SAN DIEGO -- Conservationists and an Indian tribe sued the federal government on Wednesday to indefinitely stop water transfers they claim could further threaten the dying Salton Sea.

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The suit, filed in federal court in Riverside, could delay a complicated water swap aimed at reducing California's overuse of Colorado River water.

The suit claims that Interior Secretary Gale Norton and the Bureau of Reclamation have failed to complete mandated studies on restoring the sea -- a landlocked body of water 120 miles northeast of San Diego that is a main stopping place for tens of thousands of migrating birds.

A call seeking comment from the Interior Department was not immediately returned.

The 380-square-mile lake, created in the desert by a 1905 canal breach, survives mainly through agricultural runoff. But with no outlet to the ocean, it has become increasingly salty and could be unable to support fish or wildlife in a few decades.

A 1998 act required the government to develop a plan by 2000 to restore the sea. The suit says feasibility studies and a final environmental impact statement are two years late.

The action was filed by the Sierra Club, the Center for Biological Diversity, and the Cabazon Band of Mission Indians, whose members historically have used the sea and surrounding areas for hunting, fishing, and other activities.

The sea is an "environmental treasure," said Elden Hughes, chair of the Sierra Club's California/Nevada Desert Committee.

"One day last year there were 24,000 white pelicans on the sea. That's almost every white pelican that's in the western United States," Hughes said. "There's no other place to go. Ninety percent of our wetlands along the (California) coast are gone. They've been filled in."

Hughes believes the Bureau of Reclamation has completed its report but "they're afraid of what it will show, and it might hamper their desire to transfer water," he said.

The sea is already 25 percent saltier than the Pacific Ocean and probably will need at least $1 billion in work to keep it from getting too salty to support its fish and the birds that feed on them.

The lawsuit asks a court to bar the Bureau of Reclamation from authorizing any water transfers incompatible with the restoration of California's largest lake until the mandated reports are completed.

The Salton Sea is the key in a proposed transfer of water from Imperial County agriculture to San Diego for drinking water. The plan has become crucial as California tries to shrink its take of Colorado River water by 15 percent by 2016.

Six other western states, their populations growing rapidly, want their fair share of river water.

The state has a Dec. 31 deadline to show it's on track to meet that goal or risk an immediate cutback that would be borne entirely by Southern California homes and businesses.

Last month, state senators sent Gov. Gray Davis legislation promoting the transfer. The measure would provide at least $50 million from a proposed November water bond to help restore the Salton Sea and the lower Colorado River.

Meanwhile, Imperial Valley farmers have been offered tens of millions of dollars to leave part of their fields fallow and allow irrigation water to be shipped to San Diego.