Between 2001 and 2019, the built-up landscape of America — buildings, roads and other structures — has expanded into previously undeveloped areas, adding more than 14,000 square miles of new development across the contiguous United States — an area over five times the size of Delaware.

A Washington Post analysis of data released by the U.S. Geological Survey this summer highlights where the most development has taken place. Suburbs are sprawling out in Arizona and Nevada as industries move to the Sun Belt, retirement communities are popping up in Florida as the baby boomer generation ages, and oil and natural gas wells have emerged across North Dakota and West Texas.

Uneven development across the country has been driven in large part by rising housing costs, according to Albert Saiz, an associate professor at MIT’s Department of Urban Studies and Planning.

“What explains the divergence in urban growth in the last 20, 30 years is really the housing market,” Saiz said. As cities on the coasts have become more and more expensive, there has been an influx of people and industries into other parts of the country, creating the perfect conditions for rapid sprawl into available land in the South and West.

In Harris County, Tex., which includes Houston, this dynamic has led to the addition of 200 square miles of new development since 2001 — an increase of 21 percent.

“Houston is located in a very flat part of Texas, where land development is as easy as pie,” said Margaret Wallace Brown, the city’s director of planning and development. “You could see where we are building way off into the horizon.”

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Uninhibited growth can lead to new problems for cities, as human development puts new demands on the environment.

Researchers and experts have said explosive urban development was a contributing factor for the intense flooding after Hurricane Harvey devastated Houston in 2017. The city is located on some of the country’s least permeable soil, and construction of impervious surfaces, like roads in flood-prone areas, exacerbated the inability of the city to withstand worsening natural disasters because of climate change.

“You have the two water problems: too much water or not enough,” Saiz said, referring to both the risk of flooding in low-lying developed areas in states such as South Carolina, Florida and Texas, as well as the dangers of water shortages in the Southwest United States.

Water shortages in cities like Las Vegas are the result of outdated leasing agreements for water from the Colorado River.

“We’ve seen a massive population growth, but we’ve never seen a larger proportion or allocation” of water, said Ryan Smith, Las Vegas’s acting director of economic and urban development. The population in and around Las Vegas has tripled since 1990.

Sprawl in the South and Southwest

In the South and Southwest, the cities of Phoenix, Houston and Las Vegas have seen explosive growth over the past 20 years. Maricopa County, which encompasses Phoenix, added the most developed land since 2001: more than 270 square miles.

In 1999, Houston enacted new land use regulations that set in motion two decades of the city’s expansion. Developers prioritized multifamily townhouses and encouraged migration from other states and countries. These changes led to a 21 percent increase in developed land since 2001.

“We are not hemmed in by other cities. We are not hemmed in by geographic features such as mountains or rivers,” Wallace Brown said. “Sprawl comes naturally to this landscape.”

Cities like Las Vegas have also seen a sharp uptick in growth as new residents and industries move to the Sun Belt.

“I think our low tax environment, quality of life, affordability has helped. I think professional sports has helped to bring a community together. And there’s ancillary development that happens out of that,” Smith said. “Vegas is poised for growth because it’s really a young city.”

The city faces potential downsides of its growth, as well as limitations on future growth. In 2020, a majority of Nevada’s land was owned by the federal government.

“We have a severe shortage in affordable housing, like most cities, but ours is exponentially larger,” Smith said. “We have to look at available federal lands and try to obtain them for future growth.”

While Las Vegas has historically grown horizontally, Smith expects that the future will bring more vertical development in the city center.

Fracking boom in North Dakota and West Texas

In North Dakota and West Texas, two decades of oil booms are speckled across the landscape.

In McKenzie County, N.D., just south of Williston, N.D., there was a 66 percent increase in developed land between 2001 and 2019. Even as the oil boom in western North Dakota slowed down as oil prices dropped, the remnants of the growth remain.

A similar story is unfolding across West Texas, which has seen explosive population growth alongside new oil and natural gas development. Nineteen of the top 25 fastest-developing counties in the country by percentage are in Texas.

Midsize cities in the Mountain West growing quickly

For midsize cities like Boise, Idaho, and Reno, Nev., affordability and proximity to prime spots for hiking, skiing, mountain biking and other outdoor activities are a huge draw for residents.

Across the country, housing prices have risen by almost 25 percent since March 2020, creating bidding wars for any house that goes on the market in cities like Boise. This most recent growth has generated more than $1 trillion in new wealth for existing homeowners in the United States.

Cody Riddle, deputy director of current planning in Boise, believes a major draw has been a proximity to nature with the benefits of an urban area. “It doesn’t get too hot, it doesn’t get too cold. You can float the rivers in the summer, and you can ski in the winter,” he said. “The downtown has the urban amenities that people desire as well.”

“Just this morning, I saw a bobcat [while] on my bike,” Riddle added with a chuckle.

In Reno, known as the “Biggest Little City in the World,” over half of the city’s growth has come from Californians and Texans, many of whom moved to the city for jobs in the growing advanced manufacturing and technology sectors. In 2016, Tesla opened a new gigafactory outside the city. But as Reno and Boise grow, housing affordability may hinder future growth. In Reno, the median price for an existing single-family home has recently reached half a million dollars.

Looking forward

Saiz expects that demographic shifts will continue to drive new development into previously undeveloped areas, even as the United States faces the deepening climate crisis.

“The older population is going to be growing, which sustains the growth of Florida and the Southwest,” Saiz said. “You need more services, more hospitals, more doctors, more research.”

“You have more growth in some of these areas and then you have the incredibly uncertain impact of climate change … it’s either not good or very bad.”