"If you don't change anything else, it's going to take more water to restore the ecosystem of the delta," said Bill Rukeyser, director of the state water board.
Diverting water for decades has been blamed for changing the delta's salinity and flow, contributing to the system's overall decline.
To restore the delta solely based on water flow alone, the report says sending water supplies downstream to the Central Valley and Southern California could be reduced by one-third. North of the delta to the Bay Area, deliveries could be slashed 70 percent.
Cutbacks are already happening to deal with the three-year drought and to save the endangered delta smelt fish. If delta supplies are limited, where else can the thirsty state get water for its residents and farms?
Water agencies say there is nowhere else to get water and that the report isn't meant to be implemented on its own, pointing out it doesn't even have any regulatory clout.
"The more you cut back, obviously, the more challenges you have. We have to find something that works and balances both the water supply and the ecosystem," said Jeff Kightlinger, general manager and chief executive officer of the Metropolitan Water District of Southern California.
Still, environmentalists warn the findings can't just be dismissed, because water flow is a major part of the delta's problems.
"Flow is probably the one part of the ecosystem that we have affected the most. We have changed the flows of our system more than we've actually polluted it and about as much as we've destroyed habitat," said Gary Bobker, program director of The Bay Institute.
The report will be forwarded to the Delta Stewardship Council, which has the tough job of finding solutions that consider nature's and people's needs.