Making up for a Bigger Dam at Los Vaqueros

Los Vaqueros Reservoir, sprawling improbably through the dry hills of eastern Contra Costa County, holds 100,000 acre-feet of Sacramento-San Joaquin Delta water for the 550,000 customers of the Contra Costa Water District (CCWD). “CCWD is 100 percent dependent on the Delta for water, and prior to the construction of Los Vaqueros, CCWD customers were subject to seasonal fluctuations in salinity,” says the water district’s Jennifer Allen. During high Delta flows, the reservoir is filled with low-salinity water, which is then blended with the saltier water diverted during low flows. Allen says expanding the reservoir’s capacity to 160,000 acre-feet will improve the reliability and quality of the water supply and also protect Delta fisheries by allowing CCWD to adjust the timing of intake.

But those benefits come with an extra price tag for the water district, which is buying conservation land in three counties to mitigate the flooding of 340 acres of seasonal wetlands, oak woodlands, and grasslands. Allen says negotiations are still underway for several properties and won’t say exactly how much land is being protected in exchange for the increased capacity, but the overall package may amount to as much as 6,000 acres.

Seth Adams, director of land programs for Save Mount Diablo, says it’s an unprecedented amount of mitigation. Purchases already secured include 661 acres in Contra Costa County, 573 in Alameda County, and 3,000 in San Joaquin County. Adams is particularly pleased about two properties connected by a cattle tunnel beneath Interstate 580. The tunnel will provide one of the few wildlife corridors across the freeway in the Altamont Pass. “It will provide a significant linkage, so Mount Diablo and public lands to the north of I-580 don’t get cut off from the Diablo range on the south side,” Adams says.

CCWD will maintain ownership and manage the mitigation land, but the state Department of Fish & Game will hold conservation easements on the properties. Adams says Save Mount Diablo pushed for aggressive mitigation partly because the expanded reservoir is drowning land that, according to existing conservation easements, should have been permanently protected.

During construction, hiking and fishing may be disrupted. See our hiking tips, and go to www.ccwater.com/losvaqueros for closure updates.