RALEIGH - Facing a prolonged drought and a dwindling reservoir, Raleigh leaders have received tentative approval to temporarily dip into the murky bottom layer of Falls Lake to keep water taps flowing.

Until now, the bottom of Falls Lake has been reserved for collecting sediment streaming into the reservoir over time.

Because of the drought, federal and state officials have approved in principle the provision of the surplus water to Raleigh in stages, at a cost of up to $1 million a month, officials say.

The U.S. Army Corps of Engineers, which built and operates Falls Lake, would sell the water to the state, which would make it available to Raleigh. Details, including whether the state would pass part or all of the cost to the city, must still be ironed out.

With Falls Lake down about 8 1/2 feet, Raleigh's water-supply portion of the lake has dropped to about 30 percent of its normal level. And winter rains that would replenish it haven't come.

Raleigh covets the lake's "sedimentation storage" reserve as an emergency supply in case the city's normal allocation runs out this spring or summer.

"The sedimentation pool is our safety net," Raleigh Mayor Charles Meeker said. "When you get down to that point, you're in a crisis."

Meeker has called on Raleigh residents to slash water use to 35 gallons a day and to pay more for it, even as city officials negotiate with state and federal authorities over drawing on the lake's bottom surplus. In Falls Lake's 25 years of operation, that has never before been done -- or needed.

"The fact that we're having to consider this shows that the drought is very serious," said John Morris, director of the state Division of Water Resources. "We hope we won't need it."

Falls Lake's layers

For management purposes, Falls Lake is divided into layers.

When the lake is at its normal level, about one-third of its water is for drinking -- almost 15 billion gallons. Almost half of the lake's ordinary volume is reserved for gradual releases from its dam to maintain a healthy flow downstream in the Neuse River.

That leaves the bottom one-fifth of the lake for collecting sediment over decades. It holds about 8 billion gallons of surplus water, a bit more than half as much as the city's water allocation.

More room for water

The dark, cold, murky stratum is home to bass, crappie, catfish, and the mud that washes gradually into the 24-mile-long reservoir north of the city. The water could require additional chemical treatment to make it safe to drink, which would add to the cost, city officials say.

A federal study in the 1990s found that Falls Lake's bottom is filling more slowly than expected, leaving more room for water than first planned. In theory, the lake's bottom layer holds enough water to supply Raleigh and the seven other Wake County towns it serves for two to three months.

Under the proposal, the Army Corps of Engineers would sell up to about 6 1/2 billion gallons of the bottom water in four increments as needed. That's up to 87 days' worth at the lake's current draw-down rate, assuming no rain. That would come after Raleigh exhausts its regular supply, which by the most recent estimate on Tuesday stood at 113 days left.

Raleigh's City Council voted this week to ask the corps to let the city tap permanently into the surplus at the bottom. A permanent reallocation of half the lake's bottom layer would boost the city's supply by more than one-fourth -- until inevitable sediment accumulations erased it.

The change would require a joint state and federal study that would take years, perhaps in time for the next drought.

"It's a good idea every 10 to 15 years to re-examine corps projects to see what's needed in the future," said Terry Brown, water control manager at the corps office in Wilmington, which manages Falls Lake. "Things change."

Safety net needed

In normal times, when rain keeps Falls Lake full, Raleigh's water allocation is adequate.

"It's the drought that's causing the problem here," Morris said. "It's the worst drought we've experienced. It's a real emergency."

And that's the catch: In the next severe drought the lake likely will drop again, and much of any additional water supply allocated to Raleigh will not be there to tap.

That's why water conservation is vital during any drought.

"We'd like to limit our use through conservation," Meeker said. "But we'd also like to have as big a safety net as possible."