with
vineyards in foreground According to the
U.S. Census Bureau, the county has an area of , of which is land and (10.9%) is water. The county lies in the
North Coast Ranges of northwestern California. Its ranges include the
Mayacamas and the
Sonoma Mountains, the southern peak of the latter being the prominent landform
Sears Point. The highest peak in the Mayacamas within the county and the highest peak in the county is
Mt. Saint Helena. It has uncommon occurrences of
pygmy forest, dominated by
Mendocino cypress. The highest peak of the Sonoma Mountains is
Sonoma Mountain itself, which boasts two significant public access properties:
Jack London State Historic Park and
Fairfield Osborn Preserve. The county includes the City of Sonoma and the
Sonoma Valley, in which the City of Sonoma is located. However, these are not synonymous. The City of Sonoma is merely one of nine incorporated cities in the county. The Sonoma Valley is just the southeastern portion of the county, which includes many other valleys and geographic zones, including the Petaluma Valley, the
Santa Rosa Plains, the
Russian River, the
Alexander Valley, and the
Dry Creek Valley. Distinct habitat areas within the county include oak
woodland,
redwood forest,
northern coastal scrub,
grassland,
marshland, oak
savanna and
riparian woodland. The
California oak woodland in the upper
Yulupa Creek and
Spring Creek watersheds in
Annadel State Park is a relatively undisturbed ecosystem with considerable
biodiversity. These forested areas have been characterized as some of the best examples of such woodlands. An unusual characteristic of these forests is the high content of undisturbed prehistoric
bunchgrass understory, testifying to the absence of historic
grazing or other
agriculture. Trees of the oak woodland habitat include
Pacific madrone,
Douglas fir,
coast live oak,
Garry oak, and
California laurel. Common understory plants are
toyon,
poison oak, and, at the fringes,
coast silk-tassel.
Climate Sonoma County, as is often the case with coastal counties in California, has a great degree of climatic variation and numerous, often very different, microclimates. Key determining factors for local climate are proximity to the ocean, elevation, and the presence and elevation of hills or mountains to the east and west. This is in large part due to the fact that, as throughout California, the prevailing weather systems and wind come normally from the Pacific Ocean, blowing in from the west and southwest, so that places closer to the ocean and on the windward side of higher elevations tend to receive more rain from autumn through spring and more summer wind and fog. This itself is partly a result of the presence of high and low pressures in inland California, with persistent high summer temperatures in the
Central Valley, in particular, leading to low pressures, drawing in moist air from the Pacific, cooling into damp cool breezes and fog over the cold coastal water. Those places further inland and particularly in the lee of significant elevations tend to receive less rain and less, in some cases no, fog in the summer. The coast itself is typically cool and moist throughout summer, often foggy, with fog generally blowing in during the late afternoon and evening until it clears in the later morning becoming sunny, before repeating. Coastal summer highs are typically in the mid to high 60s, warming to the low 70s further from the ocean. Certain inland areas, including the Petaluma area and the Santa Rosa Plain, are also prone to this normal fog pattern in general. Snow is exceedingly rare in Sonoma County, except in the higher elevations on and around the
Mayacamas Mountains, particularly
Mount Saint Helena, and
Cobb Mountain, whose peak is in Lake County.
Ocean, bays, rivers and streams Sonoma County is bounded on the west by the Pacific Ocean, and has of coastline. The major coastal hydrographic features are
Bodega Bay, the mouth of the Russian River, and the mouth of the
Gualala River, at the border with
Mendocino County. All of the county's beaches were listed as among the cleanest in the state in 2010. Six of the county's nine cities, from Healdsburg south through Santa Rosa to Rohnert Park and Cotati, are in the Santa Rosa Plain. The northern Plain drains directly to the Russian River, or to a tributary; the southern Plain drains to the Russian River via the
Laguna de Santa Rosa.
Russian River as viewed from the Jenner Cliffs looking south, showing the
mouth of the
Russian River at the Pacific Ocean Much of central and northern Sonoma County is in the watershed of the Russian River and its tributaries. The river rises in the coastal mountains of Mendocino County, north of the city of
Ukiah, and flows into
Lake Mendocino, a major flood control reservoir. The river flows south from the lake through Mendocino to Sonoma County, paralleled by Highway 101. It turns west at Healdsburg, receiving water from
Lake Sonoma via
Dry Creek, and empties into the Pacific Ocean at
Jenner.
