Lake Mead, the nation's largest reservoir, reached historic low levels over the weekend, another indication of the persistent drought that grips the American West.
Saturday night, even after a prolonged rainstorm, the gauges at Lake Mead settled out at 1,080.13 feet. It's the lowest recorded lake elevation since the reservoir was filled in the 1930s, said Rose Davis, a spokeswoman for the Bureau of Reclamation, a federal agency that oversees water resources.
And it didn't stop there. By mid-afternoon Tuesday, the lake was at 1,079.76 feet. If lake levels are projected to fall below 1,075 feet in January 2016, it will trigger restrictions on the amount of water than can be drawn from the lake. Additional restrictions would follow if levels reach below 1,050 feet and 1,025 feet.
The city of Las Vegas, which gets 90 percent of its water from Lake Mead, is so concerned about falling reservoir levels that it is building a new intake pipeline deeper within the lake, to ensure it will be able withdraw water even if lake levels continue to decline.
Lake Mead is part of the Colorado River Basin, which provides a crucial source of water to seven states and Mexico. The region is in the midst of a 15-year drought, while the state of California is in its fourth consecutive dry year.
The California drought is one of the worst in the state's history, prompting Gov. Jerry Brown to issue unprecedented water use restrictions earlier this month.
Climate change will only exacerbate the impacts, experts say.
Global warming is "more or less a stacking of the deck" that increases the likelihood of dry conditions in the West, said Greg Pederson, a research scientist at the U.S. Geological Survey.
"The way the climate system works out here is it tends to be sticky," Pederson said. "If it's wet, you get these generally wet conditions for 10 to 20 years at a time, and if it's dry, you typically get dry conditions 10 to 20 years at a time."
While scientists say that climate change may not be the main cause of the drought, it has made it worse. Global warming has caused the high temperatures that have dried up soils and caused early melting of the snowpack, and many scientists say it has also altered atmospheric circulation patterns that have shifted storms away from the state– meaning much less rain.
"I would guarantee you it's a combination" of natural and human causes, Pederson said. "And they're inevitably linked and related."
The bottom line, Pederson said, is that managing Western water resources is enough of a challenge given the climate's natural variability and increasing demand from a growing population. Climate change adds another layer of problems, he said.
The low levels at Lake Mead came as no surprise to Tim Barnett, a marine research physicist emeritus at the Scripps Institution of Oceanography. In 2009, Barnett co-authored a study that predicted the Colorado River would be chronically unable to provide the amount of water expected by the seven basin states and Mexico by 2050. The researchers found that reduced runoff caused by climate change would create water shortages, even under conservative global warming scenarios.
In an interview, Barnett said the latest news from Lake Mead shows the region is on track for the types of shortages he predicted.
Barnett and other experts say water resources along the Colorado River have long been over-allocated. Water rights among the seven basin states (and Mexico) are managed through a complex system of treaties and laws that limit the maximum amount of water each state can withdraw from the river.
But those limits were based on river flows from some of the wettest years in the Colorado River's history.
"The bottom line is the Colorado River is over-appropriated," Barnett said. "And any reduction in water availability, any climate change that will make the Southwest more dry, is going to cause havoc somewhere down the road."
The state of California is legally entitled to receive 4.4 million acre-feet of water per year from the Colorado River. Jeanine Jones, interstate resources manager for the California Department of Water Resources, said the state has been able to take its full share from the river, in part because some of the upper basin states, like Wyoming, have low populations and haven't used all the water they're entitled to.
Jones said she expects a higher risk of shortages as the region's population grows. Rising demand and climate change are both considered in regional water management planning, she said.
Three of the seven states in the Colorado River Basin led the nation in population growth from 2000-2010. Nevada grew 35.1 percent, Arizona 24.6 percent and Utah 23.8 percent.
The four other states in the Basin––California, Colorado, New Mexico and Wyoming––all showed double-digit growth. The region added more than 6.9 million people, accounting for one quarter of the nation's growth in the decade.
With about 65 million people, the seven Basin states account for around 20 percent of the U.S. population.
Correction: This story has been updated to more accurately describe the Lake Mead conditions that would trigger water use restrictions.