Lake Berryessa is an artificial lake in Napa County, California. It is the largest lake in the county. It was formed after the construction of Monticello Dam, in the 1950s. Since the 1960s, the lake has provided electricity to the North Bay area of San Francisco. When it is full, the lake covers an area of about 25 kilometres (16 mi) by 5 kilometres (3.1 mi). It has a maximum depth of about 84 metres (276 ft). As of 2021, the lake has the largest spillway in the world. It also became known because it is the site of one of the murders of the Zodiac Killer.