Barnett found it would be very hard to maintain at least a 1,000-ft depth for Lake Mead; consequently, the Las Vegas water system would not be able to function.

He also concluded that climate change likely would decrease Colorado River Basin runoff by up to 30%. Even a 10% reduction would render the system unable to make requested deliveries by 2040. A 20% drop would result in a water-supply shortfall in 2025.

These findings were alarming to most Colorado River stakeholders, Barnett says. "The bureaucratic folks sort of wrung their hands and said, 'What are we going to do about this?' " he says.

Concerns about Lake Mead have long been a Las Vegas priority, claims Patricia Mulroy, who led the regional water agency for nearly two decades until her retirement in early 2014. She now is a Brookings Institution senior fellow at the University of Las Vegas.

"Our philosophy [at SNWA] became one of not waiting for the worst of the drought to hit before we took action," she says. "Our strategy has been to take action early and avoid a dramatic or catastrophic shortfall that would have really caused societal and economic disruptions here in the southern Nevada region."

Several years prior, the agency had planned to construct a third lake intake that would draw water at an 860-ft depth. Although the project initially was seen as a way to reduce levels of sediment and contaminants in the water system, rapidly declining lake levels and dire predictions from researchers transformed the job from a water-quality fix to a vital supply solution.

"We didn't anticipate that the lake level was going to drop so severely," says Erika Moonin, SNWA project manager. "By 2005, with the drought persisting, it became more of a concern of capacity when we presented it to the board."

Lake Mead's new intake was not only another water infrastructure project—it broke world records for underground construction. In December, the nearly 3-mile-long tunnel was connected to the preconstructed intake, placed months before on the lake bottom. The tunnel-boring machine operated at nearly 15 bars, the highest pressure ever recorded for a TBM.

Most of the work was performed through a design-build contract with Vegas Tunnel Constructors—a joint venture of S.A. Healy Co. and Italy's Impregilo S.p.A. Additional work was completed by Renda Pacific. Parsons Corp. is the water agency's construction manager.

Work first began on the intake structure in 2008 and included extensive barge and underwater activity. Contractors used shaped charges, which direct the force of their explosions, to blast much of the area where the intake's base would be constructed. In this process, more than 23,000 charges were attached to a remotely operated frame and lowered by a crane barge to the lake bottom. After blasting and excavation, a tugboat and barge placed the intake structure within a 95-ft-wide by 159-ft-long by 83-ft-deep lake-bottom-pit excavation footprint at a depth of about 350 ft. Tremie concrete backfilled the area. The structure, including a concrete-base elbow and TBM reception eye, was assembled at the blasting site after a barge crane lowered it into place. A 14-ft stainless-steel riser sits atop the base; about 12,000 cu yd of underwater concrete was used to anchor the base.

In just under two years, crews excavated 14,300 cu yd of overburden and about 31,000 cu yd of rock.

The intake structure, completed in 2012, waited for completion of the tunnel from the shoreline. Construction began in early 2010, including TBM preparation. Vegas Tunnel Contractors began that process using a drill-blast-excavation approach to create a 600-ft-deep, 30-ft-wide access shaft to carve the TBM launch chamber, stub tunnels and mole starter tunnel. The horseshoe-shaped TBM launch chamber, directly below the access shaft, is 202 ft long, 47 ft wide and 38 ft tall. It was excavated by top heading and bench and drill-and-blast methods with a lining of shotcrete and rock bolts.