Before the pandemic, Colorado recorded some of the strongest population growth in the country, with many newcomers settling in metro Denver and up and down the northern Front Range.

But the latest Census numbers, released Thursday, show 13 of the state’s largest towns and cities, defined as those with a population of 20,000 or more, have fewer people than they did on July 1, 2020. And in the year through July 1, 2023, 11 out of the state’s larger municipalities, more than a third, lost population.

Population gains consist of natural increases, or births, which are falling in Colorado, minus deaths, which are rising due to an aging population, plus net migration, the flow of people moving in and out of the state.

“Even though our ‘in’ levels have stayed pretty high, what we have started to see is an increase in our ‘outs,’ especially among our older adults,” said Elizabeth Garner, Colorado’s state demographer. Viewing the latest numbers through a lens of older people leaving the state and younger people moving to places where they can start families helps explain much of what is happening.

Fountain, located south of Colorado Springs, lost the most people on a percentage basis of any large town or city in Colorado over the past three years. Its population is down 4.7% from where it was on July 1, 2020, and is down 1.5% over the past year to 28,489, according to the U.S. Census Bureau.

Fountain’s population ebbs and flows with activity at the nearby Fort Carson Army Base, which had 74,213 people employed there as of last August. About 77% of those working at the base live in the surrounding community and show up in population counts. The U.S. military is now much less active than it was last decade, so that might explain some of the decrease.

Louisville in Boulder County lost 3.7% of its population over the past three years, although last year’s decline was only 0.18%. It had 20,390 residents in the latest count. Littleton had the largest decline within metro Denver at 2.5% in the past three years, including a 0.74% decline last year, bringing its headcount to 44,451.

Arvada, Boulder, Wheat Ridge, Westminster, Centennial, Golden, Pueblo, and Lakewood were other cities that have lost population over the past three years, according to Census estimates.

At the other extreme were places with lots of new home construction like Windsor, which saw its population shoot up 4.8% in the last yearly count and 21.4% over the past three years. It has added more than 7,000 people in the past three years, surpassing Englewood and Northglenn in population, even though both those cities had more people than Windsor in the summer of 2020.

Erie and Castle Rock are also seeing population surges, up 16% and 10.2% respectively since July 1, 2020. Commerce City, Parker, Grand Junction and Brighton all had population growth rates between 5% and 10% in the three-year period that the Census has counts for.

“We see more new home development north of Denver in Loveland, Greeley, and other smaller cities in Weld County with lower-priced housing and people migrating in that direction. People are moving for affordability,” said Chelsea Scott, a senior manager in consulting tracking the Colorado housing market with John Burns Research and Consulting.

Denver is technically on the losing side of the ledger. Its population of 716,577 last summer didn’t surpass the 716,606 the U.S. Census Bureau estimated for July 1, 2020. But Denver’s population rose 0.44% in the 2023 count and if there is any Colorado city that could rebound strongly in the next count, it is Denver.

“The Census annual estimates are always subject to errors, especially at the municipal level, like in today’s release. But we think the last few years since 2021, the errors have been getting worse. The reason is the Census is underestimating immigration into the U.S. and Denver,” said Eric Finnegan, a vice president of demographics with John Burns Research and Consulting.

Chicago and Denver stand out as two U.S. cities with the highest number of immigration court filings and Finnegan estimates the U.S. Census Bureau may have undercounted the number of people who entered the country last year by 2.2 million.

Finnegan said the U.S. Census Bureau estimates 17,629 international immigrants have arrived over the past three years in the 10-county metro Denver area. John Burns estimates that 41,355 immigrants have arrived, including more than 20,000 last year alone.

“We think the Census undercounted immigration into the 10-county Denver metro area by 13,000 in 2023 and a total of 24,000 over the last 3 years,” he said.

Colorado Springs, the state’s second-largest city with 488,664 residents, grew its population 0.49% in the latest count and 1.73% over three years. Aurora, in third place with 395,052 residents, grew 0.32% last year and 2.14% over three years and is positioned to ramp up as master-planned communities like The Aurora Highlands gain traction.

Greeley, which took longer to recover jobs lost during the pandemic than any other metro area in the state, saw its population flatline for two years but rebounded in the latest count, adding 3,364 people to bring its total to 112,609, which represents a 3.1% growth rate.

Lakewood, the largest city in Jefferson County, saw its population rise in the first full count during the pandemic, but then lost people. The Census Bureau estimates the city had 155,961 residents on July 1, below the 156,247 it had on that day in 2020.