SACRAMENTO - A proposal to send more water to Central Valley fields and Southern California neighborhoods could undermine ecological gains in the Sacramento-San Joaquin Delta, environmentalists say.

A 10-year-old effort to balance demands on the state's most abundant source of water has improved the odds for migrating salmon with the number of winter-run Chinook swimming through the Golden Gate Bridge and returning to Sacramento River spawning grounds approaching 10,000.

That compares to only 211 in 1991, although the area once supported millions of migrating salmon.

Environmentalists and some federal biologists say that progress could be undermined by a plan to raise pumping limits at the Harvey O. Banks state plant. The plant, which is at the heart of California's water delivery system, now funnels several billion gallons of water a day from the south Delta east of San Francisco to the 444-mile-long California Aqueduct.

The controversy is raising questions about CalFed, the 10-year-old government program of environmental and water supply improvements to the San Francisco Bay Delta system.

Environmentalists like Bill Jennings of the group DeltaKeeper say more pumping will undo improvements.

"I think the Delta is facing the greatest threat it's faced in 20 years," Jennings said.

But water managers and government officials say more pumping can be done without causing environmental damage. They say the new system will create more flexibility to capture more water in rainy years.

"Nobody is going to turn those pumps on 24/7," said Tim Quinn, vice president of the Metropolitan Water District of Southern California, which would be a major beneficiary of increased pumping. "If it's not working with the fishery, (we) will be out there leading the charge to change."

Final decision on whether to raise the limits rests with the state, which is conducting an environmental review.

"It's the state's position that after we do all this, we expect the fish to be better off than they have been," said California Water Resources Director Lester Snow. He said other steps are planned, including the installation of south Delta salinity barriers.

In a draft federal biological review leaked to the press last month, scientists with the National Marine Fisheries Service concluded that additional pumping, along with a proposed change in Shasta Dam operations, would harm endangered and threatened salmon runs.

But those findings were overruled. Last month, the agency formally released an opinion that the dam operation changes and additional pumping would not seriously harm the fish.

Critics, including 19 Democrats in Congress, said the change came after the fisheries agency consulted with the U.S. Bureau of Reclamation, which supplies federal water to powerful Central Valley irrigation districts.

Jim Lecky, the marine fisheries assistant regional administrator who oversaw the opinion rewrite, denied politics intervened. He said he ordered the changes because his biologists had made mistakes.

The Delta is formed by the confluence of the Sacramento and San Joaquin rivers as well as runoff from the Sierra Nevada and northwest California. It provides water to more than 22 million people in the state, irrigates millions of acres of farmland and sustains California's most important fishery habitat and the West Coast's biggest estuary.

The system has been threatened since the Gold Rush with mud and gravel washing down from mining, farmers draining the levees to plant crops and major water projects in the mid-1900s that diverted water south.

CalFed sought to stop the damage with a $10 billion, 30-year program. Roughly $500 million has been spent so far as dams have been removed, fish ladders and screens installed, levees set back and gravel laid to create spawning grounds.

It was not clear how much more water the extra pumping would take. State planners said state and federal water projects could send an additional 200,000 acre-feet a year south. An acre-foot is about the amount that two families use annually.

Quinn said his agency believed the figure would be higher. And environmentalists say the number could go up 1 million acre-feet a year if higher pumping limits were met daily.