SAN FRANCISCO -- U.S. wildlife officials gave crucial first approval Monday to California Gov. Jerry Brown's decades-old ambitions to build two massive tunnels that would re-engineer the water system in the nation's most populous state.

The National Marine Fisheries Service and U.S. Fish and Wildlife Service said the $16 billion project likely will not endanger more than a dozen federally listed species in the largest freshwater estuary on the West Coast.

The project "is not likely to jeopardize the continued existence of any of these species, and is not likely to destroy or adversely modify designated critical habitat," Paul Souza, a regional director of the wildlife service, said in a letter.

Get Breaking News Delivered to Your Inbox

The decision is the first in a series of federal and state rulings that will determine the fate of the proposed twin 35-mile tunnels, California's biggest water project in decades that is expected to take more than 10 years to complete.

The project would suck part of the Sacramento River into two four-story-high tunnels running below the river's delta with the San Joaquin River.

Supporters say it would ensure a reliable water supply for cities, farms and tens of millions of residents, most of them in Southern and central California.

Opponents say the project would speed the demise of some species of salmon and other native wildlife, already struggling after decades of heavy state reliance on water from the delta.

Ted Smith, a historian and author who has lived on the delta for more than 40 years, says many of the local farmers are against the project.

"Certainly water is the issue here, not just because of the tunnels and all of that, tap water has always been the issue in [the area]," Smith told CBS San Francisco.

The U.S. agencies had earlier acknowledged that the tunnels would further harm California's delta smelt -- a once abundant fish species that's now nearly extinct -- and winter-run Chinook salmon, another endangered native fish.

Earlier this month, a group of water agencies in the Central Valley Project, the state's largest water supplier, asked the U.S. Bureau of Reclamation to delay its own ruling on the project until the water agencies get assurances that project won't cost them money or cut their own water supplies, in a letter obtained by The Associated Press. The bureau, whose approval is critical to the project, has not publicly responded.

The project's chances of federal approval may have received a boost from the election of President Trump, who has said he backs big infrastructure projects and more water for central California farmers.

Brown is the project's most vocal backer, telling reporters earlier this month that the tunnels would allow the state to reduce its dependence on the giant mechanical pumps that now draw water from the delta, for use by the 25 million Californians who get part or all of their water from the state's giant north-south water projects.

Along with water withdrawals, which make the delta warmer, narrower and shallower for fish, the pumps are one of the factors harming native fish.

"There's so many dams and channels and bypasses -- I don't know if people are aware of how engineered our water system is," Brown said, adding that the delta already was no longer the wild habitat of "some mythical golden past."

Brown has pushed variations of the project since his first term four decades ago, including a version in the 1980s that would have created canals instead of tunnels. Opponents argued that it would benefit Southern California at the expense of the north, and Californians defeated the proposal in a statewide vote.

It was Brown's father, the late Gov. Pat Brown, who oversaw construction of the state's complex and aging water system of aqueducts, dams, lifting stations and pumps.

Meanwhile, several regional public water agencies that would get water from the tunnels must also decide whether they will commit to paying for the project. Southern California's giant Metropolitan Water District leads the push and is expected to decide by early fall.

Three officials involved in the project have told the AP that the politically powerful water districts are now demanding to have a bigger direct role in financing, designing and construction of the tunnels, instead of the state. Proponents say that would speed construction of the tunnels.

Opponents fear the shift could lead water districts to cut corners on safety and environmental measures, as well as compel water districts to extract and sell more water at a higher rate to pay off the huge bills for the tunnels.