Reservoir Saving Water for a Rainless Day

People in Southern California are always being admonished to store extra water for emergencies.

Just in case they have ignored the warnings, officials from the Metropolitan Water District of Southern California dedicated a $2-billion man-made lake Saturday that will eventually hold a six-month supply. It is the largest earthen dam project in the United States and the seventh-largest in the world.

The new Diamond Valley Lake was shown off to about 4,000 people who gathered in this arid valley in southwest Riverside County, where farmers once harvested onions.

Water from Northern California and the Colorado River began pouring into the reservoir in January, fast enough to fill an Olympic-size swimming pool almost every second.

Even at that rate, the reservoir will not be filled for two or three years. When it’s done, Diamond Valley Lake will be 4 1/2 miles long, 2 1/2 miles wide and 260 feet deep at the western dam. It will contain 800,000 acre-feet of water, about 30% more than Lake Havasu and more than twice as much as Castaic Lake, the next largest reservoir in Southern California.

The lake’s creation was celebrated Saturday with daytime fireworks, bands and speeches.

Robert Roseen, 16, admitted he was dragged to the event by his parents. Once there, however, the teenager was impressed. “That dam is massive,” he said. MWD officials calculated that filling the lake with a garden hose would take nearly 20,000 years.

The lake will increase the total amount of water stored above ground in Southern California by about 50%. It is being paid for by MWD customers and is an insurance policy against the loss of other water supplies because of earthquakes or drought.

The MWD began construction in 1995, after 10 years of planning and study. One of the first issues was where to put it. Officials settled on Diamond Valley because there were no existing wetlands and few environmental impacts. To play it safe, the MWD bought enough nearby land for a wildlife preserve in case some species are someday declared endangered.

Fish already have been planted in the lake, which will be open to the public in about two years. The dam is visible to motorists looking east from Interstate 215 between Murrieta and Menifee.

The availability of six months’ emergency water--in case the San Andreas fault rips asunder the canals and aqueducts that bring water to Southern California--impressed an engineer from the Otay Water District in San Diego County, who attended Saturday’s ceremony.

“We’ve only got 10 days of emergency water storage,” he said. “Six months will be nice.” It was not an easy job.

Each end of the valley was plugged by two huge earthen dams, and a third, smaller dam plugged a saddle dip in one of the ridgelines. MWD officials, who calculated several analogies to show the project’s scope, said the amount of earth and rock used in the dams’ construction could build a 7-foot by 3-foot wall around the equator.

There were no rivers to fill the valley. But nearby canals carry water from the Colorado River and Northern California toward San Diego. That water is now being diverted into a holding basin, from which it is pumped about 200 feet up to a tower that serves as a two-way spigot for the lake.

When reservoir water is needed, the same valves will be opened, allowing the water to flow back into the nearby canals. From there, the water will flow by gravity to 90% of the MWD’s service area--which stretches from Ventura County to San Diego County.

The construction project unearthed 300 archeological sites, some dating back more than 7,000 years, but mostly from the last 200 to 500 years.