State officials moved one step closer to developing a tunnel system that would provide more water to millions of people in the San Joaquin Delta in Northern California, and to some in the south, after the release of the final lengthy environmental impact report of the plan.

A 36-foot-wide tunnel, buried deep underground, would carry water for 45 miles from the Sierra Nevada mountains to the existing Bethany Reservoir in the Central Valley near the city of Tracy, which would then be pumped to residents, according to the proposal.

The plan is needed to increase water supplies and improve earthquake resilience, according to officials.

“Doing nothing is not an option,” California Gov. Gavin Newsom said in a Dec. 11 press release on the project. “After the three driest years on record, we didn’t have the infrastructure to fully take advantage of an exceptionally wet year, which will become more and more critical as our weather whiplashes between extremes.”

When heavy rains poured down on Northern California in January, the project could have captured water to supply 2.3 million people, according to the press release. Instead, the freshwater flowed through watersheds and continued downriver, ultimately ending up in the ocean.

About two-thirds of the state’s water originates in the Sierra Nevada mountains, and water management projects currently provide water to 27 million people and 750,000 acres of farmland in the state, with economic benefits estimated at $400 billion, according to the governor’s office. However, the state is projected to lose 10 percent of its water supply by 2040.

“This proposed project is essential to updating our water system for millions of Californians,” Mr. Newsom said. “This new approach, redesigned following community and environmental input, is how we can build a California of the future.”

Some environmental groups have questioned the impact the project will have on fish and the surrounding area.

The approximately 3,500-page environmental review noted similar concerns with certain species, including Chinook salmon, and recommended a series of mitigation efforts, including filters on entrance ways to reduce the risk of death to certain fish, among others.

Now, the environmental review must be certified by the state, which is expected. Construction on the project could begin as early as 2030.