Lake Mead—America’s largest reservoir, Las Vegas’ main water source and an important indicator for water supplies in the Southwest—will fall this week to its lowest level since 1937 when the manmade lake was first being filled, according to forecasts from the federal Bureau of Reclamation.

Water ripples across a boat ramp at Lake Mead, the reservoir formed by Hoover Dam. The lake will drop to a record low this week, the consequence of a 14-year drought endured without a reduction in water use. Photo credit: © J. Carl Ganter / Circle of Blue
Water ripples across a boat ramp at Lake Mead, the reservoir formed by Hoover Dam. The lake will drop to a record low this week, the consequence of a 14-year drought endured without a reduction in water use. Photo credit: © J. Carl Ganter / Circle of Blue

The record-setting low water mark—a surface elevation of 1,081.8 feet above sea level—will not trigger any restrictions for the seven states in the Colorado River Basin. Restrictions will most likely come in 2016 when the lake is projected to drop below 1,075 feet, a threshold that forces cuts in water deliveries to Arizona and Nevada, states at the head of the line for rationing.

But the steadily draining lake does signal an era of new risks and urgency for an iconic and ebbing watershed that provides up to 40 million people in the U.S. and Mexico with a portion of their drinking water. The rules governing the river are complex, but the risk equation is straightforward: less supply due to a changing climate, plus increasing demands from new development, leads to greater odds of shortages.

No area is more vulnerable than Las Vegas, which draws 90 percent of its water from Lake Mead. Today, in the midst of the basin’s driest 14-year period in the historical record, the gambler’s paradise is completing an expensive triage. The regional water authority is spending at least $US 829 million of ratepayer money to dig two tunnels—one at the lake bottom that will be completed next spring and the other an emergency connection between existing intakes—to ensure that the 2 million residents of southern Nevada can still drink from Mead as more of the big lake reverts to desert.

Yet despite a shrinking lake, diminishing supplies and ardent pleas from tour guides and environmental groups to preserve a canyon-cutting marvel, the four states in the basin upriver from Lake Mead intend to increase the amount of water they take out of the Colorado River. All of the states are updating or developing new state water strategies, most of which involve using more Colorado River water, not less.

“We have mapped out how the remainder of our allocation can be used,” Eric Millis, director of the Utah Division of Water Resources, told Circle of Blue. “It’s going to happen sooner rather than later. We have a place for every drop.”

Utah—like fellow upper basin states Colorado, New Mexico and Wyoming—is not using all the Colorado River water it was granted by a 1922 interstate compact. The four states have the legal authority to increase their Colorado River diversions.

However, the water they seek may not be available. The calculations of availability stem from wetter hydrological conditions and supply forecasts made nearly a century ago. Under the 1922 compact, the upper basin is entitled to 7.5 million acre-feet. A later agreement apportioned each state a percentage of the available supply. The upper basin’s average annual use between 2007 and 2011, the most recent figures, was 4.6 million acre-feet.

The legal entitlement, granted at a time when the river’s hydrology was poorly understood, is surely too high. All the states acknowledge that fact. “We’re not pegging our hopes or analysis on the full 7.5,” said James Eklund, director of the Colorado Water Conservation Board, the state water planning agency.

Still, the much dryer conditions that exist today means that supplies are tight enough to substantially raise the risk of water shortages with every additional water consumption project.

A River of Equations

By law and tradition, the Colorado River Basin is divided into two halves. The four upper basin states have different obligations, preoccupations and goals than Arizona, California and Nevada, the three lower basin states.

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