Overview

Nacimiento Dam and Reservoir

Nacimiento Dam and its reservoir, Nacimiento Reservoir, are located in northern San Luis Obispo County, about 20 miles from the coast, in central California.

Nacimiento Dam was completed in 1957. This earthfill dam has a height of 215 feet above the streambed and a crest length of 1,650 feet. The crest elevation is 825 feet above mean sea level (msl) with a spillway elevation of 787.75 feet which can be raised to an elevation of 800 feet by the use of an inflatable Obermeyer spillway gate. The dam has two outlets. The High Level Outlet Works (HLOW) is composed of twin 8 foot x 8 foot square steel slide gates and cast concrete tunnels located under the center of the spillway at an elevation of 755 feet. The HLOW has a maximum capacity of 5,500 cubic feet per second (cfs) when the Lake elevation is 800 feet. The Low Level Outlet Works (LLOW) is a 53 inch diameter pipe located near the southern side of the Dam. The inlet to the LLOW consists of three 42 inch butterfly valves set in a concrete structure at an elevation of 670 feet. Releases from the LLOW can be made from either a manifold of 6 24 inch manually-operated valves or the Hydroelectric Power Plant. The LLOW has a maximum capacity of 460 cfs when the Lake elevation is 800 feet.

When the reservoir is full (elevation 800 feet) it has a maximum storage capacity of 377,900 acre-feet, is 18 miles long, and has about 165 miles of shoreline. The maximum elevation during flood stage is 825 feet, with a maximum temporary capacity of 538,000 acre feet and a temporary surface area of 7,149 acres.

Operational pools have been created to aid in the management of water being stored in the reservoir. The physical minimum pool or dead pool is at an elevation of 670 feet, 10,300 acre feet of storage, at the invert of the intake structure of the low level outlet works. Water cannot flow by gravity out of the reservoir below an elevation of 670 feet. The operational minimum pool is at elevation 687.8 feet, 22,300 acre feet of storage. Water below an elevation of 687.8 feet is reserved for fish and wildlife habitat as well as a water entitlement belonging to the County of San Luis Obispo. The conservation pool which extends from the minimum pool to the concrete spillway elevation of 787.75 feet is the operational pool used to store water for later release to the Salinas River for groundwater recharge, fish passage, and the operation of the Salinas Valley Water Project. The flood pool extends from the concrete spillway to an elevation of 801 feet or 1 foot above the top of the inflated Obermeyer spillway gate. The flood pool is intended to provide winter flood protection by maintaining the ability of the spillway to pass the Probable Maximum Flood without overtopping of the dam.

A 4-megawatt powerplant is constructed at the right dam abutment. The plant contains both large and small turbines that operate in the range of 25 cfs to 400 cfs. To maximize power production, two sets of runners have been provided. One set is used at low reservoir elevations and the second at high reservoir elevations.

Nacimiento Dam is under the jurisdiction of the California Department of Water Resources, Division of Safety of Dams (DSOD) and the Federal Energy Regulatory Commission (FERC). The FERC has jurisdiction over the project due to the existence of the hydroelectric plant.

San Antonio Dam and Reservoir

View from below San Antonio Dam Spillway and Control House San Antonio Dam and its reservoir, San Antonio Reservoir, are located in southern Monterey County, about 16 miles northwest from Paso Robles, in central California. San Antonio Dam was completed in 1967. This earthfill dam has a height of 201 feet above the streambed and a crest length of 1,433 feet. The crest of the dam elevation is 802 feet above mean sea level (msl) with a spillway crest elevation of 780 feet and a capacity of 35,400 cfs. San Antonio Dam has an Outlet Works consisting of an 84-inch diameter, 1,085 foot- long steel conduit located near the center of the Dam. The conduit leads through the dam embankment from a small intake structure to an outlet structure, which contains a Howell-Bunger type valve and supports a concrete house. The Outlet Works has a maximum capacity of 2,200 cfs when the reservoir elevation is 780 feet. When the reservoir is full (elevation 780 feet), which is the crest of the spillway, it has a maximum storage capacity of 335,000 acre-feet, is 16 miles long, and has about 100 miles of shoreline. The maximum elevation during flood stage is 802 feet, with a maximum temporary capacity of about 477,000 acre-feet and a temporary surface area of about 7,500 acres.

Operational pools have been created to aid in the management of water being stored in the reservoir. The physical minimum pool or dead pool is at an elevation of 645 feet, 10,000 acre feet of storage, at the invert of the intake structure of the outlet works. Water cannot flow by gravity out of the reservoir below an elevation of 645 feet. The operational minimum pool is at elevation 666 feet, 23,000 acre feet of storage. Water below an elevation of 666 feet is reserved for fish and wildlife habitat. The conservation pool which extends from minimum pool to elevation of 774.5 feet, 305,000 acre feet of storage, is the operational pool used to store water for later release to the Salinas River for groundwater recharge, fish passage, and the operation of the Salinas Valley Water Project. The flood pool extends from the conservation pool to the spillway elevation of 780 feet, 335,000 acre feet of storage. The flood pool is intended to provide winter flood protection by maintaining the ability of the spillway to pass the Probable Maximum Flood without overtopping of the dam.

San Antonio Dam is under the jurisdiction of the California Department of Water Resources, Division of Safety of Dams (DSOD).

Pages Under Dams and Reservoirs

• Reservoir Data

• Nacimiento and San Antonio Dam Breach inundation maps

• Feb 2019 presentation of the Nacimiento and San Antonio Dam breach inundation maps to the Board of Directors

• Nacimiento Dam Operation Policy

• Links