How do you move up to 7 million gallons of water a day?
Santa Margarita Lake stores water in one of the best places in San Luis Obispo County to catch rainfall. It is less than 10 miles from San Luis Obispo as the crow flies, but the 2,500-foot-high ridge top of the Santa Lucia Range stands astride the path.
The terrain is so folded and creased that a canyon between ridge lines actually drains south east into another watershed — Lopez Lake above Arroyo Grande — rather than into the Salinas River.
Also known as Salinas Reservoir, Santa Margarita Lake was created by the construction of Salinas Dam at the southern end of the Salinas River. Deemed a military necessity, the U.S. Army Corps of Engineers project was designed to provide water to Camp San Luis Obispo in the early days of World War II, which soon brought thousands of troops to the camp.
A hidden engineering masterpiece connected the dam with the military base — a seldom seen, mile-long tunnel under the mountain range. It is a testament to the Army Corps’ work that the Cuesta Tunnel now carries water from three different projects: Nacimento Lake, California State Water and Salinas Reservoir.
Ann Fairbanks wrote this story, which ran in The Telegram-Tribune on May 4, 1994:
Field trip through water tunnel
CUESTA RIDGE — Sloshing through foot-high water, 17 city workers trekked through a one-mile tunnel Tuesday that normally carries up to 7 million gallons of water a day to San Luis Obispo residents.
Blasted through the Santa Lucia coastal range 53 years ago by the Army Corps of Engineers, the Cuesta Tunnel was the unlikely destination of a city field trip.
“I’ve been with the city 13 years and this is the first time I’ve been through here,” said John Rosson, a utilities maintenance worker with San Luis Obispo’s water department. “I’ve been told for years, ‘You’re going to get to come up,’ and it finally came through.”
Rosson and fellow employees of the city’s water and utilities departments donned rubber boots, picked up flashlights, and headed to the tunnel that will also eventually carry water from the State Water Project and, perhaps, from Lake Nacimento.
Tuesday’s tunnel tour and one planned next week for more city workers and council members are “the last trips through the tunnel before it’s concreted-in for the State Water Project,” said Ron Munds, the city’s water conservation coordinator.
This is an educational tour, because we talk about these facilities all the time and most of us haven’t seen them before.”
The four carloads of workers split into two groups. One group drove to the tunnel’s southern entrance behind the California Men’s Colony. The other group headed up to the north portal at the top of the Cuesta Grade.
Nearby is a storage tank which holds 3 million gallons of water from Salinas Dam, the city’s main water source that’s also known as Santa Margarita Lake. The stored water is then routed to a booster station and pumped to the tunnel. It normally flows as deep as 5 feet.
On Tuesday, the water was only up to a foot deep. Because the city is currently pumping water from its supplemental source at Whale Rock Reservoir, the tour participants sloshed only through accumulated spring water that seeps through the rock.
“What kind of rock is this?” one of the workers asked as he trained his flashlight on the exposed rock some 9 feet overhead.
“I think it’s serpentine,” Dan Gilmore, the city’s utilities engineer said as he reached up and scraped out a little sample.
Walking two abreast in the 5-foot-wide tunnel, the group examined the concrete-lined walls, dodged trickles of spring water, and quizzed Gilmore about the state’s plans for the tunnel. The coastal branch of the State Water Project includes a 42-inch pipeline that will travel through the tunnel. At the same time that pipeline is laid, the state — working under a $1 million contract with the county — will install a 24-inch pipeline that will carry water from Lake Nacimento, assuming that project gets the go-ahead.
Above the two pipelines, a trough will be built to carry Santa Margarita Lake water. Together the two pipelines and the trough will allows only enough space in the tunnel for a maintenance worker on a cart.
“Watch out for the jeep coming through,” Richard Brown, a water department employee, joked as a horn he brought along blared “ooga ooga” into the darkness.
“Hey what are those lights coming our way?” someone else quipped as the two groups approached the halfway meeting point.
Keys were exchanged for the cars at either end amid comments like, “How did you get in here?” and “Did you guys pass that jeep?”
With final instructions on locking the tunnel’s doors, the groups proceeded in opposite directions, heading toward the light at the ends of the tunnel.
“The tide’s coming in,” someone remarked as the southbound group approached the lower end of the tunnel where the water flows the deepest.
After several yards of foot-deep water and two steps up and out of the tunnel, the tour participants emerged into a brilliant vista of green hills high above the California Men’s Colony.
Driving on a dirt road through ranchland, state and federal land, the group passed Chorro Reservoir — the water source for CMC and Camp San Luis Obispo that was originally built to accept Salinas Dam water via the tunnel. It’s now fed by springs and Whale Rock Reservoir.
Chorro Reservoir and the Salinas Dam were built by the Army Corps of Engineers in 1941 to supply Camp San Luis Obispo during World War II. The dam and tunnel were built in record time, proceeding from design in May 1941 to completion before the end of that year.
In 1947 the dam was declared surplus and the county has leased it from the Corps of Engineers since 1952.
“I kind of hate to see them plug the tunnel up,” said Brown, a San Luis Obispo native. “Its a piece of history.”