San Diego Bays. In addition to providing habitat for a variety of fishes and invertebrates, the eelgrass in Morro Bay supports the largest gathering of migratory and wintering black brant between northern California and Baja California; and
• The estuary and surrounding dunes, freshwater wetlands, riparian corridors, wooded shorelines, and Morro Rock provide habitat for more than 20 threatened, endangered, and other special status species.
The diversity of plant, invertebrate, fish, and wildlife species in and around Morro Bay reflects both the variety and extent of habitats present (Mooney 1992; Putnam 1992; Morro Group and Tenera 1990; Ruppert 1992; Worcester 1992; Tetra Tech 1998). The fishes and wildlife using the estuary include habitat specialists and generalists. Habitat generalists include shorebirds and other waterfowl that move between habitats with the tides. Several rare and endangered habitat specialists, among them the salt marsh bird’s beak, California sea-blite, and the black rail, are closely tied to a single habitat or unique combination of habitat features. Approximately 2,300 acres of mudflats, tidal wetlands and open water habitats in the estuary support many sensitive plant and animal species (MBNEP 2000a). State and federally listed plants and wildlife known or expected to occur in the project vicinity are listed and discussed further in the Environmental Appendix. Figure 4 shows the designated critical habitats for federally listed species. An Essential Fish Habitat Appendix also accompanies this document.
The Morro Bay Estuary is also notable for the absence of some species. An absence of native cordgrass (Spartina foliosa) – never recorded in Morro Bay – allows for a greater expanse of mudflat to be used by a larger number of migratory and wintering shorebirds. Cordgrass characterizes the lower, mid-marsh habitat in the San Francisco Bay Estuary and in a few of the larger, Southern California salt marshes. Cordgrass establishment in Morro Bay would significantly expand marsh coverage at the expense of existing mudflats and the shorebirds they support.
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Figure 4 Morro Bay Habitats: Existing Conditions
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Figure 5 Designated Critical Habitats for Federally Listed Species
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Marine, estuarine, and terrestrial habitats exist within the Morro Bay Estuary and specifically include (MBNEP 2000a):
• Subtidal open water and channels,
• Eelgrass beds,
• Mudflats,
• Salt marsh,
• Fringing freshwater wetlands,
• Riparian habitats associated with Chorro and Los Osos creeks, and
• Upland habitats surrounding the estuary.
These habitats provide food, shelter, nursery, migratory corridors, and stopover functions for a variety of migratory and resident fish and wildlife species (Tetra Tech 1999). Substantial human development is associated with residential and commercial areas, parks, roads and recreational access.
3.3.1 Open-water and Sub-tidal Habitats Subtidal habitats include open water areas and associated benthic habitats, predominantly within channels, that are normally submerged, except at the upper edges during extreme low tides. Subtidal habitat occurs below mean lower low water (MLLW = the zero datum) to about –5 feet and comprises about 175 acres. Subtidal benthic habitats consist predominantly of sandy to muddy (silt + clay) sediments on the bottoms and side slopes of channels, and in the deep water area at the mouth of the Bay, although localized areas of artificial and natural hard-bottom subtidal habitat exist in several locations, associated with piers, wharves, the Morro Bay Power Plant water intake structure, and riprap for breakwaters at the mouth of the Bay. Natural rocky substrates occur at Fairbanks Point and White Point.
The depth of the main channel (not Federally maintained) in the Morro Bay Estuary ranges from 0.6 to 9.0 m, with an average depth of five meters (Tenera 2000). This subtidal zone supports relatively few macroinvertebrates. Only two invertebrate species – the sea hare (Aplysia californica) and a shrimp (Crangon alaskensis) – were collected in 1998 from this habitat (Tetra Tech 1999).
