Central Sonoma Watershed Project

In 1958, the Sonoma County Flood Control and Water Conservation District (now Sonoma Water) and the USDA Soil Conservation Service (now Natural Resources Conservation Service [NRCS]) developed the Central Sonoma Watershed Work Plan (1958 Plan) to combat recurring flood damage in Sonoma County. The 1958 Plan covered 163,000 acres of the Laguna de Santa Rosa, Mark West, and Santa Rosa Creek watersheds (Figure 1); however, it explicitly focused on mitigating flooding in the 50,000-acre Santa Rosa Creek subwatershed through an integrated network of channels, detention reservoirs, and diversion structures (flood facilities) that were completed between 1962 and 1988. Collectively, these flood facilities are known as the Central Sonoma Watershed Project (CSWP). Sonoma Water maintains and operates the CSWP, which includes facilities along Santa Rosa, Matanzas, Paulin, Spring, and Brush Creeks that protect the City of Santa Rosa and surrounding rural areas (Figure 2).

The CSWP has exceeded its original 50-year design life and needs a comprehensive system-wide assessment to ensure it can reliably mitigate flood hazards in the City of Santa Rosa and surrounding unincorporated areas for future generations. Given the age of CSWP facilities, development and land-use changes that have occurred over the last 60 years, population growth, and newly available seismic risk and future climate data, Sonoma Water has entered into a cooperative funding agreement with the NRCS to conduct a vulnerability assessment of CSWP facilities and to develop a new Watershed Plan – Environmental Document (Watershed Plan-ED) under the Public Law 83-566 (PL-566) Program. The purpose of the Watershed Plan-ED is to identify and evaluate feasible alternatives that restore or improve flood protection and mitigate vulnerabilities to increase community resilience over the next 50 years. The Watershed Plan-ED will document potential costs, benefits, and environmental impacts of the alternatives.

As the Federal Lead Agency, NRCS will assist Sonoma Water and publish a combined Watershed Plan – Environmental Document (Watershed Plan – ED) that complies with the National Environmental Policy Act (NEPA). Sonoma Water will complete a separate California Environmental Quality Act, AB 52, and state and federal regulatory permit process.

The Central Sonoma Watershed project includes flood protection reservoirs that were built in the late 1960s to reduce flooding in the Santa Rosa area. These reservoirs are:

  • Santa Rosa Creek Reservoir (also known as Spring Lake)
  • Matanzas Creek Reservoir
  • Piner Creek Reservoir (on Paulin Creek)
  • Brush Creek Middle Fork Reservoir

Figure 1. Historic Map

Figure 2. Project facilities and Santa Rosa Creek subwatershed map

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Spring Lake

Spring Lake was constructed between 1961 and 1964 as a flood protection reservoir by Sonoma Water. The project, which consists of three steel and concrete dams, spillways, channels, and flow regulators and represents one of Sonoma Water's most ambitious flood protection projects, diverts floodwaters from Spring Creek and Santa Rosa Creek into Spring Lake, alleviating much of downtown Santa Rosa's flooding. The project is designed to accommodate water equal to a 100-year flood (in other words, a storm that, based on past records, has a 1 in 100, or 1 percent, chance of occurring in any year). Only once since the lake's construction, in 1986, has flooding been so severe as to exceed Spring Lake's capacity.

In 1974, Sonoma Water began plans to develop the land surrounding the reservoir into parkland. Today, Spring Lake Park is one of the most popular recreation areas in Sonoma County. The park's 320 acres accommodate a 72-acre lake, a 3-acre swimming lagoon, and miles of walking, hiking, and horseback-riding trails. The park also includes a visitors' center, campsites, picnic areas, and boating facilities. Under a contract with Sonoma Water, the Sonoma County Regional Parks Department operates Spring Lake Park.