San Antonio Dam
This page provides a brief overview of the history, purpose, and design of the San Antonio Dam.
The San Antonio Dam was completed in 1967 and is owned by the Monterey County Water Resources Agency.The San Antonio Dam is located in Southern Monterey County, about 16 miles northwest from Paso Robles, in Central California. It is 4.4 miles from the Nacimiento Dam. The San Antonio Dam creates the San Antonio Reservoir, which has the ability to store water from winter storms for the dry season. At an elevation of 780, the reservoir has the capacity to store 335,000 acre-feet of water when it is full. It is roughly 16 miles long, and has about 100 miles of shoreline available for recreation. The maximum flood stage elevation is 802 feet. The maximum temporary capacity during a flood event is 477,000 acre-feet, with a surface area of 7,500 acres. 
The primary function of the San Antonio Dam is to provide an instrument for groundwater recharge and flood control in the Salinas Valley.  The San Antonio Dam and Nacimiento Dam are also involved in the recently created Salinas Valley Water Project and they provide opportunities for water recreation in southern Monterey County and northern San Luis Obispo County.  The San Antonio Dam stores water during the winter and releases it during the summer. Water released from the dam travels down the San Antonio River to join the Salinas River groundwater system, which contributes to the Salinas Valley water supply. The Salinas River Diversion Facility captures some of this released water, and creates an artificial dam near the mouth of the Salinas River.  This timed-release and gradual groundwater recharge helps to slow the rate of seawater intrusion in the Salinas Valley. 
- The San Antonio Dam is located at 35°48′25″ N, 120°54′00″ W. 
- Dam Type as determined by the California Department of Water Resources: ERTH – Earth
- Dam Usage as determined by the California Department of Water Resources: STO,FC,DOM,IRR,MUN - Storage, Flood Control, Domestic, Irrigation, Municpial 
An audit of the 60-year-old dam found significant cracking in the concrete surface of the spillway, and the integrity of the understructure has been compromised by erosion and clogged drainage pipes — a “poor condition” that the state Division of Safety of Dams said in an April 2019 letter is “unsafe for use under high flow conditions.” The San Antonio Dam is currently classified as an “Extremely High” downstream hazard by the Division of Safety of Dams of the California Department of Water Resources.  Failure of the spillway would lead to massive flooding downriver, first hitting the community of Bradley. Backwater in the Salinas River could also cause inundation at Camp Roberts. Monterey County has five years to completely overhaul the spillway or replace it at a cost that will likely reach above $40 million, some of which San Luis Obispo County might have to pay. 
The peak flow through the breach is estimated to be almost 1,500,000 cubic feet per second (cfs) and is estimated to occur 2.75 hours after the dam begins to fail. Models predict that almost all the reservoir would be drained within one day, and that the flood wave would take about one day to reach the mouth of the Salinas River. At the mouth, the flow rate is projected to be 90,000 cfs. The towns of Bradley, Spreckels, San Ardo, King City, Salinas, and Castroville would experience full or partial flooding. 
In the San Antonio Dam, operational pools refer to management of water being stored in the reservoir. There are a few different operational pools related to storage, release, and conservation.
- Physical minimum pool - maximum height: 645 feet. At this elevation or lower, water cannot flow out of the reservoir.
- Operational minimum pool - maximum height: 666 feet. Any water below this elevation is reserved for fish habitat.
- Conservation pool - maximum height: 774.5 feet. Water is "conserved" to be later released into the Salinas River.
The conservation pool can store 305,000 acre-feet of water. There is one more pool known as the flood pool, with a water elevation of 780 feet and 335,000 acre-feet of storage. The flood pool is meant to provide protection from a historic flood event, and water can flow from the flood pool to the spillway. 
San Antonio Dam Measurements
The Monterey County Water Resources Agency publishes observed data for preceding 10-day periods for the following metrics: Percentage of Capacity, Elevation(feet), Depth(feet), Storage(acre-feet), Lake Surface Area(acres), Length of Lake(miles), Releases(cfs), Rainfall Past 24 hours(in.), Rainfall to Date Last year(in.), and Total Rain Last Season(in.). They also include historical data from 1958-present for the same parameters. This information can be found at (MCWRA Reservoir Data)
- Monterey County Water Resources Agency MCWRA: Dams and Reservoirs Updated 2020.
- Monterey County Water Resources Agency Monterey County Water Resources Agency Engineering Reports Updated July 21, 2017.
- CalWater Nacimiento Dam Report #79 Published 2014.
- Monterey County Water Resources Agency Monterey County Water Resources Agency: Seawater Intrusion Monitoring Updated 2020.
- USGS Feature Report: Lake San Antonio USGS Feature Report: Lake San Antonio
- California Data Exchange Center CDEC: Dam Definitions Updated 2017.
- Monterey County Office of Emergency Services Monterey County Dam Safety Updated 2020.
- Vaughan, Monica. Dam spillway near SLO County has significant cracks Published August 1, 2019.
- Monterey County Water Resources Agency (MCWRA)
- Salinas Valley Water Project (SVWP)
- Salinas River Diversion Facility (SRDF)
- Salinas River
- Nacimiento River
- Salinas Valley Seawater Intrusion
- Monterey County
- San Luis Obispo County
- San Clemente Dam
- Interlake Tunnel
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