Over 60 fish species have been recorded from all habitats within Morro Bay throughout the year, although these assemblages are typically dominated by only a few species (Fierstine et al. 1973, Horn 1980, and CDFG unpublished data). Within the main channel, several flatfish species are common, including speckled sanddabs (Citharichthys stigmaeus), California halibut (Paralichthys californicus) and English sole (Pleuronectes vetulus) (CDFG unpubl. otter trawl data). Surfperches such as the shiner perch (Cymatogaster aggregata) and pile surfperch (Damalichthys vacca) occupy back portions of the bay and pier piling areas adjacent to the main channels. Other fish species commonly found throughout deep portions of the bay and main
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channels include bat rays (Myliobatis californica), lingcod (Ophiodon elongatus), and staghorn sculpin (Leptocottus armatus) (Tetra Tech 1999). Pelagic schooling species such as topsmelt (Atherinops affinis) and northern anchovy (Engraulis mordax) may use the main channels as a pathway to back areas of the bay that are used for nursery grounds.
A total of 26 Essential Fish Habitat (EFH) species have been collected in the Morro Bay estuary (Essential Fish Habitat Appendix , Table-1), with most of these utilizing open water habitats. Three coastal pelagic species – the northern anchovy, Pacific sardine, and jack mackerel – may travel throughout the estuary via these channels. Similarly, many of the Pacific Groundfish (FMP species), including skates, sharks, rockfishes, and flatfishes can be found in the main channels. Additional details on EFH and managed species is presented in the Essential Fish Habitat Appendix.
Birds utilizing the open water habitats of the Bay include both resident and migratory diving species that feed on mid-water and benthic fishes and invertebrates, and other species that feed and/or rest on the surface. The large numbers of species that over-winter in the Morro Bay region are reflected in the annual Christmas Bird Count conducted by the Morro Coast Audubon Society, which traditionally has recorded about 200 species within a 15-mile radius of Morro Bay (Morro Coast Audubon Society 2003). Among the noteworthy year-round resident species are about 40-200 American white pelicans (Pelicanus erythrorynchos), which are uncommon in coastal California. They forage in shallow water and frequently roost on Grassy Island, a small vegetated mudflat within the bay (Josselyn et al. 1989).
Marine mammals that feed on fishes and invertebrates in subtidal and open waters of Morro Bay include California sea lions (Zalophus californicus), harbor seals (Phoca vitulina) and southern sea otters (Enhydra lutris). Sea otters are discussed further under Special Status Species.
California sea lions are common in California coastal waters and are frequently sighted in and around Morro Bay. The local CDFG office also conducts periodic counts of harbor seals in the Bay, identifying at least three major haul-out areas on mudflats in the southern bay, where harbor seals are commonly observed during low-tides. The greatest numbers and frequency of harbor seal sightings occur during the late-spring pupping period. Annual counts between 1982 and 1995 indicated an average of 30 to 40 seals hauling out at low tides. Although lower numbers (26 individuals) were reported in 1999 (Hardy 1999 pers. comm., cited in MBNEP 2000a), more recent (2002) observations by NEP personnel indicated the presence of more than 35 individuals (Multari 2003, pers. comm.). The estuary is an important habitat for this species because of the safety provided by the isolation of mudflat “islands”, by the deeper channels, and possibly because of the abundance of fish that are easily available to the seals (MBNEP 2000a).
3.3.2 Eelgrass Beds and Related Habitat Eelgrass (Zostera marina) is a flowering plant that forms beds of varying density at low- intertidal to shallow-subtidal elevations. Eelgrass beds are considered one of the most important habitats in the estuary. Morro Bay has the largest remaining eelgrass acreage south of San Francisco Bay. Eelgrass is critical to many aquatic biota due to its provision of shelter for invertebrates and juvenile fish, contribution to the detrital food chain, and due to its function as a
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substrate for attached (epiphytic) organisms that serve as food for many larger species (Tetra Tech 1999; MBNEP 2000a).
Eelgrass beds in Morro Bay are some of the largest and healthiest of any in Central or Southern California (MBNEP 2000b). The areal extent of intertidal eelgrass beds in Morro Bay has fluctuated greatly over time, diminishing sharply in the aftermath of heavy runoff and turbidity during the mid-1990s. It has rebounded in recent years (Chestnut 1999). The majority of eelgrass is found between 0.0 and +1.0 MLLW (246 acres), although significant eelgrass beds are also found in the shallow subtidal zone. There are currently about 330 acres of mapped eelgrass in the inter-tidal zone and about 650 acres of potential eelgrass habitat in the estuary. Throughout the depth range of eelgrass beds, sedimentation is an important factor that can cause burial of plants in the short term and raising of substrate elevation (more exposure to desiccation) in the longer term.
