More than a dozen congressional Democrats called for an investigation Friday into allegations that an analysis of how California salmon might be affected by the state's water system was politically manipulated.

The 300-page study examines how politically charged plans to rejigger operation of dams and pumps that deliver water through the Delta from Northern California to Southern California will affect several species of threatened and endangered salmon and steelhead.

The lawmakers said they feared there was an ongoing "catastrophic failure of oversight" that could drive salmon and steelhead toward extinction.

"I would hope the inspectors general would investigate these allegations immediately," said Rep. George Miller, one of 19 members to seek the investigation. "There is a great deal at stake."

Miller, D-Martinez, was reacting to the differences between two versions of the salmon report: one written late this summer by biologists at the National Oceanic and Atmospheric Administration's fisheries office and a second version that contained revisions by the agency's managers.

The latter version, if made final, would make it far easier to renew long-term water contracts in the Central Valley and boost the capacity of Delta pumps that deliver water to Southern California.

Although the study, called a "biological opinion," has yet to be finalized, the Times obtained portions of the earlier draft and a full copy of the most recent draft.

The versions have key differences, most notably that the earlier version says water operations will jeopardize the continued existence of some fish species, and the later draft does not. The differences were first reported in the Sacramento Bee last week in a story that prompted the congressional letter.

In addition, the earlier draft contains a requirement that would have forced the Contra Costa Water District to shut down one of its water supply canals for six months a year. The revised report says only that the agency must monitor salmon caught in the canal.

A Contra Costa water official said the earlier version was in error because biologists had wrongly assumed the canal at Rock Slough was used for all of the district's water supply, an assumption that led them to conclude more fish were being killed there than actually were.

"That was a goof," said Contra Costa Water District assistant general manager Greg Gartrell.

Jim Lecky, the assistant regional administrator at the National Oceanic and Atmospheric Administration who oversees the salmon report, said there were other errors.

"I reviewed my staff's work and I didn't think they did a good job," Lecky said. "There were a bunch of errors in their assumptions about the project."

The congressional letter is the latest in a series of efforts by Miller to slow down and examine plans by federal water managers in California. For weeks, he has been trying to get the U.S. Bureau of Reclamation to reconsider plans to renew dozens of long-term water contracts for farmers and others throughout the Central Valley that Miller considers unduly favorable to water users.

In addition, water officials are looking to increase the capacity of pumps that move water from the Delta to Southern California.

Both the contract renewals and the increased pumping hinge on the salmon study, which technically is a review of a U.S. Bureau of Reclamation document called the "Operations, Criteria and Plan" that describes how the state's two largest water delivery projects, the Central Valley Project and the State Water Project, will be operated.

The congressional letter suggests the bureau, "in its haste to finalize water contracts in California, has improperly undermined the required NOAA fisheries environmental review process."

Reclamation Bureau spokesman Jeff McCracken said his agency had no improper influence on the biological opinion.

"We never saw the earlier draft that had the alleged different opinion in it," McCracken said.

Earlier in the week, state Sen. Mike Machado, D-Stockton, asked for an independent scientific review of the biological opinion.

Lecky said the issue was being blown out of proportion.

"This is a typical consultation process," he said. "It's nothing out of the ordinary."