PHOENIX - After a decade-long drought on the Colorado River, the Lake Mead reservoir reached a historic low level last November and water users in Arizona and Nevada braced for shortages within the year.
But the federal agency that operates the reservoir recently declared that enough water would flow down the river this spring to raise levels above the shortage triggers and ease the threat of rationing through at least 2016.
It wasn't enough water to end the drought, however.
Lake Mead will still finish the year about 100 feet below its high-water mark and still less than half-full. But it was enough to satisfy the terms of a 2007 river-management plan that gives the federal government the leeway it needs to better stretch water supplies on an already over-allocated river.
"This year was key," Scott Huntley, spokesman for the Southern Nevada Water Authority in Las Vegas, told The Arizona Republic. "It could have gone one of two ways: either with the lake gaining water or, had we had another bad year, with shortages. What we're seeing now is definitely good news."
The U.S. Bureau of Reclamation, which manages the lower Colorado River, determined in early April that runoff from winter snow on the upper river would produce enough water to adjust storage levels at Lake Mead and Lake Powell, the river's other big reservoir on the Arizona-Utah line near Page.
What that means under the 2007 river-management plan, adopted by the seven Colorado River states, is that the bureau will move extra water from Powell downstream into Mead.
In recent years, the bureau has released 8.23 million acre-feet annually for use in Arizona, Nevada, California and Mexico. For this water year, the bureau will release 11.56 million acre-feet, over 3 million extra acre-feet. That's enough to raise water levels at Lake Mead out of the shortage-danger zone.
"Drought conditions over the past 11 years had raised the possibility of water shortages in the Lower Basin over the next year," Interior Secretary Ken Salazar said in a written statement, "but, thanks to good precipitation, wise planning and strong collaboration among the states, we are able to release additional water and avert those shortages."
The river-management plan, known as the interim-surplus guidelines, was negotiated in 2007 among the seven river states - Arizona, California, Nevada, Colorado, New Mexico, Utah and Wyoming - as a way to deal with potential shortages.
Its key element was a set of rules to jointly manage Lake Mead, which stores water for the lower-river states, and Lake Powell, which stores water on behalf of the four upper-river states.
Under the agreement, water levels at Lake Mead are used to determine shortages. If the reservoir drops below a certain level, Arizona and Nevada lose some of their allocations for a year.
Arizona agreed to shoulder the first shortages on the river as a condition for construction of the Central Arizona Project Canal, which delivers water to Phoenix and Tucson.
Nevada agreed to absorb some of the shortages as part of a water-banking deal with Arizona.
Lake Powell, meanwhile, is used to balance the contents of the two reservoirs. If Powell rises above a certain level, extra water is released downstream to Mead. That's what Arizona and Nevada were hoping for after Lake Mead dropped to within 7 feet of the first drought trigger in November.
The effects at the lake last year were shocking: Bleached bathtub rings grew on the rocks around the reservoir, marinas were abandoned as water receded, and marina owners moved boat docks farther out into the lake almost weekly.
Had this past winter stayed dry, a shortage could have been declared as early as 2012, costing Arizona about 11 percent of its allocation for one year and Nevada a little over 4 percent of its allocation.
Instead, snow piled up in the Rocky Mountains, the water source of the upper Colorado.
Hydrologists estimate that the runoff for April through July, when snowmelt is the highest, will total 120 percent of the long-term average. By the end of the water year (similar to a fiscal year) on Sept. 30, inflow into Lake Powell is projected to total 109 percent of average, only the third above-normal year since 2000, when the drought began.
More important, hydrologists determined that the runoff would boost Lake Powell above the threshold needed to release more water downstream.