More jobs. More homes. More tax revenue.

And a lot more people.

Utah's largest counties have seen unprecedented growth in the past year, according to 2005 estimates released Monday by the state's leading demo- graphers.

Salt Lake County grew by 23,000 people, or 2.4 percent. Utah County increased by 18,400 (4.2 percent) and Washington by 10,000 (8.4 percent).

Even the statisticians were surprised by the spikes.

"We were taken off-guard by the magnitude," said Robert Spendlove, chairman of the Utah Population Estimates Committee. "But the numbers make sense."

Two colossal factors are slamming Utah at the same time and the ripple effects will touch everything from the water supply to the school system.

Grandchildren of the baby boomers are in their most fertile years, causing what demographers call an Echo Boom, a third-generation, mini-baby boom.

As a result, Utah's births minus deaths have equaled about 37,000 more Utahns for the second year in a row.

But that doesn't even account for half of the population growth in the past year. Most of the increase results from new move-ins.

The Beehive State recorded a net migration of 40,647 people after subtracting the number who moved out from those who moved in.

Not since 1992 has Utah recorded such a high growth rate, which coincides with the last big economic expansion.

The population explosion "represents both a challenge and an opportunity to some extent," said Walt Busse, an employee of The Church of Jesus Christ of Latter-day Saints, who is a member of the committee. The committee includes both government representatives and people from the private sector.

"Tax resources are going up but so is the demand for it," Busse said.

Construction workers and real estate agents are probably excited, while school teachers and prison guards are feeling the pinch.

Those who live in such places as St. George, Herriman and Eagle Mountain should continue to see an influx of residents.

Washington County showed the fastest growth rate.

"Retirees do not explain the growth in Washington County anymore," Spendlove said.

While those in their golden years continue to move to the warmer climates of southern Utah, so do thousands of people from the more costly home markets of California and Las Vegas.

Those transplants have started to move up Interstate 15 to Iron County, too. Iron recorded an additional 2,472 people or an increase of 6.4 percent, the second biggest in the state.

Committee member Pam Perlich, who works at the University of Utah, expects the high growth to stay around for awhile, but probably at a slightly slower rate.

The Echo Boom is expected to last another 10 years or so, but the key to such strong growth is the economy, according to Spendlove.

"You can't have sustained population growth without economic growth."

mcanham@sltrib.com