Solano Project
History
The Solano Project is a federal project with the Bureau of Reclamation that stores water in Lake Berryessa for delivery to users throughout the region. The idea for the development of the Solano Project was conceived in the 1940’s and 1950’s to meet the water demands of agriculture, municipalities and military facilities within Solano County. As agriculture developed in the County, use of groundwater increased substantially. Groundwater overdraft persisted in several parts of the County. This overdraft condition provided the impetus for a surface water supply to offset the overdraft. The population of Solano County in the 40’s and 50’s was also expected to grow; however, planners at that time had no way of knowing that the urban population growth in Solano County would increase as dramatically as it has over the past three decades.
During the planning of the Solano Project, Napa County and Yolo County were asked if they wished to participate in a larger Solano Project. Napa and Yolo declined, so the Solano Project was sized to meet only the projected water needs of Solano County. Congressional authorization was granted for the construction of the Solano Project and the first water was delivered in 1959. The total construction cost for the Solano Project was $38 million. For a more detailed history of the Solano Project, see the book by the Solano Irrigation District entitled “The Solano Water Story: A History of the Solano Irrigation District and the Solano Project.”
Monticello Dam was constructed from 1953 to 1957 and has a height of 304 feet and a crest length of 1023 feet. The dam is able to hold a capacity of 1.55 million acre-feet of water. (For reference, two households use 1 acre-foot of water a year.)
Monticello Dam Spillway
What is a Spillway?
As part of the contruction of the Monticello Dam, engineers included a spillway into the design of the dam. The spillway is an engineered structure resembling a funnel. During very wet years, the volume of water stored in Lake Berryessa will exceed the capacity of the dam and the excess water is diverted by a unique spillway, affectionately referred to as the “Glory Hole” by locals. The spillway essentially acts like a funnel and diverts water underneath the dam and releases into the creek below.
Why Was the Spillway Designed This Way?
Monticello Dam was built to contain the waters of Putah Creek and its tributaries. Due to the narrow canyon where the dam was built, also known as Devil’s Gate, a traditional channel-type spillway going over the top of the dam could not be used. Therefore, a special type of spillway known as a bellmouth, or morning glory (owing to its resemblance to the eponymous flower), was created to accommodate excess water during very wet periods.
The spillway is 72 feet in diameter at the top and narrows down to 28 a foot diameter at the bottom over the course of a two-hundred-foot drop. When the surface elevation of Lake Berryessa exceeds 440 feet, water flows over the rim of spillway and is discharged below the dam. This prevents flooding of the lake itself. Highway 128 acts as an emergency spillway so that water will not overtop Monticello Dam.
How Often Does the Lake Spill?
Since 1957, Lake Berryessa has spilled on numerous occasions, most recently on February 4, 2025. This most recent spill was the first spill recorded since 2019. Overall, Lake Berryessa has spilled 26 times including the 2025 spill. Lake Berryessa spilled during the years 1963, 1965, 1967-1971, 1973-1975,1982-1984, 1986, 1996-2000, 2003-2006, 201, 2019, and 2025.

Photo depicting most recent spill as of February 2025. Photo courtesy Solano County Water Agency
Where Does the Water Go?
Water funneled through the spillway is sent down Putah Creek, through Lake Solano and makes its way through the Yolo Bypass. The Yolo Bypass is a major flood control system created to help prevent flooding of the Sacramento Metropolitan Area. The Bypass is also a rich wildlife habitat, home to hundreds of bird species and one of the largest Mexican free-tailed bat colonies in California.