A total of 77 invertebrate taxa were collected by Tetra Tech (1999) in eelgrass habitats within the Morro Bay Estuary. Invertebrates in these eelgrass habitats had the highest species richness, mean density, and biomass compared to all other habitats in the estuary. Further, stability among the most common species (annelid worms, arthropods, and mollusks) was higher than for any other habitat. Tetra Tech (1999) describes abundances of fishes collected in eelgrass habitats in the Morro Bay Estuary. Topsmelt (Atherinops affinis) was the most abundant species, with other common fishes including Pacific staghorn sculpin (Leptocottus armatus), shiner surfperch (Cymatogaster aggregata), arrow goby (Clevlandia ios), and Bay pipefish (Syngnathus leptorhinchus). Recent eelgrass surveys by the National Marine Fisheries Service (NMFS) collected high abundances of young-of-year (YOY) rockfishes (Sebastes spp.) and cabezon (Scorpanichthys marmaratus), but collected no topsmelt (R. Hoffman, pers. comm. 2002). Less common species in eelgrass habitats include California halibut (Paralichthys californicus), jacksmelt (Atherinopsis californiensis), and longjaw mudsucker (Gillichthys mirabilis) (Tetra Tech 1999).
Eelgrass beds provide important foraging habitat for shorebirds at low tide, and for diving birds at high tide, when the beds are submerged. Eelgrass is the preferred food source for wintering populations of the black brant (Branta bernicla nigricans) (MBNEP 2000a,b).
3.3.3 Mudflat Habitat Mudflat habitat includes intertidal non-vegetated flats composed of sand, silt, clay and organic matter. Mudflats overlap the eelgrass zone and extend to the lower edge of salt marsh or fringing wetlands, an elevation range of approximately MLLW to mean high water (MHW, +5.0 ft above MLLW). Along non-vegetated portions of the sand spit, intertidal mudflats grade into sandy beach. Throughout the year, mudflats are exposed by most low tides and submerged by most high tides. The substrate is unconsolidated and generally devoid of vegetation except for algal mats that can develop seasonally in response to warm water and high nutrient levels. There are currently approximately 1,200 acres of mudflats in the estuary.
A total of 44 invertebrate taxa were identified by hand coring, beach seining and beam and otter trawling of the mudflats during 1998 surveys (Tetra Tech 1999). Some commercially important invertebrates occur within the estuary’s mudflats. Pacific Oysters (Crassostrea gigas) have been
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cultivated and harvested for commercial purposes since 1946 (Richards, no date in MBNEP 2000b) – the California Department of Health Services (CDHS) and the California Department of Fish and Game (CDFG) regulate and monitor all phases of the harvesting. Other, less intensively harvested species that have become rarer in recent times include Geoduck (Panopea generosa), littleneck (Protothaca staminea), razor (Siliqua patula) and bent-nosed clams (Macoma nasuta) and basket cockles (Clinocardium nuttallii). In addition, ghost shrimp (Callianassa californiensis) and fat innkeeper worms (Urechis caupo), both of which are common on the mudflats, are collected and used for bait by local fishermen.
The extensive mudflats of Morro Bay provide important foraging and resting habitat for migratory and wintering shorebirds. Morro Bay is a critical stopover for migratory and wintering shorebirds along the Pacific Coast (Page et al. 1999). Bird numbers increase and diminish as flocks of different species arrive and depart, with large numbers of individuals and species present from September through April, when roughly 20,000 shorebirds can be present (Page et al. 1992, 1999). Waterfowl also use the mudflats as important resting areas during low tide, and as shallow-water foraging habitat during high tide.
3.3.4 Salt Marsh Habitat Salt marsh habitat occurs in the upper intertidal zone of the Morro Bay estuary. This habitat in Morro Bay occurs from approximately mean high water (+4.8 ft MLLW) to extreme high water (+7.5 ft MLLW) (Tetra Tech 1999). Typically, tidal salt marsh is established on broad, very gently sloping marsh plains whose elevation is stabilized at approximately mean higher high water (MHHW = +5.3 ft MLLW) by opposing forces of sedimentation and subsidence (Tetra Tech 1999). In the Chorro-Los Osos Delta, however, fluvial sedimentation has resulted in most of the salt marsh being elevated above MHHW, (Haltiner 1988; Haltiner and Thor 1991; Josselyn et al. 1991). There are currently approximately 376 acres of saltmarsh habitat in the estuary. Weedy riparian habitat has replaced salt marsh in a portion of the upper Chorro Creek delta where sedimentation has raised elevations above tidal inundation, and in recent years riparian habitat has expanded downstream into the creek channels.
The typical west coast salt marsh profile, consisting of a progression from mud flat to low marsh cord grass (Spartina foliosa), middle marsh pickleweed, and high marsh salt grass (Distichlis spicata), is not fully represented at Morro Bay. Salt marshes at Morro Bay do not contain cord grass. The lower edges of the salt marsh tend to be vegetated with pickleweed. Salt marsh within the Morro Bay Estuary had been historically characterized as a combination of low marsh, dominated by pickleweed and jaumea (Jaumea carnosa), with smaller areas of high marsh, supporting a more diverse plant community (Tetra Tech 1999). Surveys conducted in 2002, indicate that Morro Bay’s salt marsh habitat supports a mosaic of pickleweed, jaumea, alkali heath (Frankenia salina), arrow grass (Triglochin concinna), salt grass and marsh-rosemary (Limonium californicum).
A total of 14 invertebrate taxa were identified in hand cores and beach seines during 1998 surveys of this habitat (Tetra Tech 1999). Species richness and abundance were highly variable among sites and replicate samples. California horn snails (Cerithidea californica) were one of the most abundant epifaunal invertebrates collected, while the most common infauna included the amphipod Corophium spinicorne and the polychaete Polydora nuchalis. A total of eight fish
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species were collected in the salt marshes at Morro Bay during 1998 surveys (Tetra Tech 1999). Pacific staghorn sculpin and topsmelt were the most abundant species, with other species in lower abundance, including arrow goby, bat ray, California halibut, and yellowfin goby.
Salt marshes in Morro Bay provide additional foraging and resting areas for shorebirds and waterfowl that also utilize other habitats of the estuary – particularly when mudflats are submerged at high tide. Savanna sparrows (Passerculus sandwichensis), saltmarsh common yellowthroats (Geothypis trichas sinuosa), soras (Porzana carolina), Virginia (Rallus limicola), clapper (Rallus longirostris obsoletus) and California black rails (Lateralis jamaicensis coturniculus) are known or expected to nest in or adjacent to Morro Bay salt marsh habitats. Predators such as coyotes (Canis latrans) and racoons (Procyon lotor) frequently venture from adjacent upland areas into salt marsh habitats to forage.
3.3.5 Fringing Wetland Habitat Fringing wetlands comprise a variety of wetland plant assemblages that occur under the influence of freshwater inflows, from groundwater and/or localized surface runoff, around the fringes of the Bay in transition zones between estuarine (salt marsh or mudflat) and upland habitats. Fringing wetlands occur along the shoreline below the Elfin Forest and along the South Bay and sand spit. The Sweet Springs Preserve contains a variety of habitats mapped in this category, including a freshwater pond and marsh and saltwater ponds (e.g., Perkins 2002). Although some areas of fringing wetlands are subject to tidal influence, these habitats develop where low-salinity groundwater or freshwater springs and seeps dilute the more saline, tidal waters of the Bay and allow the establishment of freshwater and brackish marsh plant species.
The vegetation of the fringing wetlands is highly variable and may include a combination of salt marsh, freshwater wetland and brackish water marsh species (Mooney 1992). Three habitat subtypes have been differentiated, including woody fringe, robust emergent fringe (dominated by tall monocots such as cattails and bulrushes), and herbaceous emergent fringe (dominated by low herbaceous species).
Woody fringe vegetation occurs along the Elfin Forest shoreline and at the Sweet Springs Preserve. This habitat is dominated by species that form a dense thicket, such as arroyo willow (Salix lasiolepis), and includes other riparian and mesic (north-facing slope) species such as marsh baccharis (Baccharis douglasii), wax myrtle (Myrica californica), coast live oak (Quercus agrifolia), California blackberry (Rubus ursinus), and blue elderberry (Sambucus mexicana).
