Prior to the construction of Nacimiento Dam, the lower Nacimiento River only flowed during the wet season between December and May. Without enough time to be absorbed into the Salinas Valley aquifer, much of the stormwater flowed directly into the Pacific Ocean. As the Salinas Valley developed as a major agricultural region dependent on groundwater, the Monterey County Flood Control and Water Conservation District proposed a reservoir on the Nacimiento River to capture winter floods, and release it at a low enough rate throughout the year to maximize groundwater recharge. Because the Nacimiento River is the biggest tributary of the Salinas River, the dam would capture up to half the annual peak flows in the entire Salinas River Basin. Under the county plan, water would only be stored when the flow rate at the confluence of the Nacimiento and Salinas Rivers exceeded, which was estimated as the highest volume the riverbed can naturally absorb. When natural runoff exceeded this amount, no water would be released. The California Department of Fish and Game opposed the project due to the detrimental impacts this would have on winter run steelhead trout. The DFG sought a minimum flow of in the Nacimiento River, which the county believed would place too great a demand on the new reservoir. Ultimately, the county won and was granted the water rights for the reservoir in 1955 without any provisions for fishery flows. Construction of Nacimiento Dam began in 1955 and was completed in 1957. The reservoir filled to capacity for the first time in 1958. After further petitioning from the DFG, the MCWRA now maintains a minimum flow of in the Nacimiento River except in years of severe drought. Monterey County is entitled to most of the water supply from Lake Nacimiento. The dam releases an average of of water per year for groundwater recharge and instream flows in the Nacimiento River. San Luis Obispo County has a smaller allocation of per year which is used for domestic water supply via the Nacimiento Water Project, providing water to Paso Robles, Templeton, Atascadero and San Luis Obispo. Although proposed since the 1950s, the Nacimiento Water Project was not completed until 2011.
Specifications
Nacimiento Dam is an earthfill dam with a height of and a crest length of. Altogether, the dam contains of material. The crest of the dam is above sea level, and the spillway crest elevation is. The concrete spillway is located on the north side of the dam and is controlled by an inflatable Obermeyer gate. The dam also has two outlet works, a low level valve for normal water releases with a design capacity of, and a high level valve for flood control with a capacity of. There is also a small hydroelectric plant at the dam with a capacity of 4,000 kilowatts. Lake Nacimiento has a design capacity of at a water level of . The lake receives water from a drainage basin of and covers up to. The conservation pool, which is used for irrigation, groundwater recharge and environmental flows, extends from the minimum operating level of to the spillway crest of ; the flood control pool is all storage above the spillway crest. The surcharge pool extends from the spillway gate to the top of the dam at. At this maximum level, the lake holds and increases to. Due to the flooding tendency of the Nacimiento River, which has been called "the most active watershed in the state", the reservoir can fill quickly to capacity during wet winters. The spillway gates are left open between November 1 and May 1 in order to prevent the lake from overfilling. In contrast, it is often at low levels for multiple consecutive years during droughts. The lake spilled in 1958, 1969, 1983, 2011 and 2017.
