A surge of winter runoff that could not be captured has buttressed objections to building a huge aqueduct to route Sierra water around the Delta.
For several years, the Contra Costa Water District and others have argued it would be foolhardy to build a large, expensive and highly contested water diversion that winter rains or spring snow melt could rarely fill.
A smaller version -- perhaps one-fifth the size contemplated -- would be cheaper, deliver nearly as much water and could not be misused in ways that would harm drinking water quality and fisheries, they contend.
But backers of the larger version, primarily major Delta water users in the San Joaquin Valley and Southern California, insist a big aqueduct would maximize flexibility -- that it is important to be able to take large gulps during high flows when such diversions have little environmental effect.
This year, when a wet winter ended a three-year, reservoir-draining drought, is a case in point.
When the big flows came through this spring, Delta water managers' pumps diverted no more than about half what it would take to fill the proposed 15,000-cubic-foot-per-second aqueduct under study. Most days, the pumping rate was far less.
As it turned out, there was nowhere to send the water, except to let it flow through the Delta to the San Francisco Bay and the Pacific. The state's reservoirs filled quickly, and groundwater storage banks were taking water as fast
"Deliveries have been limited by the number of places to put the water," said Mark Cowin, director of the state Department of Water Resources. "If there were more opportunities to store water, we would have exported more water."
Cowin said more reservoirs and groundwater storage banks should be built. He also said the Brown administration has changed the course charted by the Schwarzenegger administration and will consider smaller Delta diversion alternatives equally with the larger version.
"This is an issue that will be thoroughly scrutinized," said Richard Stapler, spokesman for the state Natural Resources Agency.
Still, bigger is better, say the major water agencies that propose to pay for the project.
A big canal could take advantage of early-season storms to fill reservoirs sooner and stretch water supplies, said Laura King Moon, assistant general manager of State Water Contractors, which represents water agencies in the Bay Area, Southern California and elsewhere.
"The reasons for it can get pretty complicated," King Moon said. "The contractors are convinced that 15,000 cfs is the optimal size for both water supply and ecosystem reasons.
"I expect there to be a lot of debate over this in the coming months," she said.
The aqueduct is being proposed as part of a Bay Delta Conservation Plan, which seeks to stabilize water supplies and protect the environment.
By taking water from the Sacramento River through a new set of tunnels or a canal and restoring wetlands, supporters of the idea say they can ensure more reliable delivery of Delta water and eliminate the environmental problems of using huge pumps in the south Delta.
After nearly $150 million in planning, though, the conservation plan was severely criticized this month by a panel convened by the National Academy of Sciences, which concluded in part that water agencies apparently decided they wanted to build a large aqueduct and then set out to rationalize it as a strategy for protecting the environment.
Although some critics of replumbing the Delta contend no new aqueduct is needed, others find the idea of a smaller one attractive because it guarantees a limit on how much water can flow through it, unlike a larger canal that might be overused.
That concern would seem to be well-founded.
For example, the state Department of Water Resources operated its Delta pumps for years in flagrant disregard of a legal requirement to obtain a permit under the state's endangered species law, and water agencies have sued frequently to weaken environmental standards.
An increase in Delta pumping during the 2000s coincided with the collapse of several fish species, and just last week, House Republicans introduced legislation to reverse environmental safeguards in the Delta to increase water deliveries.
Contra Costa leaders looked to additional storage as a buffer against those types of threats 30 years ago when they won statewide rejection of the Peripheral Canal during Gov. Jerry Brown's first administration.
The worry then, as now, was that building a big canal without sufficient storage capacity would create an incentive to fill the aqueduct whenever water agencies in the San Joaquin Valley or Southern California were thirsty, and not necessarily when Mother Nature is generous.
"It will allow them to take Delta water when it's needed south as opposed to when it's truly surplus," said former Contra Costa County supervisor Sunne McPeak.
At what point water can be considered surplus has always been a tricky question, and it is an issue that's under scrutiny.
As a new supervisor in 1980, McPeak offered lawmakers a deal to test their commitment to balancing Southern California thirst against the Delta's health.
Contra Costa, she said, would not oppose the Peripheral Canal they were about to approve if they agreed to not allow water to be delivered through it until a new reservoir was built.
Lawmakers declined, and McPeak launched the successful campaign to kill the Peripheral Canal by referendum in 1982.
"They didn't understand what we were offering when we said we would go neutral," she said recently. In retrospect, she's glad they rejected the idea.
She was convinced that without new reservoirs, the canal would be used to drain the Delta at great cost to fisheries, recreation and water quality in Contra Costa County.
"It exposed their real intentions," McPeak said, charging that any plan to build a big new aqueduct without more water storage threatens the estuary.
McPeak helped write a nonbinding but influential plan known as Delta Vision and now heads the foundation that promotes that plan, which was completed in 2009.
Largely at her insistence, the Delta Vision plan says a new aqueduct should be accompanied by more reservoirs.
McPeak does not object to a large aqueduct if more storage is made available, saying that laws, contracts and bond-financing covenants can safeguard against abuse.
Others argue against new storage and an aqueduct, saying that the Delta is already overtapped.
The State Water Resources Control Board last year reported that, ideally, the use of Delta water -- both water pumped out of the south Delta and water diverted upstream -- would have to be roughly halved to adequately protect its environment.