The difference will be decided upstream, where Lake Powell soon might contain enough water to allow for surplus deliveries to Lake Mead next year.
Federal forecasters now rate the chances of that happening at 50-50.
But if Powell fails to reach that all-important "equalization elevation" -- if the coin should fall the other way -- Lake Mead will get no extra water and its decline will continue.
The surface of the Las Vegas Valley's primary water source has fallen 120 feet over the past decade, as record drought reduced snowfall in the Rocky Mountains and flows in the Colorado River.
In January 2000, Lake Mead was at 96 percent of capacity. It now stands at 42 percent.
The latest two-year projection by the U.S. Bureau of Reclamation assumes that Lake Powell will top its equalization elevation and release about 2.4 million acre-feet more water downstream than usual. That's enough water to supply the entire Las Vegas Valley for eight years, and bureau officials expect it to raise the level of Lake Mead by about 16 feet by December 2010.
Southern Nevada Water Authority officials would love to see that happen, but they aren't betting on it.
"When you're a water manager, you hope for the best and plan for the worst," authority spokesman J.C. Davis said. "We cross our fingers and toes and all that. The fact is you have to prepare for every eventuality, and some of the options out there are pretty ugly."
Deputy General Manager Kay Brothers laid it all out for authority board members earlier this month: If Powell doesn't reach the equalization mark, Lake Mead will get 8.23 million acre-feet next year, which is roughly 770,000 acre-feet less than what Nevada, Arizona, California and Mexico take from Lake Mead annually.
As a result, a lake already at its lowest level since 1965 could drop another 9 feet or more, depending on the flows from tributaries that join the Colorado downstream from Lake Powell.
The lake's surface now rests at about 1,094 feet above sea level, less than 20 feet away from the trigger point for a federal shortage declaration that would force Nevada, Arizona and Mexico to reduce their combined water use by 400,000 acre-feet a year.
Nevada's share of such a cut would be 13,000 acre-feet, roughly the amount of water used each year by 26,000 average Las Vegas homes.
Brothers warned board members that the lake could hit elevation 1,080, just 5 feet away from the shortage trigger, in January 2011.
Lake Mead hasn't slipped beneath elevation 1,085 since a severe drought in 1956. It hasn't been below 1,083 since it was first filled in 1937.
The Bureau of Reclamation's current projection for next year is also drawing a mix of optimism and skepticism from National Park Service officials and business owners at Lake Mead National Recreation Area. After all, they have heard this line before.
The bureau also predicted surplus water for Lake Mead last year, but unseasonably hot, dry conditions in March erased all that.
The sudden change sent park officials and marina operators scrambling. Instead of slightly higher water, they suddenly found themselves with just a few weeks to prepare for a sharp 10-foot drop.
For Las Vegas Boat Harbor, that meant pushing the entire marina 250 feet farther out into the lake in three separate moves, all executed with very little warning, said Gail Gripentog-Kaiser, manager and part-owner of the operation.
The job required divers in the water and cranes in the air to reposition roughly 200 anchors each weighing more than a ton.
"They're massive concrete blocks is what they are," Gripentog-Kaiser said.
Each move also required the extension of utility lines that link the floating facilities to power and sewer services back on shore.
Gripentog-Kaiser said last year's sudden change in the projections for Lake Mead caused a "big hullaballoo" and prompted the bureau to start holding monthly briefings with marina operators.
At those meetings, bureau officials present "the worst-case, probable, most probable and best-case" scenarios for the reservoir, she said.
"We want the best information we can get, obviously. We're much more informed now."
Depending on topography, the shoreline can recede 10 to 20 feet for every 1-foot decline in the water level.
The park service spent last summer extending boat ramps left high and dry by the shrinking lake. Some of those ramps soon might need to be lengthened again.
"We're looking at the what-if scenario, and we're not holding back just because the projections look good right now," said Andrew Munoz, spokesman for the 1.5 million acre recreation area.
For one thing, it's too soon to tell what's really going to happen.
Federal officials just started measuring this year's snowpack in the Rockies, and the heaviest accumulations will not arrive until after the first of the year.
"It's like trying to predict the final score of a baseball game in the top of the second inning," Davis said. "The point is, don't get too excited, don't get too down. It's early yet."
A final decision about how much water to release from Lake Powell won't be made until April 1, Munoz said. "The critical months are going to be March and April."
Gripentog-Kaiser said the falling water level has caused problems with access, but it has not changed the overall experience once you get out on Lake Mead.
Even at 42 percent full, the reservoir contains more than 85,000 acres of open water that is up to 300 feet deep in places.
Some old coves are "hiking spots now," she said, but there are plenty of new coves where boaters can "get back out of the wind."
As always, the key is to treat every visit to the lake as if it's your first because the shoreline is constantly changing and so are the boating hazards lurking just below the surface.
"It's not like there's no water out there," said Gripentog-Kaiser, whose family has been in the marina business at Lake Mead for 57 years. "It's a whole new lake of adventure."
Contact reporter Henry Brean at hbrean@reviewjournal.com or 702-383-0350.
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