The late comedian Jimmy Durante used to do a Broadway shtick in which he led a live elephant down the street and then was confronted by a police officer.

"What are you doing with that elephant?" the policeman would ask.

Durante's reply: "What elephant?"

As state and federal officials push ahead with their Bay Delta Conservation Plan, the unavoidable elephant in the room is the 35-mile twin tunnels they propose to build through the Sacramento-San Joaquin Delta. BDCP supporters would prefer the media not focus so much on these tunnels. They note that the conservation plan seeks to restore 57 different wildlife species and create roughly 145,000 acres of wetlands and other habitat. The new "conveyance" system, they argue, would improve water reliability to areas south of the Delta and provide an insurance policy against earthquakes and saltwater intrusion.

Yet the footprint of these tunnels is pretty hard to ignore, especially with new details released Thursday by state and federal officials. Each tunnel would be 40 feet in diameter, and the construction shafts needed to build the tunnels would be 60 feet in diameter, bored roughly 150 feet beneath the Delta and crossing beneath the Sacramento River two times.

Boring of these tunnels would produce "tunnel muck," as it is artfully described in Chapter 4 of the BDCP revised administrative draft. This is not just soil, but conditioning agents (such as bentonite and polymers) that would help make the job easier for massive boring machines. About 7,000 cubic yards of tunnel muck would be produced each day. Overall the project expects to generate a total of 22 million cubic yards of tunnel muck, enough to cover 100 football fields to a height of roughly 100 feet.

So what are state and federal officials going to do with all this muck?

"Before the muck, or elements of the muck, can be reused or returned to the environment, the muck must be managed, and at a minimum, go through a drying/water-solids separation process," the report states. To do this, construction crews would deposit the muck in storage areas along the tunnel "ranging in size from approximately 100 to 570 acres. In total approximately 1,595 acres will be devoted to tunnel muck storage," with some of the muck left there permanently.

BDCP supporters note that the project has been downsized. Originally the tunnels were designed to move 15,000 cubic feet per second of water. Now the gravity-fed twin tunnels would carry a maximum of 9,000 cfs, and only when the river is running at more than four times that flow.

Even so, the total acreage needed just for muck storage is more than six times the size of the downtown Sacramento railyard. And this is just part of the footprint. In Sacramento County, in the stretch of the river between Clarksburg and Courtland, contractors would construct three water intakes covering 2,700 acres of riverfront. Below the intakes would be a 925-acre forebay, requiring the excavation of 6 million cubic yards of earth. At the end of the tunnels would be another new forebay, of 840 acres, requiring excavation of 14 million cubic yards of earth.

At Thursday's press conference, state Fish and Wildlife Director Chuck Bonham touted all the acres of marshes, grasslands and other habitats that would be restored under the plan. "We are talking about a restoration project potentially observable from space," Bonham said.

Yet before all that restoration happens, an ugly construction project would be visible from space. A bucolic stretch of southwest Sacramento County would be transformed into vast industrial site, with new electric power lines, access roads, pumps, pipelines and tunnel muck storage sites.

In much of the north state, the biggest concern over BDCP has been the potential impact on water rights. One of my colleagues on the editorial board is convinced that the tunnels and export of water to Southern California "will suck Northern California dry."

I don't share that fear. Even with the tunnels, the State Water Project and Central Valley Project would still be subject to the Endangered Species Act, flow regulations set by the State Water Resources Control Board and court rulings on water rights. Department of Water Resources Director Mark Cowin anticipates the tunnel project would deliver an average of 4.8 million to 5.6 million acre-feet of water each year.

Depending upon which amount is ultimately diverted on an annual basis, that would be either 10 percent less than the average diversion the last two decades, or a mere 5 percent more.

Neither scenario would dry up Northern California, where we are less than efficient in our water use, especially in the city of Sacramento.

For me, the real questions about his project are at least threefold:

• Does the payoff for Southern California justify the construction impact on Sacramento County and the Delta? How much energy will be needed to construct these massive tunnels and dispose of the muck? What pollution will result, including carbon emissions in a state committed to reducing its carbon footprint?

• Can water contractors pay for it? The project is expected to cost $24.5 billion (including operations and maintenance over 50 years), but every large construction project has cost overruns. How large will they be? Will water contractors cover those or attempt to pass them onto taxpayers?

• What will be the impact on salmon? Reducing diversions in the south Delta might help endangered smelt, but new intakes on the Sacramento River could directly harm salmon and also reduce flows through the north Delta. Will those reduced flow affect their mysterious migrations back to spawning grounds in the Sacramento Valley? Can that even be scientifically analyzed?

We will get some more answers in coming weeks as new BDCP reports are rolled out. But this may not be a situation where more information will necessarily make us feel more comfortable about the 9,000-cfs elephant in the room.

Follow Stuart Leavenworth on Twitter @ SacBeeEditBoard .

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