SALT LAKE CITY — The window-like arches on the outside of the towering Mountain Dell Dam provide an artistic facade that fits with the natural beauty of the Utah mountain canyon where the century-old structure sits.

The design’s historical significance earned it a spot on the National Register of Historic Places and a designation as an American Water Works Association Landmark.

But the concrete structure also sits on a less desirable list: It is one of five dams in Utah and at least 1,688 in the United States that are in poor or unsatisfactory condition and located in places where a breach would likely kill at least one person, according to an investigation by The Associated Press.

A more than two-year investigation by the AP has found scores of dams nationwide in even worse condition, and in equally dangerous locations. They loom over homes, businesses, highways or entire communities that could face life-threatening floods if the dams don’t hold.

A review of federal data and reports obtained under state open records laws identified 1,688 high-hazard dams rated in poor or unsatisfactory condition as of last year in 44 states and Puerto Rico. The actual number is almost certainly higher: Some states declined to provide condition ratings for their dams, claiming exemptions to public record requests. Others simply haven’t rated all their dams due to lack of funding, staffing or authority to do so.

The AP’s database of individual dams shows hazard levels and condition ratings as of 2018. It’s possible that circumstances have changed since then.

The story of Mountain Dell

The Mountain Dell Dam is 5 miles east of Salt Lake City next to a heavily traveled stretch of Interstate 80 that connects residents and tourists to the mountain resort town of Park City, where many events in the 2002 Winter Olympics were held. The dam is listed in poor condition because of deteriorating concrete that allows seepage from the reservoir, said David Marble, Utah assistant state engineer. The dam is only allowed to be about half full in the winter because of what is known as a “freeze-thaw problem,” which refers to erosion that occurs when concrete cracks get worse over time after freezing and thawing, he said.

Salt Lake City, which owns the dam, is currently installing a liner system on the upstream face of the dam in an attempt to slow the leaks. The $3.1 million project is one of several done in recent years to ensure the dam doesn’t reach a critical level, said Bernard Mo, the city’s storm water capital improvement program manager who is responsible for dam safety.

”We’ve been monitoring this for years,” Mo said. “It’s not like we’re leaving alone an unsafe dam.”

State officials had been “leaning on” on the city to address the dam, and the repairs are substantial, Marble said. “This is not just a little glue,” he said.

A dam failure would close the highway in the canyon and lead to potential flooding downhill in the eastern parts of Salt Lake City, including near a popular park, according to an emergency action plan produced in 1994 and updated in 2010.

But Marble and Mo insist the dam isn’t at risk of that, which is why it’s not in the “unsatisfactory” category for dams. Utah doesn’t have any dams in that category.

Deaths on decline buts failures top 1,000

Deaths from dam failures have declined since a series of catastrophic collapses in the 1970s prompted the federal and state governments to step up their safety efforts. Yet about 1,000 dams have failed over the past four decades, killing 34 people, according to Stanford University’s National Performance of Dams Program. Built for flood control, irrigation, water supply, hydropower, recreation or industrial waste storage, the nation’s dams are over a half-century old on average. Some are no longer adequate to handle the intense rainfall and floods of a changing climate. Yet they are being relied upon to protect more and more people as housing developments spring up nearby.

”There are thousands of people in this country that are living downstream from dams that are probably considered deficient given current safety standards,” said Mark Ogden, a former Ohio dam safety official who is now a technical specialist with the Association of State Dam Safety Officials.

The association estimates it would take more than $70 billion to repair and modernize the nation’s more than 90,000 dams. But unlike much other infrastructure, most U.S. dams are privately owned. That makes it difficult for regulators to require improvements from operators who are unable or unwilling to pay the steep costs.

”Most people have no clue about the vulnerabilities when they live downstream from these private dams,” said Craig Fugate, a former administrator at the Federal Emergency Management Agency. “When they fail, they don’t fail with warning. They just fail, and suddenly you can find yourself in a situation where you have a wall of water and debris racing toward your house with very little time, if any, to get out.”

A National Climate Assessment released by the White House last year noted growing frequency and intensity of storms as the climate changes. That can push some dams beyond what they were designed to handle.

Even if kept in good condition, thousands of dams could be at risk because of extreme rainstorms, said Fugate, the former FEMA official.

”These are like ticking bombs just sitting there, waiting for the wrong conditions to occur to cause catastrophic failure,” he said.

Utah fares better than many other states

Overall, Utah dams are in pretty good shape compared to other states. Just 2% of Utah’s 241 high-hazard dams are in poor condition, one of the lowest rates among 44 states and the U.S. territory of Puerto Rico that provided the AP with full data. High hazard is a category that doesn’t refer to the dam’s condition, but rather the fact that a failure could lead to loss of life.

Utah’s dam safety budget has increased 18 percent since 2010 to $866,000 this year, according to data obtained by the AP. The state has repaired 55 dams over the past two decades at a cost of $198 million in state and federal money. Marble said 84 dams remain on the state’s fix-it list.

The program has shored up some of the state’s most at-risk dams, including one at Fairview Lake in central Utah that had a leak that was letting out enough water per second to fill a basketball, Marble said.

”We had quite a few that you would call a ‘leaker’ and a ‘saggers’ that we were very concerned about,” Marble said. “I sleep a lot better at night because of the work we’ve done over the last period of time.”

The other four Utah high-hazard dams listed in poor condition are smaller irrigation dams that are privately owned.

The top of the concrete wall is deteriorating on the aging Red Pine dam deep in the mountains east of Salt Lake City, a few miles downhill from the Snowbird ski resort, Marble said. Some temporary repairs are being done.

A state restriction has been in place for about a decade limiting the owners of Jones dam east of Heber City from filling it more than about halfway due to seepage issues, Marble said. The small dam is near a smattering of homes.

State officials didn’t even know about the Alton Reservoir irrigation dam in southern Utah until a few years ago and are working with the owners to make sure it’s in satisfactory condition, Marble said.

The Palisades Lake dam in central Utah near a state park that offers hiking, camping and fishing has some seepage, he said. The dam is on a state list to be repaired sometime in the future, Marble said.

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