Endangered salmon and steelhead in Central Valley rivers must have access again to historic spawning grounds above major California dams, according to sweeping new federal rules that could boost water bills for millions statewide.

The National Marine Fisheries Service unveiled the complex set of rules, called a biological opinion, Thursday in response to a lawsuit by environmental groups. Affected species are winter- and spring-run salmon, Central Valley steelhead and green sturgeon.

The rules require the U.S. Bureau of Reclamation to restore access for fish to waters above Nimbus and Folsom dams on the American River, Shasta Dam on the Sacramento, and New Melones Dam on the Stanislaus.

Those dams were built decades ago without fish ladders and have blocked access to hundreds of miles of historic spawning grounds.

So dire is the situation that experts have concluded the rules are also necessary to save an endangered population of killer whales that range from British Columbia to California and primarily eat salmon. If California's salmon disappear, killer whales could be next.

"They've addressed the big issues," said Kate Poole, attorney at the Natural Resources Defense Council. "There's no question any more about the fact that the Bay-Delta ecosystem is in dire need of significant changes and fixes. This is one big step to do that."

The environmental group American Rivers, not a party to the lawsuit, said the new rules are unprecedented.

"This is the most significant single order for fish passage that we're aware of," said Steve Rothert, the group's California director.

Water agencies can appeal the rules. They argue that, over the long term, a state and federal habitat conservation plan they're now drafting will achieve the same goals, yet allow for more flexibility in managing water.

Under Thursday's new rules, water diversions from the Sacramento-San Joaquin Delta must be cut 5 percent to 7 percent under certain conditions, which may worsen water shortages in some areas.

The new federal rules mark the latest episode in the drama over a California aquatic environment spinning out of control. It comes on the heels of similar rules imposed in December to protect the threatened Delta smelt, which also reduced water availability for farms and cities.

Officials on Thursday said Californians may have simply pushed the limit of the state's available freshwater supplies.

"You're going to see less reliable water as it relates to farming in the Central Valley, and it will become more difficult to find replacement water for urban growth," said Donald Glaser, the Bureau of Reclamation's regional director. "We have to just find better ways to make efficient use of the water we have."

The rules also require changes at salmon hatcheries, including Nimbus Hatchery on the American River, to improve survival of wild salmon.

Reclamation also must adopt a new water flow standard for the American River, and find a way to flood the Yolo Bypass more often to improve salmon habitat.

But retrofitting the dams for fish passage is by far the most costly and significant measure. Building traditional fish ladders is likely to cost billions of dollars, though the rules don't require this. Instead, the fisheries service is ordering a multi-agency task force to recommend ways to restore fish above the dams by 2016, and then to carry out the best options by 2020.

The ruling also governs water operations of the California Department of Water Resources. DWR will share the cost of the new orders, agency spokesman Matt Notley said.

Glaser said costs will likely be passed down through water contractors to consumers throughout California. This could drive up water bills for millions of farmers and urban Californians from Red Bluff to San Diego.

"We are acutely aware of the significance of this opinion for the region's farmers and residents," said Maria Rea, manager of the fisheries service's Sacramento office, which prepared the rules. "What is at stake here is not just survival of the species but the health of the entire ecosystems that depend on them."

NRDC and other environmental and fishing groups sued the government to overturn prior federal rules protecting Central Valley salmon and steelhead. Subsequent investigations showed those rules, adopted during the Bush administration, were influenced by politics and lacked scientific rigor.

Thursday's new rules went through two independent reviews, but that didn't stop politicians and interest groups from pushing back.

"This federal biological opinion puts fish above the needs of millions of Californians and the health and security of the world's eighth largest economy," Gov. Arnold Schwarzenegger said.

Western Growers, a farm group, said the rules would cause "real and very serious harms to the human species."

Others, however, said restoring salmon could bring enormous benefits to the environment and the economy.

Will Templin, of the Upper American River Foundation, said there is still good habitat to welcome back migrating fish. A recent genetic study, Templin said, showed some rainbow trout on the upper American River are actually remnants of steelhead that once migrated from the ocean.

"For me, it'll feel like something long overdue," he said.

Call The Bee's Matt Weiser, (916) 321-1264.