Robust emergent fringe vegetation occurs in scattered patches along the South Bay and sand spit shorelines and is dominated by tall bulrushes and cattails. These vegetation communities can extend well into the intertidal zone, below the elevation at which pickleweed can survive, and provide clear indications of low-salinity groundwater discharge (Morro Group and Tenera 1990).
Herbaceous emergent fringe vegetation occurs near the upper limit of tidal influence, where salt and freshwater mix, and is characterized by a mixture salt marsh and brackish marsh plants. Characteristic species include those of the salt marsh (described in section 4.4.5), as well as spearscale (Atriplex triangularis), southwest spiny rush (Juncus acutus spp. leopoldii), other rushes (Juncus effusus, J. patens), creeping wildrye (Leymus triticoides), water hemlock
Morro Bay Estuary Ecosystem Restoration Feasibility Study 27 Draft - F3 Conference Report
(Oenanthe sarmentosa), silverweed (Potentilla anserina) and hedge nettle (Stachys chamissonis).
The invertebrate communities of fringing wetlands have not been sampled. A mixture of salt marsh, mudflat, and fresh- to brackish water benthic species is expected, as described in previous sections. Local benthic species composition probably varies strongly depending on salinity gradients. Insect diversity and production is presumably high and likely provides abundant food for wildlife. During the highest tides, estuarine fishes are expected to migrate into the shallows and forage in fringing wetlands.
Fringing wetlands greatly increase the diversity of habitats available to wildlife at Morro Bay by providing cover, foraging, and nesting habitat for marsh birds such as the marsh wren, common yellowthroat, and red-winged blackbirds. Wading birds (herons, egrets, rails), as well as raccoons typically forage at the interface between fringing wetlands and tidal marsh and mudflat habitats. Low-salinity aquatic and wetland habitats on the edges of the Bay also support the Pacific treefrog (Pseudacris regilla), southwestern pond turtle (Clemmys marmorata, a sensitive species), and the California red-legged frog (Rana aurora draytoni, a federally-listed threatened species).
3.3.6 Riparian Habitats Riparian habitats within the study area comprise corridors of vegetation and aquatic habitat associated with the channels and bordering floodplains of Chorro and Los Osos creeks. The area described in this and past reports extends approximately one mile upstream from the Bay and includes the riparian habitats of the two creeks. In contrast to estuarine (tidal) salt marshes, mudflats, and open waters of the bay, fluvial processes associated with freshwater stream flow control the structure and function of riparian habitats. Generally, the downstream limit of riparian habitat in the estuary is approximately the point at which tidal influence overrides freshwater stream flow in terms of physical (e.g., channel geomorphology, salinity, currents) and biological (e.g., vegetation, fish community) effects. The transition from riparian to estuarine habitats occurs farther downstream in the estuary along Chorro Creek than along Los Osos Creek because Chorro Creek flows with much greater volumes than Los Osos Creek.
The riparian habitats associated with Chorro and Los Osos Creeks are dominated by dense stands of willows, primarily arroyo willow (Salix lasiolepis). Velvet willow (Salix stichensis) and red willow (Salix laevigata) are also scattered in the riparian habitat along with other native trees such as black cottonwood (Populus balsamifera sspp. trichocarpa), western sycamore (Platanus racemosa), and wax myrtle (Myrica californica). Marsh baccharis and mugwort (Artemisia douglasiana) are common native shrubs in open areas. The forest understory in Chorro Creek is dominated by the non-native, invasive exotic cape ivy (Delairea odorata). In Los Osos Creek the understory is dominated by native California blackberry (Rubus ursinus) with non-native species that include cape ivy, fennel (Foeniculum vulgare), Italian thistle (Carduus pycnocephalus) and annual beardgrass (Polypogon monspeliensis).
Degraded riparian habitat occurs near the head of the delta surrounding the northern Chorro Creek channel. This area is built up by sedimentation above tidal influence, and is dominated by non-native weeds, notably hoary cress (Cardaria draba) and fennel.
