A plan to build gates across two Delta channels has strong support from state and federal leaders, though little is known about how the project would affect the environment.

The so-called "two gates" project would build moveable gates across Old River and Connection Slough in the Sacramento-San Joaquin Delta.

The channels bracket Bacon Island in the heart of the estuary. They are key passages for water and aquatic life moving between San Francisco Bay and the south Delta, where powerful state and federal water export pumps divert water to the San Joaquin Valley and Southern California.

Water officials argue that blocking those channels at key times could prevent threatened Delta smelt from being sucked to their deaths in the pumps. This might allow water diversions to continue even when smelt migrate into the central Delta in winter. Pumping is often reduced now to protect fish, contributing to statewide water shortages.

The project's main proponent is the Metropolitan Water District of Southern California, which imports about 30 percent of its water supply from the Delta to serve 18 million people.

"The work we have done shows that by temporarily opening and closing these gates, we could improve the level of smelt protection," said Roger Peterson, Metropolitan Water's assistant general manager. "That will eventually result in us being able to operate water supplies with more reliability."

Other supporters include the San Luis & Delta Mendota Water Authority, the Contra Costa Water District, Kern County Water Agency and Westlands Water District.

The project was supported last year by a broad committee of Delta interests acting as advisers to the Delta Vision Task Force. Appointed by Gov. Arnold Schwarzenegger, the task force recommended building the two gates by next year as a pilot project.

The $30 million plan calls for building hinged gates on surplus cargo barges. The gates would swivel open and closed like revolving doors on a hotel. The barges would be sunk to the river bottom, with the gates extending to the surface.

Metal sheet piling would be installed between the barges and levees on each side to close off the channel.

The whole apparatus could be removed if the project proved unsuccessful, and moved to another location to try again.

From December through March, Metropolitan Water proposes to close the gates an hour each day. From March through June, the gates could be closed as long as 10 hours a day. They would stay open the rest of the year.

The idea, in short, is to control smelt habitat.

Patterson said smelt prefer to move with pulses of silty water. They follow this cloudy water as it moves deeper into the estuary, and closer to the pumps, in winter and spring.

The gates could limit how far this cloudy water flows, and thus how far smelt move.

It is unknown whether restricting the smelt's movement would harm them. It's also unknown whether other species would be affected.

One side effect is that predators, such as striped bass, could learn to lurk near the closed gates to eat their fill of smelt that might become trapped there.

"We propose it as a five-year test," Patterson said. "But, clearly, if it's not working after the first few months, you can refloat these barges and try it at a different location."

The project is supported by some environmental groups, but not all.

"This might be a project that could provide some incremental improvements," said Jonas Minton of the Planning and Conservation League. "A main reason we are open to this concept is the ability to monitor it and very quickly change or terminate the project."

Water officials want to build the project by December. This would require expedited environmental review.

Interior Secretary Ken Salazar recently told a rally in Fresno he supports it. Plans call for the gates to be operated by the U.S. Bureau of Reclamation, which is overseen by the Interior Department and which operates one of the two Delta water export systems.

Schwarzenegger said on June 19 that he also supports fast review. The state Department of Water Resources operates the other Delta water pump system.

The project is among a package of proposed Delta fixes being drafted by the ongoing Bay Delta Conservation Plan process. Its centerpiece is a massive new water canal that would divert a portion of the Sacramento River's flow out of the Delta and directly to the export pumps.

A canal could take 10 to 15 years to build. The gates are seen as a quick fix that may help in the meantime.

The CalFed Bay-Delta Authority will convene an independent panel of scientists to review the project at an Aug. 6 meeting in Sacramento. It will issue findings a month later.

Minton of the Planning and Conservation League supports expedited review. But Bill Jennings, executive director of the California Sportfishing Protection Alliance, does not. He said it should be subject to a full environmental impact study.

"We will definitely go to court over that, because we don't know what the adverse impacts will be," he said.

Paying for the project also may be controversial. The Metropolitan Water District has spent $5 million on initial studies, Patterson said. It wants the remaining $25 million to come from state bond funding for habitat projects.

Patterson said the project aims to benefit fish and therefore qualifies as a habitat project.

Minton disagreed.

"I would say the project is to mitigate the impact of the pumping that serves those water districts," he said. "They should provide the funding for projects that benefit them."

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