Six months ago, Lake Mead was turning to dust, its water levels receding so rapidly that marina operators were moving boat docks weekly.

The reservoir, victim of a 10-year drought on the Colorado River, reached a historic low in November, and water users in Arizona and Nevada braced for shortages within the year.

Low water levels at Lake Mead

Then last week, the federal agency that operates the reservoir declared that enough water would flow down the river this spring - the most in over a decade - to raise water levels above the shortage triggers and ease the threat of rationing through at least 2016.

It wasn't enough water to end the drought.

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 overallocated river.

"This year was key," said Scott Huntley, spokesman for the Southern Nevada Water Authority in Las Vegas.

"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 good news arrived April 8. That's when the U.S. Bureau of Reclamation, which manages the lower Colorado River, determined 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."

Shortage planning

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.

"It wasn't unexpected," said Tom McCann, assistant general manager of the CAP, whose supplies would have been hit hardest in a shortage.

"We've been monitoring the snowpack in the Upper Basin, and it was looking good. The only thing we were concerned about was an early warm-up or too much wind."

Favorable snowpack

Bureau hydrologists had been watching the snowpack since the first flakes flew and had started increasing the flow from Powell into Lake Mead several months ago, said Malcolm Wilson, chief of the bureau's water-resources group in Salt Lake City.

The bureau uses its April projections to reach a decision on the annual release total. On April 1, Lake Powell was projected to rise about 27 feet by Sept. 30, pushing the reservoir above the threshold to start boosting levels in Lake Mead.

As a result, the bureau determined it could release an estimated 11.56 million acre-feet for the current water year. The extra water, beyond the annual set amount, will help Lake Mead rise about 23 feet above last year's record-low level and 30 feet above the drought trigger by Sept. 30.

The water will also help Mead start to make up losses from years of lower releases. As long as Lake Powell is below the annual 8.23 million acre-feet threshold for added releases, Lake Mead operates in a deficit situation because demand from the three lower-river states and Mexico exceeds 9 million acre-feet.

An acre-foot of water is 325,851 gallons, enough to serve two average households for one year.

After last year's close call, some people began to wonder why Mead was allowed to drain as Powell seemed to recover.

The answer, again, was in the 2007 plan. Lake Powell dropped faster at the drought's onset and has begun to recover first.

"We were not immune from getting criticism," Nevada's Huntley said. "It appeared in the immediate onset of it that Lake Mead was actually suffering more than Powell, and some people up here were asking us why did we make that kind of deal. But it's what that whole idea of managing the lakes together is about."

Powell will still finish the water year about where it was last year at the same time. What happens next year again depends on the winter runoff.

If Powell reaches the threshold again next spring, the bureau will again release extra water into Mead. Even if that doesn't happen, Mead is expected to remain above the drought triggers until at least 2016.

McCann said water managers were anxious until the final numbers came in. Although snowpack measurements looked good, a sudden rise in temperatures or a windy spring could have reduced runoff.

Even with the promise of more water, no one at the CAP or any other agency that relies on the river is letting down their guard.

"We're looking to see if we're going to start coming out of dry cycle into a wet cycle or whether we will continue in a dry cycle with a couple of wet years," McCann said.

"That's the nature of the Colorado River. That's why we have storage."