Lake County was formed in 1861 from portions of Napa, Mendocino and Colusa counties. The county is named after Clear Lake, the largest natural lake wholly located within the state of California (Lake Tahoe is larger but is partially in Nevada, while the Salton Sea was formed after a breached irrigation canal flooded the desert for two years). The lake is believed to be the oldest lake in North America, with evidence indicating that a lake has existed at the site for 2.5 million years.
The county is also home to Mount Konocti, a 4,305 foot volcano that is part of a volcanic field that contains seven vents ranging in age from 10,000 to 2.5 million years old. The field's underground magma chamber has given rise to the world's largest complex of geothermal power plants, with 22 plants compromising a geothermal field known as The Geysers.
The county has been primarily an agricultural region since its inception, with wine being an important product from the earliest days of Spanish settlement. The Prohibition era essentially ended wine production until vineyards were reestablished in the 1960s, and today about thirty-five wineries can be found in the county.
The county's hilly geography resulted in it being the only county in the state to never be served by a railroad line. The same hills, as well as its location away from cities, have given the county the best air quality in the state, and the American Lung Association ranked it as having the best air quality in the entire nation in 2013, 2014 and 2015.
- 1 Mendocino County
- 2 Glenn County - Bordering Lake County to the northeast, Glenn County is an agricultural region that is most likely to be experienced by travelers passing through on Interstate 5. While it lacks significant amenities, there are a few options for travelers to experience small-town America at events such as the Glenn County Fair, held in mid-May in the town of Orland, or at the Thunderhill Raceway Park outside of Willows, home to the longest automobile race in the United States: the 25 Hours of Thunderhill.
- 3 Colusa County - Located east of Lake County, visitors to rural Colusa County will find an abundance of rice fields and almond trees, but a limited number of amenities and attractions. Sights that may be of interest include four national wildlife refuges, as well as the Colusa County Courthouse in the town of Colusa, which was erected in 1861 and is the oldest remaining courthouse in the Sacramento Valley.
- 4 Yolo County - With extensive farmlands, Lake County's southeastern neighbor offers numerous opportunities for visitors to engage in agritourism: farmer's markets are held regularly, organic farms offer tours and the opportunity to pick your own produce, and more than 35 wineries can be found in the county. The college town of Davis is home to California's third-largest state university and boasts the highest number of bikes per capita in the USA, a statistic that led the US Bicycling Hall of Fame to move to the town in 2010.
- 5 Napa County - Neighboring Lake County to the southeast, America's preeminent wine-producing region attracts more than five million annual visitors to over two hundred wineries, often overcrowding the roadways on summer weekends. Travelers will find world-famous restaurants to complement the wines, and lodging that includes luxury spas, B&Bs, and upscale hotels. Those uninterested in viticulture may choose to enjoy the hot springs of Calistoga or hike/bike the many parks and trails in the area's beautiful rolling hills.
- 6 Sonoma County - Although its wineries may not be as famous as those in the Napa Valley, Lake County's southwestern neighbor is actually the largest wine producer in California Wine Country and home to over 250 wineries. More than seven million visitors each year explore the county's open spaces and beautiful coastline, including the big trees at Armstrong Redwoods State Natural Reserve and the seaside town of Bodega Bay where Alfred Hitchcock's thriller The Birds was filmed. For those interested in early California history, Fort Ross is a state historic site that preserves a fur trading outpost that was operated by Russia from 1812-1841.