Scott County is the fifth-fastest growing county in Iowa since the 2010 Census, new population estimates say.

The estimates, which are being released today, show moderate growth in the county, with the population increasing to 168,799 as of July 1, 2012. That’s a 3,575-person increase, or just shy of 2.2 percent, since April 1, 2010.

That rate of growth is higher than what the county saw over the past decade, and what’s notable is that  unlike some other years, the county’s population is going up not just because of what is called natural increase, or a greater

number of births over deaths. More people are moving here, too.

Scott County had the fourth-highest number of net migrants of the 99 counties in the state.

The picture is different across the Mississippi River. Rock Island County experienced a small loss of 89 people over the two years to settle at a population of 147,457 as of July 1, 2012. It also saw a steady flow of people leaving the county, according to the Census Bureau. During the two-year period, a net 1,096 people moved out of the county. A greater number of births over deaths helped to offset most of the loss.

The new figures aren’t actual head counts, like the 2010 Census. They’re government estimates, which also figure in birth, death and administrative records.

In Iowa, 35 counties gained population, while 64 lost people over the two-year period.

Scott County’s population increase isn’t of the bin-busting variety. It still is only a quarter of the state’s fastest-growing county, the Des Moines-area Dallas County, which grew by 8.8 percent. But it is more than twice the statewide average.

Tara Barney, chief executive officer of the Quad-Cities Chamber of Commerce, said strength in certain economic sectors is likely drawing people.

“I know a lot of businesses have been very aggressive recruiting in very specialized areas,” she said.

Those include agriculture and ag-related businesses, as well as manufacturing and engineering, she said.

Historically, people who move out of either Scott or Rock Island counties have tended to relocate across the river rather than going to counties farther away. That could account for some of the higher net in-migration Scott County is experiencing. The new data says a net 1,600 new people relocated to Scott County between 2010 and 2012.

Barney, however, said she didn’t think that kind of reshuffling is driving the increase here. She said companies such as Alcoa Davenport Works and Cobham Mission Systems are drawing people from outside the area.

Overall, the Quad-City metropolitan area, which also includes Mercer and Henry counties in Illinois, grew by 2,940 people to 382,630 over the two-year period. It moved from the 164th most populous metro area in the country to 165th.

Nationwide, the Census Bureau says, some of the strongest growth among metro areas was in or near the Great Plains and West Texas, as well as in counties in North Dakota. It attributed some of the growth to the oil and gas boom.

Midland, Texas, had the fastest rate of increase of the nation’s metro areas between 2011 and 2012, while two counties in North Dakota ranked among the top 10.

“After a long period of out-migration, some parts of the Great Plains — from just south of the Canadian border all the way down to West Texas — are experiencing rapid population growth,” said Thomas Mesenbourg, the Census Bureau’s senior adviser. “There are probably many factors fueling this growth on the prairie, but no doubt the energy boom is playing a role.”