San Luis Obispo Creek Stormwater Resource Plan Final Draft February 1, 2019 Prepared for the City of San Luis Obispo, Natural Resources Program, and Cal Poly San Luis Obispo 990 Palm St. San Luis Obispo, CA 93401 Prepared by Stillwater Sciences, Inc. 895 Napa Avenue, Suite B‐4 Morro Bay, CA 93442 EXECUTIVE SUMMARY This report constitutes the San Luis Obispo Creek Watershed Stormwater Resource Plan (henceforth, the “SRP”), whose overarching purpose is to develop strategies to best manage the risks and opportunities presented by stormwater runoff from this coastal, mixed‐land‐use watershed along California’s Central Coast. This Resource Plan follows the guidance of the State Water Board in analyzing the San Luis Obispo Creek watershed as a whole, integrating the current knowledge of the watershed and its receiving waters’ condition to guide recommendations for the multi‐benefit management of stormwater to improve overall watershed health. The San Luis Obispo Creek watershed drains approximately 84 square miles, with the City of San Luis Obispo at its geographic center. The creek and its tributaries are used by south‐central California steelhead (Oncorhynchus mykiss, listed as “threatened” under the federal Endangered Species Act), although only few areas remain with high‐quality spawning and rearing habitat. Resource quality and stream health decline monotonically down the channel network, with the most abrupt decline associated with the urban center of the City of San Luis Obispo. Although a variety of direct channel impacts coincide with this zone, the well‐documented decline in various in‐stream conditions through this area, particularly water quality, is undoubtedly a primary result of urban stormwater runoff. Although potable water is supplied almost exclusively from out‐of‐basin reservoirs, groundwater is used extensively for agriculture and much of the watershed overlies a designated groundwater basin. The protection or recovery of “watershed processes,” encompassing the storage, movement, and delivery of water, chemical constituents, and/or sediment to receiving waters, should be the fundamental goal of stormwater management, and this principle guides the analyses and recommendations of this SRP. This approach leads to multi‐benefit outcomes because the focus is on correcting the underlying cause(s) of resource degradation, not its variety of symptoms. Across areas displaying the greatest impacts to resources, their clearest commonality is a loss of infiltration from the contributing watershed area. Thus, the stormwater management strategies most responsive to this condition will be those that are most effective at recovering this watershed process to the greatest benefit of all water resources. In summary, using the guidance provided by the characterization of the watershed (Section 1), the lens of impaired watershed processes (Section 3), and the identification of sites with the highest rating for addressing those impaired processes through stormwater management (Section 4), key areas plus a list of 3 planned projects sites have been identified that emphasize the types of structural stormwater projects (regional capital improvement projects, small‐ and large‐scale low impact development projects, and green streets) that are particularly amenable to addressing the critical impairments to watershed processes. Through coordination with other entities and the public in both the development (Section 2) and implementation (Section 5) of the SRP, this document details the methodology for developing its findings and identifies multiple opportunities for multi‐benefit watershed improvements. Table of Contents EXECUTIVE SUMMARY .................................................................................................................................. 1 1 EXISTING CONDITIONS IN THE SAN LUIS OBISPO CREEK WATERSHED .................................................. 1 1.1 INTRODUCTION ............................................................................................................................. 1 1.1.1 Purpose and Scope .................................................................................................................... 1 1.1.2 Elements of the SRP .................................................................................................................. 1 1.1.3 Previous Studies and Plans ....................................................................................................... 2 1.2 THE SAN LUIS OBISPO CREEK WATERSHED ................................................................................... 2 1.2.1 Watershed Setting and Boundaries .......................................................................................... 2 1.2.2 Watershed Topography and Geology ....................................................................................... 4 1.2.3 Stream Channels and Surface‐Water Hydrography .................................................................. 6 1.2.4 Water Supply ........................................................................................................................... 11 1.2.5 Groundwater ........................................................................................................................... 11 1.2.6 Land Cover and Land Use ........................................................................................................ 15 1.2.7 Additional Map Information ................................................................................................... 17 1.3 RECEIVING WATER CONDITIONS ................................................................................................ 17 1.3.1 Approach ................................................................................................................................. 17 1.3.2 Habitat Structure .................................................................................................................... 18 1.3.3 Flow Regime ............................................................................................................................ 19 1.3.4 Water Quality .......................................................................................................................... 20 1.3.5 Energy Sources and Biotic Interactions ................................................................................... 24 1.3.6 Integrative Measures of Watershed Health ........................................................................... 24 1.4 IMPLICATIONS FOR STORMWATER MANAGEMENT ................................................................... 27 1.4.1 Watershed Conditions and Pollution‐Generating Activities ................................................... 27 1.4.2 Habitat Conditions .................................................................................................................. 28 1.4.3 Groundwater ........................................................................................................................... 28 1.5 APPLICABLE PERMITS .................................................................................................................. 28 1.5.1 NPDES Phase 2 stormwater (MS4) permit .............................................................................. 28 1.5.2 Pathogen TMDL ....................................................................................................................... 29 1.5.3 Nutrient TMDL ........................................................................................................................ 29 2 COORDINATION AND COLLABORATION IN PLAN DEVELOPMENT ....................................................... 30 2.1 PUBLIC ENGAGEMENT ................................................................................................................ 30 2.2 AGENCY AND COMMUNITY CONSULTATION .............................................................................. 30 3 TYPES AND LOCATIONS OF PRIORITY PROJECTS .................................................................................. 31 3.1 PROCESS‐BASED WATERSHED MANAGEMENT ........................................................................... 31 3.1.1 Watershed Processes .............................................................................................................. 31 i 3.1.2 Watershed Management Zones of the San Luis Obispo Creek Watershed ............................ 32 3.2 CALCULATION OF RUNOFF AND POLLUTANT LOADING ............................................................. 34 3.3 APPROACH TO ADDRESSING WATER‐QUALITY NEEDS ............................................................... 35 3.3.1 Pollution‐Generating Activities ............................................................................................... 35 3.3.2 Strategies to Address Polluted Runoff .................................................................................... 36 3.3.3 Consistency with NPDES Permits ............................................................................................ 37 3.3.4 Consistency with TMDLs ......................................................................................................... 37 3.4 CONCEPTUAL PROJECT TYPES ..................................................................................................... 38 4 SCREENING, SCORING, AND PRIORITIZING OF SCMs ........................................................................... 41 4.1 METHODOLOGY .........................................................................................................................