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Few data are available on the invertebrates of the riparian habitats. Tetra Tech (1999) reported unidentified flatworms, freshwater snails, and typical freshwater insects in Chorro Creek (e.g. Notonectid backswimmers, predaceous Dytiscid diving beetles, Corixid water boatmen, and unidentified flies). Crayfish (Procambarus sp.) were also found. Unidentified flies were observed in Los Osos Creek. The lower portions of Chorro and Los Osos Creeks support assemblages of native and non-native fishes, with Chorro Creek supporting a stronger freshwater component (Tetra Tech 1999; Aspen 2002). Species found or expected to occur in the lower brackish portions of both streams include: threespine stickleback (Gasterosteus aculeatus), prickly sculpin (Cottus asper) and California killifish. Speckled dace (Rhinichthys osculus) and the Sacramento pike minnow (Ptychocheilus grandis), a non-native predatory species, have also been reported in lower Chorro Creek (Tetra Tech 1999). Chorro and Los Osos Creeks both support steelhead trout (Oncorhynchus mykiss, a federally-listed threatened species as discussed below). The tidewater goby has been reported historically, and may still be present in the transitional brackish zones of both creeks.
The riparian habitats of Chorro and Los Osos Creeks support a diverse assemblage of wildlife species, many of which are considered sensitive. Amphibians and reptiles associated with aquatic habitats of the Chorro Creek and Los Osos Creek watersheds may occur in the riparian zones of these creeks, where they enter the Morro Bay estuary, include: garter snakes (Thamnophus spp.), California toad (Bufoboreas halophilus), coast range newt (Taricha torosa), Pacific treefrog, California red-legged frog, and southwestern pond turtle. Characteristic bird species of riparian thickets that are expected to nest along Chorro and Los Osos creeks include: common yellowthroat, song sparrow, black phoebe, Wilson’s warbler, yellow warbler, and yellow-breasted chat. Mammals known and/or expected to occur in these habitats include: Virginia oppossum (Didelphis virginiana), California ground squirrel (Spermophilus beechyi), California vole (Microtus californicus), coyote, raccoon, striped skunk (Mephitis mephitis), and mule deer (Odocoileus hemionus) (FHWA 1995).
3.3.7 Upland Habitats Upland habitats that surround the estuary include (Mooney 1992), central foredunes (including the non-vegetated beach above the high tide line), coastal dune scrub, coastal sage scrub, central maritime chaparral, pygmy oak forest (known locally as the “elfin” forest), and non-native (eucalyptus and/or cypress) woodland. Disturbed habitats and developed lands also fall into this category, but are not biologically significant.
Foredunes occur along the sand spit and along the causeway to Morro Rock on the north shore of the entrance channel. The dunes are relatively unstable and sparsely vegetated with low growing, perennial species such as red (Abronia maritima, CNPS List 4), yellow (Abronia latifolia) and purple (Abronia umbellata) sand verbenas, beach bur (Ambrosia chamissonis), beach evening primrose (Camissonia cheiranthifolia), and sea rocket (Cakile maritima) (Mooney 1992).
Sandy beach and foredune habitats provide resting areas for migratory shorebirds and waterbirds that forage in the estuarine habitats of the bay (discussed above). They also provide nesting habitat for the federally listed threatened western snowy plover (Charadrius alexandrinus) (MBPP 2002). The foredunes at the northern end of the sand spit are included within federally
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designated critical habitat for snowy plovers and provide an important nesting location for the species.
Coastal dune scrub typically occurs on the relatively flat terraces adjacent to the ocean. Coastal development has eliminated much of this habitat in the vicinity of Morro Bay, but extensive, high quality dune scrub habitat remains along the sand spit (MBPP 2002). Coastal dune scrub is threatened by the invasive exotic, veldtgrass (Ehrharta calycina), which can carpet the dunes and promote grass fires, to the exclusion of native species (Mooney 1992). Coastal dune scrub supports several special status plant species (see below).
Common wildlife occurring in Central Coast dune scrub habitat include mule deer (Odocoileus hemionus), brush rabbits (Sylvilagus bachmani), coyotes (Canis latrans), and a variety of songbirds (see coastal sage scrub and chaparral below). Special status wildlife that occur in this habitat include the silvery legless lizard (Anniella pulchra pulchra, a state and federal species of concern), the Morro shoulderband snail (Helminthoglypta walkeriana, federally-listed endangered), the Morro Bay kangaroo rat (Dipodomus heermanni morroensis, federally and state-listed endangered) and the Morro blue butterfly (Icariacia icariodes ssp. morroensis) (Mooney 1992; MBPP 2002).
