SACRAMENTO  (AP) — More planning is needed on a $25 billion proposal to build two massive water tunnels in Northern California, state officials said this week.

Some 30,000 pages of environmental reviews and draft plans have already been generated on what would be the biggest water supply project in California in decades, the Los Angeles Times reported .

On Wednesday, the Department of Water Resources said it will do further work with state and federal agencies on revising parts of the drafts. They will be reissued next year for additional public comment.

That will push a final decision on the tunnels — originally scheduled for late this year — well into 2015.

Backed by major urban and agricultural water districts, the project would change the way some Northern California supplies are sent south to the San Joaquin Valley and Southern California, the Times said.

Sacramento River water would be diverted into two 30-mile-long tunnels and conveyed under the Sacramento-San Joaquin delta to existing pumping facilities.

The $25 billion project also includes extensive restoration of fish and wildlife habitat in the delta. Supporters say the changes will improve environmental conditions and ease pumping restrictions that have cut water exports.

But delta interests and some environmental groups are opposed, arguing that the tunnels will rob the delta of more water, adding to its many ecological problems.