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Here’s how $500M new reservoir planned near Patterson would work

The area west of Patterson Calif. along Del Puerto Canyon Road is pictured on Wednesday afternoon May 29, 2019. A proposal for an 800 acre reservoir that would include a lake in the scenic canyon in the foothills west of Patterson is underway.
The area west of Patterson Calif. along Del Puerto Canyon Road is pictured on Wednesday afternoon May 29, 2019. A proposal for an 800 acre reservoir that would include a lake in the scenic canyon in the foothills west of Patterson is underway. jlee@modbee.com

A proposed reservoir in Del Puerto Canyon, just west of Patterson, promises reliable water deliveries for farms in western Stanislaus County and nearby counties.

It could serve to recharge groundwater for Patterson, a city of 23,750 residents, while other proposed benefits are water deliveries for wildlife refuges and flood control on occasions when storms threaten flash floods on Del Puerto Creek.

Proponents including Del Puerto Water District and the San Joaquin River Exchange Contractors Water Authority discussed the multiple benefits of the Del Puerto Canyon Reservoir at a press briefing Monday.

A draft environmental study on one of the newest water storage projects in California will be released in late November.

Anthea Hansen, Del Puerto’s general manager, said no issues have arisen that would stop the canyon reservoir project, which has drawn little public attention until recently. Estimated to cost $400 million to $500 million, the dam and reservoir just west of Interstate 5 could be built over a six-year period starting as early as 2022.

The environmental study is assessing the impacts on wildlife, cultural resources, traffic, air quality and other issues. The canyon reservoir could store as much as 85,000 acre feet of water pumped from the nearby Delta Mendota Canal.

Del Puerto and four districts in the exchange contractors authority have contracts for water from the federal Central Valley Project, a massive complex of dams and canals in Northern and Central California. Water for those farmers in Stanislaus, San Joaquin, Merced, Fresno and Madera counties needs to be pumped south of the Sacramento-San Joaquin delta, but dry years and protections for endangered fish have resulted in unreliable deliveries or none at all.

Chris White, executive director of the exchange contractors, said the canyon reservoir will hold water in wetter years and release it for irrigation in drier periods.

Water released into Del Puerto Creek downstream from the dam would filter through the permeable creek bed to replenish groundwater. The city of Patterson also could put the additional creek water in a recharge basin to prop up the level in wells.

City officials have asked for studies on the safety of a reservoir larger than Turlock Lake and a 200-foot-high earthen dam plugging the historic “gateway” or mouth of Del Puerto Canyon.

White said the state Department of Water Resources division of dam safety will require a safe design and uphold the highest standards in reviewing the design.

Barbara Barrigan-Parrilla, executive director of Restore the Delta, a delta protection group, said a project taking delta water in wet periods won’t likely raise issues, but filling the reservoir in dry years could spark a reaction from environmental groups.

“If taking more water means gutting protections for the delta than it becomes problematic,” she said.

Barrigan-Parrilla said her group will probably look at the operational details in the environmental study. She added it may be wise to spend money on upgrades to the San Luis Reservoir dam before funding a new reservoir. Some of the funding for canyon reservoir would come from the Water Infrastructure Improvements for the Nation Act, plus multiple other sources.

Hansen said one of the challenging issues will be access for heavy equipment building the dam just off Interstate 5. The only access point now is the limited Sperry Avenue interchange.

In addition, the districts will need to move a Shell Oil Company line and reroute Del Puerto Canyon Road.

There no plans for boating or other on-the-water recreation at the reservoir.

After the environmental study is released, meetings will be held for public comments. The public comments will be addressed in the final study.

Ken Carlson covers county government and health care for The Modesto Bee. His coverage of public health, medicine, consumer health issues and the business of health care has appeared in The Bee for 15 years.
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