Coastal sage scrub occurs in drier areas on hillsides around the bay. Vegetation consists of a variety of small shrubs, dominated by California sagebrush (Artemisia californica) and black sage (Salvia mellifera). Central maritime chaparral typically includes dense, large woody shrubs such as Morro manzanita (Arctostaphylos morroensis, federally-listed threatened and CNPS List 1B), chamise (Adenostoma fasciculatum), ceanothus (Ceanothus sp), holly-leaved cherry (Prunus ilicifolia), sand almond (Prunus fasciculata var. punctata, CNPS List 4), bracken fern (Pteridium aquilinum), and coast live oak (Quercus agrifolia) (Mooney 1992). This habitat is scattered on the hillsides south of Los Osos and on north-facing slopes of the marine terraces south of Los Osos creek (MBPP 2002).
Wildlife common in coastal sage scrub and chaparral along California’s Central Coast include bobcats (Lynx rufus) mule deer, coyotes, dusky-footed woodrats (Neotoma fuscipes), Anna’s (Calypte anna) and Allen’s (Selasphorus sasin) hummingbirds, wrentits (Chamaea fasciata), California thrashers (Toxostoma redivivum), rufous-sided (Pipilo erythrophthalmus) and California (P. crissalis) towhees, brush lizards (Urosaurus ornatus), western whiptails (Cnemidophorus tigris), side-blotched lizards (Uta stansburiana), and gopher snakes (Pituophus melanoleucus) (e.g., FHWA 1995).
Oak woodlands, dominated by coast live oak, occur on mesic soils of north-facing slopes and canyons within the watershed. This habitat type is variable and typically grades into others such as riparian and chaparral, especially in the southern portion of the bay. The canopy of the oak woodland may vary and create a mosaic of habitat types, ranging from the open oak savannah (widely scattered oak trees with a grassy understory) to oak woodland (scattered oaks with a chaparral understory) to dense oak forest (closed oak canopy with a shaded understory).
A unique pygmy oak phase of oak woodland habitat – locally known as the elfin forest – occurs only in the South Bay area. The elfin forest is dominated by a stunted, wind-pruned variety of coast live oak (Q. a. var. frutescens), along with other native shrub species including wood fern
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(Dryopteris arguta), manroot (Marah fabaceous), bracken fern, California blackberry, gooseberry (Ribes spp.) and poison oak (Mooney 1992; Holland 2002). The Elfin Forest is managed as a wilderness preserve and public access is allowed via a boardwalk.
Oak woodland provides dense cover, structural complexity, and high vegetative and insect production that, in turn, support a high diversity of wildlife. Typical wildlife of coastal oak woodlands around Morro Bay include mule deer, bobcat, California quail, dusky-footed woodrat, striped skunk (Spilogale gracilis) alligator lizard (Gerrhonotus multicarinatus), hermit thrush, California quail, western screech owl, acorn and Nuttall’s woodpeckers, common titmouse, and bushtit.
Non-native woodlands near Morro Bay include planted stands of eucalyptus and/or cypress along the northeastern side of the Bay on City and State Parks property. These large trees provide roosting and nesting sites for herons, cormorants, and raptors and provide winter roosting areas for monarch butterflies.
3.3.8 Special Status Species A total of 26 Essential Fish Habitat (EFH) species have been collected in the Morro Bay estuary (Essential Fish Habitat Appendix, Table-1), with most of these utilizing open channel habitats. Three coastal pelagic species, the northern anchovy, Pacific sardine, and jack mackerel may travel throughout the estuary via these channels. Similarly, many of the Pacific Groundfish FMP species, including skates, sharks, rockfishes, and flatfishes can be found in the main channels. Some EFH species can be found in eelgrass beds, while others occupy adjacent habitats. Recent surveys by NMFS collected juvenile rockfishes and cabezon (Pacific Groundfish FMP species) in eelgrass beds (R. Hoffman, pers. comm. 2002). Other EFH species occurring in or near eelgrass beds include northern anchovy and Pacific herring (Coastal Pelagic FMP species). EFH fishes found in mudflat habitats include schooling coastal pelagic species such as northern anchovy and Pacific herring, as well as some Pacific Groundfish FMP species, including leopard sharks, which feed in the mudflats on fat innkeeper worms and crustaceans. Transient visitors in salt marsh habitats include some Coastal Pelagic FMP species, such as northern anchovy, which moves between different habitats in the estuary, including salt marshes. Northern anchovy are able to tolerate shifts in salinity better than most other FMP species and can better accommodate a broad range of seasonal and inter-annual differences within the estuary.
