Gov. Jerry Brown has billed his $25 billion plan to build two massive tunnels under the Delta as a way to not just make it easier to move water from north to south, but also increase the reliability of water supplies and bring back salmon and other endangered species.

But now the Brown administration is proposing a major and politically risky change: dropping a 50-year guarantee to restore the Sacramento-San Joaquin River Delta’s environment. A centerpiece of the project, the environmental plan included $8 billion to preserve 100,000 acres of wetlands and dozens of other restoration efforts.

The dramatic course correction, whose details have not yet been made public, comes after biologists at the U.S. Fish and Wildlife Service and other federal agencies told the state they won’t issue permits for the environmental plan. The reasons, the biologists say, is that the state cannot prove it will restore salmon, smelt, sturgeon and other wildlife struggling for survival in the Delta.

Losing the guarantee of 50 years of environmental restoration would create a substantial political problem for one of Brown’s signature projects.

First, it would be easier for environmentalists and other opponents to describe the twin-tunnels plan as little more than a water grab by Southern California, a perception that could make it untenable to Northern Californians.

At the same time, it would also be harder to gain support from water districts around the state — whom Brown is counting on to pick up the $17 billion cost to build and operate the tunnels. That’s because the 50-year “habitat conservation plan” was supposed to guarantee them reliability from endangered species lawsuits and decisions by the federal government that have limited Delta pumping in recent years to protect endangered fish.

Richard Stapler, a spokesman for the California Natural Resources Agency, confirmed Friday that the state in the next few months will release an addendum to the project’s environmental impact report that reflects the proposed change.

Stapler insisted that efforts to restore endangered salmon and other species in the Delta will still be part of the project. “We will be taking a broader look and more thoughtful approach to habitat restoration,” he said.

But asked if that means the roughly $8 billion in the plan now earmarked for environmental restoration will still be guaranteed up front, he acknowledged: “We still don’t have a determination as to that piece of it.”

The governor himself on Saturday downplayed the issue. “I think it’s kind of a technical point,” Brown told reporters at a Colusa County cook-off. “Somehow it satisfies the federal fish and other authorities, so that’s good with me.”

The two tunnels would each be 40 feet wide and 35 miles long. They would take water from the Sacramento River north of Sacramento and move it under the Delta to state and federal pumps in Tracy.

The pumps send water down the State Water Project and federal Central Valley Project to 25 million Californians from Silicon Valley to San Diego — and to 3 million acres of Central Valley farmland.

The Brown administration says the tunnels will not provide more water than California now takes from the Delta. But by moving the intake structures north, supporters say, the plan will make the water more reliable by reducing reliance on the huge pumps, which grind up and kill fish and make parts of the Delta run backward.

Environmentalists say that without the 50-year environmental guarantee up front in a legally binding document, promises that the Delta will be restored after the tunnels are built mean little.

“I’m reminded of what Wimpy in the old Popeye cartoons used to say: ‘I will gladly pay you Tuesday for a hamburger today,'” said Jonas Minton, water policy adviser for the Planning and Conservation League.

Environmentalists say the solution to the Delta’s collapsing wildlife, water quality and fisheries is for cities and large Central Valley farms to take less water from the Delta. Water supplies can be boosted, they say, by more conservation, more water recycling, drip irrigation and other ways. “Why are they wasting more of the public’s money pursuing the worst half of a failed project?” Minton asked.

Over the past eight years, California’s major water districts have paid $240 million to fund studies of the tunnels plan. Among them: the Santa Clara Valley Water District, Metropolitan Water District of Southern California, and Westlands Water District in Fresno.

This year, water districts are being asked to kick in more money for planning. But questions are growing.

“I think that it would be a concern for the public if the twin tunnels project would move ahead without the long-term environmental protections in place,” said Barbara Keegan, a board member of the Santa Clara Valley Water District. “The project has been defined as having two co-equal goals — improving water supply reliability and restoring the environment. And if we are proceeding with one goal and not the other, obviously that’s going to be an issue.”

The state’s largest water agency, the Metropolitan Water District of Southern California, which provides water to 19 million people from Los Angeles to San Diego, was hoping the 50-year plan would fly, said General Manager Jeff Kightlinger.

Although he still supports the project, it will now be harder to fund, Kightlinger said, because rather than having a 50-year guarantee that districts could take water from the Delta more reliably, water agencies could be essentially back to a year-to-year situation in which the pumps can be turned down any time by federal agencies and courts if endangered fish are harmed.

“The goal was to do something better than what we are doing today,” he said. “This would not meet that goal. So that’s something I know our board is going to have to take a hard look at. We’re going to have to really scrub that and say does it really make sense to reinvest this kind of money in a system and not really know what you are getting?”

Farm water groups also are increasingly perturbed.

“The construct of using a 50-year habitat conservation plan is done. It’s off the table,” said Jason Peltier, Westlands’ deputy general manager. “The simple fact is that we don’t have a viable project. We have never seen the state develop a plan that’s affordable and improves water reliability and supply.”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN