BOULDER CITY, Nev. -- Straddling the border between Nevada and Arizona, the Hoover Dam is a symbol of human engineering might.
For more than 70 years, its massive walls have tamed the flows of the Colorado River, fueling the growth of cities like Las Vegas that depend on it to supply water and power from its generating station.
But these days, what's most striking is the lack of water stored behind the dam's concrete arch. A thick white band of mineral deposits marks the walls of Black Canyon above the water line. Locals call it the "bathtub ring." It's where the water used to be, before the start of the current decade-long drought.
For officials charged with keeping water flowing to Las Vegas and other Colorado River communities, the bathtub ring isn't a curiosity. It's yet another reminder that worries about climate change are reshaping their future.
"Around 2002, we really began to look at whether this was one of those traditional droughts the Colorado River has experienced -- or are we looking at something very different?" said Patricia Mulroy, general manager of the Southern Nevada Water Authority.
Spring warming is coming earlier and harder, evaporating mountain snowpack that feeds the river and its two main reservoirs, Lake Mead and Lake Powell.
Mountain snowpack evaporates
"In two weeks in April, we lost the equivalent of 14 feet of Lake Powell in snowpack," said Mulroy. "So it's a pretty daunting and disconcerting reality that we're beginning to get our heads around."
Mulroy is not alone. Across the United States, water managers are beginning to grapple with climate change. And it's changing the way they think about almost everything.
For the utilities that supply the nation's drinking water, one of the first casualties is the idea that the conditions of the past can predict the future, said David Behar, deputy to the assistant general manager for water at the San Francisco Public Utilities Commission.
"It's a game changer for water managers," he said. "It takes the variability that we understand, and can live with, and amplifies it by an order of magnitude."
For planners who are used to taking the long view -- constructing expensive infrastructure that's expected to last 30, 40, 50 years, or even longer -- climate change is changing their rules, introducing new and hard-to-define levels of risk.
"You have to plan decades ahead, in terms of water supply and source water needs," said Dan Hartnett, director of legislative affairs for the Association of Metropolitan Water Agencies. "It's too late if you wake up one morning and the tap runs dry."
A tale of tree rings and 'Sin City'
Walking along Las Vegas Boulevard, it's hard to believe Sin City is facing a water crisis. The dancing fountains at the Bellagio Hotel and Casino spring to life every half-hour, captivating tourists who gather to gawk.
The nearby Mirage features a lagoon complete with towering waterfalls and a fake volcano. And on a hot summer's day, visitors who find the scorching desert sun discomforting can still have their drinks outside at bars equipped with machines that cool the air with a fine mist.
But those are illusions. For Mulroy, it is Lake Mead that tells the tale of Las Vegas' water future. The reservoir's water level fell this summer to its lowest point since 1965, when officials diverted Colorado River flows to fill the newly constructed Lake Powell. By late August, Lake Mead's elevation hovered at just 1,092 feet above sea level.
Under the complex system of allotments that governs how Colorado River water is distributed to seven U.S. states and Mexico, that leaves just 17 feet of wiggle room before the Southern Nevada Water Authority must, by law, begin seeking alternate sources of water.
If the lake dips to 1,050 feet, the Hoover Dam's hydroelectric power plant will shut down and Las Vegas will have to begin weaning itself off the river. It's hard to tell how soon Lake Mead could hit this ominous milestone.
The pact that governs the Colorado's supply was negotiated early last century, one of the wettest periods in the past 1,200 years, based on climate records drawn from tree rings across the southwestern United States. Many scientists say it's nearly inevitable that the region will become drier, moving closer to the historical average.
Right now, the best climate models predict a drop of anywhere from 5 to 25 percent in the mountain runoff that supplies the Colorado by midcentury. But a recent study by researchers at the University of Colorado suggests that the decisions water managers make can -- temporarily -- blunt the effects of climate change and continuing population growth.
Bigger mission looms for the wastewater patrol
Under the harshest scenario, in which climate change reduces the runoff that feeds the Colorado by 20 percent midcentury, water managers could cut the risk that the river's reservoirs would run dry by as much as one-third. But they would have to take aggressive steps to cut releases from Lake Mead and Lake Powell in times of drought.
"Until 2026 or 2027, the risk is pretty low ... something water managers can plan for," said the study's lead author, Balaji Rajagopalan, a hydroclimatologist. "But after 2027, through 2050 -- that's when the risk pretty much explodes."
Kevin Perry isn't thinking about the Colorado River's future as he deftly navigates Las Vegas' sprawling web of gated communities and fresh tract houses, symbols of the bygone '90s boom. He's looking for scofflaws.
Perry is part of the Las Vegas Valley Water District's wastewater patrol, looking for businesses and homeowners with sprinklers spraying the sidewalks and streets along with their plants, or evidence that a utility customer is violating the summer ban on midday watering.
The wastewater patrol, more than a decade old, is part of the Southern Nevada Water Authority's efforts to conserve water, which also include recycling wastewater that flows from household drains and a popular new program that pays customers to replace grass with less thirsty trees and plants.
"People like their landscapes to look like where they came from," he said. "They come from somewhere that's far greener, where rain takes care of their needs, and when they get here, they want to green it up."
But converting that greenery to something more appropriate for a city that gets an average of just 4.5 inches a year of precipitation comes with a price. The success of the turf removal program, combined with the local effects of the national economic recession, has reduced the utility's revenues at a time when climate change is prompting new investments in infrastructure.
Preparing the 'third straw'
The water authority has embarked on a massive construction project to build a new intake pipe to pull water from Lake Mead. Known locally as the "third straw," it will ensure the utility can siphon water if the lake level dips below an elevation of 1,000 feet, rendering two existing intakes useless.
SNWA is also seeking permission to build a controversial $3.5 billion pipeline to transport groundwater from rural eastern Nevada, a plan that has drawn ire from ranchers and environmental groups and is currently tied up in court.
"Right now is the perfect storm," Mulroy said in late August. "You have a daunting drought in the Colorado River that is forcing us to spend $800 million to build a third intake we never anticipated having to build, which is an excruciatingly difficult construction project. And you have connection charges that were 57 percent of the revenue for our capital funding plan that evaporated. And you have to balance the two, but you have to build the third intake. You don't have a choice ... not without endangering the community as a whole."
If a recent study by two water utility trade groups is correct, Las Vegas' spending could be just an opening wager in a game of climate change poker.
The report, by the Association of Metropolitan Water Agencies and the National Association of Clean Water Agencies, estimates that the United States' drinking water and wastewater utilities will spend $448 billion to $944 billion between now and 2050 to adapt their infrastructure and operations to cope with the shifting climate. This estimate, large as it is, doesn't include big societal costs likely to spring from climate-induced changes to the nation's water supply, including impacts on ecosystems and public health.
Living with uncertainty
While the fate of America's energy and transportation systems has dominated the country's climate debate, water utilities have begun making their case on Capitol Hill and in academic circles. "Water utilities, right now, feel like we're going to be among the first sectors affected by climate change," said Marc Waage, manager of water resource planning at Denver Water.
Just two years after AMWA's climate committee met for the first time, between a fifth and a quarter of the group's members serve on the panel. Utilities joining in the climate discussion run the gamut from rural to urban, small to large, east to west, said Erica Brown, AMWA's director of regulatory affairs and scientific program development.
Their concerns range from climate-driven drought to an increase in extreme rainstorms that can overwhelm sewers and impair water quality. "In places like Arkansas, politically, people might not want to talk about climate change," she said, "but you talk to our members there and they say, 'Yeah, we see changes in our watershed. We're aware of this.'"
Meanwhile, a group of larger urban systems -- including Las Vegas, New York, Denver, Seattle and San Francisco -- has formed the Water Utility Climate Alliance. The group is less than 2 years old, and its members' general managers meet every three weeks via conference call as they pursue an aggressive research strategy. WUCA is finishing two white papers: one examining the latest science on how climate change will affect the water cycle, and a second that examines strategies for making decisions in the absence of precise information about what the future will look like.
Said Behar, the San Francisco water official who serves as WUCA's staff chairman, "We're not able to get as specific climate information as we'd like, but we still think that we are facing a need to make decisions about infrastructure -- in some cases, expensive decisions about infrastructure -- in the context of uncertainty."
What that means is that some timeworn practices in the water business "have to be challenged," concluded Mulroy, Las Vegas' water chief. "I think it's a conversation for the country that's almost unavoidable. ... The worst case, as far as I'm concerned, is if we are surprised by the magnitude of climate change. We can't move fast enough."