ST. GEORGE — When the first 300 Mormon families arrived here in 1861, it rained for 40 days, according to accounts from the time.

But during most years since then, finding enough water to sustain life has been a struggle for those who have made the desert their home.

"Building dams to divert water from the Virgin and Santa Clara rivers was a constant battle," said Doug Alder, past president of Dixie College and a St. George resident. "The whole history of the county has been water development. Every 10 years you had a water project. Many of them didn't succeed."

Yet at least in the short term, water concerns do not seem to be slowing new residential development in Dixie.

In fact, the Washington County Water Conservancy District has 25,000 acre feet of unallocated water reserves, enough to support at least 20,000 new households or 60,000 people, according to a report commissioned by the Deseret Morning News and prepared by James Wood, director of the University of Utah's Bureau of Economic and Business Research.

That should be enough water to last until 2020, according to Barbara Hjelle, assistant manager and general counsel of the Washington County Water Conservancy District. And with plans under way to build a 120-mile pipeline from Lake Powell to St. George, the county's water needs could be assured until 2039, Hjelle said.

The area will need every drop it can find.

With a growth rate of 8 percent, Washington County is the nation's fifth-fastest-growing county.

More than 600,000 people will live in Washington County by 2050, according to the Governor's Office of Planning and Budget. But some believe the state's projections are off. Allan Carter, director of developer services for Southern Utah Title Co., which tracks real estate transactions on a daily basis, estimates that the county will surpass the 600,000 mark by 2038.

"There's a lot of development going on down here and a lot of people coming to us for water," Hjelle said. "A lot of folks come to us and say, 'You should not develop water, because you should stop growth in Washington County.' Those sorts of decisions should be made by the people who are elected. We are not trying to drive policies of growth through water."

Without the Lake Powell pipeline, which will channel about 70,000 acre feet of water annually and support 200,000 people, water could become a constraint by 2020, Wood said.

The district's biggest customer is St. George, which purchases 10,000 acre feet of water annually. Another 2,000 acre feet of water is sold to Washington city, with smaller blocks of 500 to 1,000 acre feet sold to other municipalities.