Brown's administration is pushing for final federal and state approval of the 35-mile-long, 40-foot-wide tunnels, touted to ensure more reliable water deliveries to city and farm water agencies in Central and Southern California.
The state's environmental report concludes the tunnels, while taking 5 percent more water from the Sacramento River, would be the least disruptive of all possible options for water deliveries from California's largest river.
Brown's earlier proposals to redo water delivery from the Sacramento River near its meeting with the San Joaquin River included a canal plan rejected by voters in 1982 and a broader version of the tunnels that federal regulators objected to in 2014, saying it could threaten endangered species.
Brown said Thursday that the proposed tunnels and the discarded earlier versions of the project had been subjected to "more environmental review than any other project in the history of the world."
The tunnels project "is absolutely essential if California is to maintain a reliable water supply," Brown said in a statement.
Brown's administration and water agencies in Central and Southern California are the main backers of the project.
Opponents include some Northern California water districts and farmers, and environmental groups, which fear losing more water and habitat for salmon, steelhead, sturgeon and dozens of other native fish and other wildlife already suffering under the delta's more than half-century-old waterworks of pumps, pipes and canals.
"We just don't think that the only answer is to take more water out of a river in crisis," said Osha Meserve, a lawyer working with Northern California farmers and conservationists opposed to the project. "Ninety-thousand pages, or a million pages, don't explain why that's a good idea."
The tunnels project still needs an agreement on financing by the water districts that would benefit from it, plus federal and state decisions on whether the project would comply with endangered species laws.