Lake Berryessa is the largest lake in Napa County, California. This reservoir is formed by the Monticello Dam, which provides water and hydroelectricity to the North Bay region of the San Francisco Bay Area.The lake was named for the first European settlers in the Berryessa Valley, José Jesús and Sexto "Sisto" Berrelleza (a Basque surname, anglicised to Berreyesa then later respelled Berryessa), who were granted Rancho Las Putas in 1843.