Lake Berryessa, Artificial reservoir in Napa County, California.

Lake Berryessa is a reservoir in Napa County, California, covering roughly 20,000 acres and forming a shoreline that extends around 165 miles (265 kilometers). The water fills a long narrow valley framed by brown hills dotted with oak trees and grassland.

Monticello Dam was built between 1953 and 1958, flooding the farming community of Monticello along with its schoolhouse, cemetery, and orchards. Families left their homesteads before the rising water filled the valley and shaped the new reservoir.

The reservoir takes its name from the Berryessa family: José Jesús and Sexto "Sisto" Berryessa were granted Rancho Las Putas in 1843. The valley flooded for the dam was the traditional land of the Pomo tribe, and the town of Monticello, founded by Ezra Peacock in 1867, lay beneath the water. Photographers Dorothea Lange and Pirkle Jones recorded the valley's abandonment in Death of a Valley, published in Aperture in 1960.

The reservoir lies in the Vaca Mountains, and the Green Valley Fault runs parallel to it in the hills to the west. Summer days are hot and mostly dry, with normal July highs near 94.8 °F (34.9 °C); winters are cool and wet, with January lows around 36.4 °F (2.4 °C). The lake level rises unpredictably because its tributaries, Putah, Capell, Pope and Eticuera creeks, drain separate areas.

The Glory Hole spillway is 72 feet (22 m) in diameter at the rim and narrows to about 28 feet (8.5 m), dropping 200 feet (61 m) straight down. It first flowed in 2017 after heavy rains, the first time since 2006, and again in 2019. In 1997 a woman was pulled inside and killed.