Already the $23.7 billion proposal is facing heavy criticism. Opponents say the tunnels will suck more water from the already fragile delta, further harming its fisheries, increasing costs for water users and devastating the area's agricultural-based economy by destroying water quality.
Last week, 11 members of Congress from the area sent a letter to the governor and federal Interior Secretary Ken Salazar, urging them to conduct a statewide analysis before proceeding with the plan. They questioned the thoroughness of a report done for the state that found the project's benefits outweigh the costs.
Supporters say the project is the long-overdue answer to pleas for a steady water supply to adequately supply farmers and municipalities south of the delta. They also claim the project's location actually will help the region's endangered fish species, especially the salmon and smelt.
State and federal officials acknowledge the plan has holes, but believe they will be able to address concerns as the project is built over the next 10 to 15 years.
"We decided to embrace scientific uncertainty regarding the facility's operation, water flows, habitat restoration and the response of fish," said Karla Nemeth, program manager for the plan at the California Natural Resources Agency.
Precisely how much water is diverted will depend on what is determined by the scientific studies that will accompany construction, she said.
"Ultimately what passes muster is what will work for the fish," Nemeth said.
Environmental groups said it's unacceptable to proceed with a project that has so many unanswered questions.
"We need to know upfront what the impacts of the project are and how they are going to mitigate these impacts or to actually improve conditions for fish," said Jim Metropulos, an advocate at Sierra Club California.
The delta, an inland estuary where hundreds of species live, is the hub of California's water delivery system. Both the state and federal governments run massive pumps on the south end that siphon drinking and irrigation water to more than 25 million Californians and to farms that produce half the nation's fruits and vegetables.
But the system doesn't provide all the water that's needed and state officials have for decades tried to come up with a plan to provide a better supply. Voters rejected a canal plan in 1982 and various other ideas were bandied about until the tunnel project emerged.
The broad outline for the tunnels is known, but Brown and Salazar are expected to provide greater detail when they make their announcement Wednesday.
The tunnels would siphon water using three intakes on the Sacramento River below Freeport, carry it some 60 miles underground to pumping facilities near Tracy, and then use existing canals to move it to farms in the Central Valley and cities such as Los Angeles and San Diego.
The system would be capable of diverting 67,500 gallons of water per second, a pace that would fill six Olympic-sized swimming pools every minute. The tunnels would be sized to handle even more water and the extra space would allow the water to move by force of gravity, saving on costs.
The project also includes plans for more than 100,000 acres of floodplain and tidal marsh habitat restoration. Water users would pay for the tunnels and related infrastructure, while taxpayers would finance the restoration portion. A key funding mechanism is an $11 billion water bond that will go before voters in 2014.
Balancing the needs of water users against preservation of the ecosystem is the heart of the project and the center of the controversy.
The decline of once-abundant fish species like salmon and smelt has triggered regulations to protect them, and court decisions have curtailed water deliveries in recent years. Farmers in the Central Valley say the restrictions have forced them to fallow productive land.
But Bill Jennings, an executive committee member of Restore the Delta, a coalition of farming, urban and environmental groups, said the tunnel project could make things worse for delta farmers. They now irrigate with water that is cleansed as it flows through the delta. If the tunnel project moves ahead, they will use water that has more salts and toxins that could kill or damage crops.
Recreational and commercial fisheries would be hard hit because less water will make it through the river, he said. Boating, swimming and water skiing also would suffer.
"The delta is in a biological meltdown. Taking more water won't restore an ecosystem that's already hemorrhaging from lack of flows," Jennings said. "This plan is not a path to restoration; it's a death sentence for one of the world's greatest estuaries."
But other groups said the project has merit—if it is run as efficiently as promised.
"There's ample reason to believe that we can divert the same amount of water out of a new system with far less environmental impacts," said John Cain, director of conservation at the nonprofit American Rivers.
Taking water from the north of the delta instead of the south would greatly benefit the estuary, Cain said. In the south, where in-flows are low, pumps entrain and kill fish and their food resources.
The huge tunnel project could actually prove more beneficial to fish.
"It's the big gulp, little sip theory," he said. "A large facility would be able fill up and take a large gulp during wet periods, while taking less during dry periods."