"That projection assumes the growth rate in the county will slow down," Spendlove said. "Washington County will still be one of the major growth counties in the state and nation."

Meanwhile, the census estimates showed continued growth along the Wasatch Front. Salt Lake County, with an estimated population of 948,172, ranked 38th nationally in numerical growth. The state's largest county gained 13,334 people, a growth rate of 1.4 percent.

Utah County ranked 64th, gaining an estimated 9,624 people to grow to 443,738 — a growth rate of 2.2 percent. And Davis County ranked 97th, gaining an estimated 6,792 people to grow to 268,187.

Spendlove said the Wasatch Front is experiencing what demographers call a "doughnut effect." Much of the growth is in suburban areas within large counties and in counties that border the anchoring metropolitan areas.

Spendlove said plans, such as a proposed pipeline from Lake Powell to bring in more water, are in the works to ease the impact of the growth.

"It's something that we've been thinking about for several years," Spendlove said. "For the entire history of state projections, we've underestimated Washington County. No one saw the growth we're seeing could continue."

Spendlove said although the county's growth will slow, according to Utah Population Estimates Committee projection, it's expected to continue outpacing statewide growth.

By 2050, the state's growth is expected to slow to 1.3 percent per year, but Washington County's isn't forecast to drop below 2.4 percent.

Spendlove said Salt Lake County is also expected to continue to grow — reaching 1.6 million people by 2050.

Washington County's rapid growth has worried conservationists, who note that St. George has one of the highest per capita water usage rates in the West.

Merritt Frey, executive director of the Utah Rivers Council, said in 2000 St. George used 391 gallons per capita per day. Phoenix used 240.

Frey said in 2003 the average water usage for Utah was 265 gallons per person per day.

While there are indications that the numbers have lowered some, there's nothing to suggest the spread has changed, she said.

"There really needs to be a huge focus in that county on water conservation," she said. "The advantage of new development is they have a chance to do it right the first time . . . to install efficient systems up front."

Ron Thompson, district manager of the Washington County Conservancy District, said growth is something his county has been dealing with for several years and is focusing on in its long-term planning.

Communities are working on water conservation plans and are working hard at defining transportation corridors. And schools are talking about long-term growth.

"St. George is probably more equipped than some of the smaller communities, but I think they'll get there," he said. "I think we're headed in the right direction."

The census calculates the estimates using administrative records and estimates for births, deaths and net migration.

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E-mail: dbulkeley@desnews.com