(Updated 8/5/10)

The expansion of the Los Vaqueros Reservoir will be a boon to the humans who rely on it for their drinking and irrigation water supply, but it could affect the wildlife living in and around the reservoir, particularly during construction, according to a recent study.

The Contra Costa Water District (CCWD) approved the expansion by 60 percent in March. Construction planned to begin early next year will increase its capacity to 160,000 acre-feet of water from the current 100,000 acre-feet.

The benefits include providing a reliable water supply in drought periods, improving water quality, helping the environment and creating jobs, according to a CCWD press release. “The expansion to 160,000 acre-feet will provide immediate local benefits, but does not preclude future consideration of further expansion with commitments from partners regionally and statewide,” the release states.

That further expansion could total 275,000 acre-feet, nearly tripling the current size of the reservoir, which would increase the impacts on wildlife in the area, according to a U.S. Fish and Wildlife Service (USFWS) report.

Animals in the 18,000-acre Los Vaqueros Watershed include endangered or threatened species such as the San Joaquin kit fox, bald eagle, California red-legged frog, Alameda whipsnake, California tiger salamander and vernal pool fairy shrimp.

Expanding the reservoir to 275,000 acre- feet “would have the most extensive terrestrial impacts, including permanent and temporary impacts to existing conservation easements, existing mitigation areas, critical habitat for federally-listed species, proposed critical habitat for federally-listed species, and potential impacts to sensitive fish, wildlife, and plant species,” concludes the USFWS report.

In addition, the increased diversion of water from the Delta to the reservoir “may incrementally affect aquatic habitat characteristics in the Delta, and contribute to conditions that negatively impact sensitive species in the Delta.”

Expanding the reservoir to just 160,000 acre-feet would reduce the wildlife impacts, but it “would still result in impacts to grassland habitat within the Los Vaqueros watershed, in addition to impacts to other habitats within the watershed,” the report states.

The USFWS is withholding support for either option for now, awaiting more information on what CCWD and its possible partners plan to do to minimize the impacts, “including where and how much mitigation habitat would be acquired, loss of dedicated conservation easement lands preserving movement corridors, uncertainty regarding water operations agreements” between CCWD and other agencies and completion of further studies.

CCWD Spokeswoman Jennifer Allen said the district will work cooperatively with USFWS "to come up with a mitigation strategy that would more than offset any impacts.”

On the plus side, the expansion is expected to reduce the current impacts to Delta fish by changing the timing that water is diverted from the Delta, improving flow conditions during certain times of the year and during drought years and improving other aquatic characteristics, according to the report.

You can download a copy of the USFWS report, the reservoir expansion environmental impact report and other documents at www.lvstudies.com.