Laguna de Santa Rosa The Laguna de Santa Rosa is the largest tributary of the Russian River. It is long, running north from Cotati to the Russian River near Forestville. Its flood plain is more than . It drains a watershed, including most of the Santa Rosa Plain. The Laguna de Santa Rosa Foundation says: The Laguna de Santa Rosa is Sonoma County's richest area of wildlife habitat, and the most biologically diverse region of Sonoma County (itself the second-most biologically diverse county in California)... It is a unique ecological system covering more than and a mosaic of creeks, open water, perennial marshes, seasonal wetlands, riparian forests, oak woodlands, and grasslands... As the receiving water of a watershed where most of the county's human population lives, it is a landscape feature of critical importance to Sonoma County's water quality, flood control, and biodiversity. The Laguna's largest tributary is
Santa Rosa Creek, which runs through Santa Rosa. Its major tributaries are
Brush Creek, Mark West Creek,
Matanzas Creek,
Spring Creek, and
Piner Creek. Santa Rosa Creek was shown to be polluted in Sonoma county first flush results.
Other water bodies The boundary with
Marin County runs from the mouth of the
Estero Americano at
Bodega Bay, up Americano Creek, then overland to
San Antonio Creek and down the Petaluma River to its mouth at the northwest corner of
San Pablo Bay, which adjoins
San Francisco Bay. The southern edge of Sonoma County comprises the northern shore of San Pablo Bay between the Marin County border at the Petaluma River and the border with Solano County at
Sonoma Creek. Sonoma County has no incorporated communities directly on the shore of San Pablo Bay. The Petaluma River,
Tolay Creek, and Sonoma Creek enter the bay at the county's southernmost tip. The intertidal zone where they join the bay is the vast
Napa Sonoma Marsh.
Americano Creek, the Petaluma River, Tolay Creek, and Sonoma Creek are the principal streams draining the southern portion of the county. The Sonoma Valley is drained by Sonoma Creek, whose major tributaries are
Yulupa Creek,
Graham Creek,
Calabazas Creek, Schell Creek, and
Carriger Creek;
Arroyo Seco Creek is a tributary to Schell Creek. Other creeks include Foss, Felta, and Mill. Lakes and reservoirs in the county include Lake Sonoma,
Tolay Lake,
Lake Ilsanjo,
Santa Rosa Creek Reservoir,
Lake Ralphine, and
Fountaingrove Lake.
Marine protected areas Like underwater parks, these
marine protected areas help conserve ocean wildlife and marine ecosystems. •
Del Mar Landing State Marine Reserve •
Stewarts Point State Marine Reserve & Stewarts Point State Marine Conservation Area •
Salt Point State Marine Conservation Area •
Gerstle Cove State Marine Reserve •
Russian River State Marine Reserve and Russian River State Marine Conservation Area •
Bodega Head State Marine Reserve & Bodega Head State Marine Conservation Area •
Estero Americano State Marine Recreational Management Area Threatened/endangered species A number of endangered plants and animals are found in Sonoma County, including the
California clapper rail (
Rallus longirostris obsoletus),
salt marsh harvest mouse (
Reithrodontomys raviventris),
northern red-legged frog (
Rana aurora),
Sacramento splittail (
Pogonichthys macrolepidotus), California freshwater shrimp (
Syncaris pacifica), showy Indian clover (
Trifolium amoenum), Hickman's potentilla (
Potentilla hickmanii),
northern spotted owl (Strix occidentalis caurina), and
marbled murrelet (Brachyramphus marmoratus). Species of special local concern include the
California tiger salamander (
Ambystoma californiense),
coho salmon, and some endangered plants, including Burke's goldfields (
Lasthenia burkei), Sebastopol meadowfoam (
Limnanthes vinculans), and Sonoma sunshine or Baker's stickyseed (
Blennosperma bakeri). Endangered species that are
endemic to Sonoma County include Sebastopol meadowfoam, Sonoma sunshine, and Pitkin Marsh lily (
Lilium pardalinum subsp. pitkinense). The
Sonoma County Water Agency has had a Fisheries Enhancement Program since 1996. Its website says: "The primary focus of the FEP is to enhance habitat for three
salmonids:
Steelhead,
Chinook salmon, and
Coho salmon. These three species are listed as threatened under the U.S.
Endangered Species Act. The California Department of Fish and Game considers the Coho salmon endangered."
Adjacent counties •
Mendocino County, California – north •
Lake County, California – northeast •
Napa County, California – east •
Solano County, California – southeast •
Marin County, California – south •
Contra Costa County, California – south-southeast (water only, single point)
National protected area •
San Pablo Bay National Wildlife Refuge (part) ==Transportation==