As mentioned in previous sections, numerous special status species, including federally and state listed threatened and endangered species occur in each of the Morro Bay habitats. A more extensive discussion of special status species in the study area is presented in the Environmental Appendix, including listings of special status plant species and wildlife species that are known to occur in the Morro Bay Watershed and Estuary.
4 Sedimentation Analysis Throughout the central coast of California, sedimentation rates have been influenced by human activity. In the Morro Bay watershed, there has been little documentation of the changes that have been made to creeks and floodplains that have accelerated sediment delivery to the bay. Two exceptions to this lack of documentation, identified by Haltiner (1988) include channel
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reconfiguration and levee construction along Chorro Creek that destroyed riparian forest and open wetlands formerly known as the “Cienga”, and an observation from 1894, indicating that cultivation of bottom lands has increased sedimentation (Haltiner, 1988).
For this analysis, the historical sedimentation rates in Morro Bay were determined by conducting a series of field investigations and historical data analyses. Sediment enters the system via three major transport mechanisms: littoral transport through the Bay entrance, fluvial transport from Chorro and Los Osos Creeks, and aeolian transport from the sand spit.
4.1 Historical Sedimentation
Bathymetric changes in the bay were evaluated by comparing the 1935 U.S. Coast and Geodetic Survey (USCGS) and 1998 Tetra Tech Inc. surveys. Neither the 1935 USCGS nor the 1998 Tetra Tech Inc. data include information on the Chorro Creek delta. Bathymetric maps made in 1873, 1919, and 1978 were also reviewed but were not included in the analysis because of inadequacy and uncertainty of the methods and accuracy of the historical charts. Philip Williams and Associates undertook a topographic survey of Chorro Creek delta in 1987 at scale 1 inch = 500 ft with vertical datum in NGVD 1929 (feet). The delta topographic data was reduced to metric units with respect to MLLW datum and was merged with the 1998 Tetra Tech Inc. bathymetric data
Historical variations of sedimentation rates in Morro Bay during 1873 through 1998 have been analyzed. Volumetric estimates of Morro Bay below 2 meter elevation were calculated for the years 1873, 1919, 1935, 1978, and 1998. Following the work of Haltiner (1991), the bay has been divided into four deposition zones (Zone 1-4) as shown in Figure 6. Net sedimentation rates in Morro Bay were determined by comparing 1935 and 1998 bathymetric maps. Comparisons were made by creating digital terrain models from the contours and elevation data of the 1935 and 1998 bathymetric survey maps. Volumetric changes between 1935 and 1998 were determined separately for each of the four zones, and then combined to estimate sediment rate for the entire bay. It was found that the total sediment rate estimated by this study is 50,951 cy/yr during 1935 through 1998 and the corresponding cumulative average shoaling for the entire area is about 1.2 ft. Much of the intertidal mudflats throughout the bay have been raised approximately by 1.11 ft to 1.49 ft.
The mean tidal prism, the volume of water between MHW and MLW, in Morro Bay was determined for the 1935 and 1998 bathymetric data. Mean tidal prism was calculated separately for the 1935 and 1998 data for each of the four zones and then combined to estimate the total tidal prism for the entire bay. These calculations indicate that the overall decrease of tidal prism during the last 63 years (1998-1935) is about 17%, which is equivalent to 0.27% per year. Haltiner (1991) estimated an overall decrease of tidal prism of 25% during the past 103 years (1987-1884), which is equivalent to 0.24% per year. Most of the sediment depositions occurred around the Central and South Bay areas.
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Figure 6 Four Physiographic Zones
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