Benjamin Spillman, Jill Castellano and Tracy Loew

USA TODAY Network

As the nation's 84,000 dams continue to age, a growing number of people downstream are at risk, experts say.

That's not only because of older infrastructure but also because of population growth around some of the dams. More than a quarter were developed primarily for recreational purposes, according to National Inventory of Dams data from 2016.

"The nation’s dams are aging, and the number of high-hazard dams is on the rise," according to a 2013 report from the American Society of Civil Engineers. "Many of these dams were built as low-hazard dams protecting undeveloped agricultural land. However, with an increasing population and greater development below dams, the overall number of high-hazard dams continues to increase."

That problem was highlighted this week as nearly 200,000 people evacuated an area near California's Oroville Dam, about 150 miles northeast of San Francisco. California water officials were worried that erosion they discovered Sunday at the top of its emergency spillway could send a 30-foot tall wall of water down the Feather River and through the Northern California cities of Oroville, Yuba City and Marysville.

► Related: New storms could imperil Oroville where 200,000 were evacuated

The population of Oroville, the county seat of Butte County that's less than 10 miles downriver from Oroville Dam, has more than doubled since the dam was completed in 1968.

Most U.S. dams were completed between 1950 and 1980. A small fraction of dams, 2.8%, were built before 1900. More than 4,000 dams have been built since 2000, accounting for 4.5% of all U.S. dams.

The latest data in the dams inventory, which the Army Corps of Engineers compiles, shows almost 15,500 dams across the USA are characterized as high hazard, meaning at least one person could die if the dam were to fail.

Among those high-hazard dams, nearly 1 in 5 lack an emergency action plan, a document dam owners maintain that includes critical information such as emergency contacts, details about the dam and an inundation map.

“If you have a good emergency action plan, you are going to reduce the consequences if the dam fails,” said Lori Spragens, executive director of the Association of State Dam Safety Officials.

The average age of the United States' 84,000 dams is 52, according to the American Society of Civil Engineers' report.

“We can’t seem to get the federal government or the states interested in funding the most fundamental part of what makes us go, which is infrastructure,” Spragens said. “It just needs so much more attention at a national level.”

In seven states, more than half of the high-hazard dams are operating without emergency action plans, according to the National Inventory of Dams:

• In South Carolina, it’s 96%.

• Mississippi, 88%

• Rhode Island, 82%

• Alabama, 79%

• New Mexico, 61%

• Florida, 58%

• North Carolina, 57%

Only three states — Louisiana, Maine and Tennessee — and Puerto Rico have emergency plans in place for all the dams with high-hazard potential.

In California, the problem is especially widespread.

California has more than 1,500 dams, according to the National Inventory of Dams database. Of those, 52% are considered high hazard, the fourth highest of any state.

► Related: What is the Oroville Dam and what will happen if its spillway fails?

Nationwide, 17% of dams are considered high hazard.

In California, more than a third, 36%, of the high-hazard dams don't have an emergency plan, which would kick into gear if the dam appeared to be a threat. Oroville Dam does have an emergency plan.

Nationwide, 31% of high-hazard dams lack an emergency plan.

Almost two-thirds of U.S. dams are privately owned. Among the rest: Local governments own 20%; states, 7%; public utilities, 4%; the federal government, also 4%; and the remainder don’t have their ownership listed.

Understanding the level of risk associated with the nation’s dams can be difficult.

States have oversight of about 7 in 10 of the nation’s 84,000 dams. The federal government through the Army Corps of Engineers, the Bureau of Reclamation and the Federal Energy Regulatory Commission regulates others.

More than three quarters of the high-hazard dams are state regulated, according to Association of State Dam Safety Officials. Of those, about 1 in 7 are in need of remediation.

“That is one of our biggest national challenges, to try and get these dams inspected more regularly,” Spragens said. In 2015, states spent $49.4 million regulating and inspecting more than 81,000 dams, including the high-hazard dams under state regulation.

The Federal Emergency Management Agency operates the National Dam Safety Program, which is an attempt to support standards for dam safety. The program supports research and training related to dam safety and inspection, including grants to states to improve their own regulatory systems.

► Related: Dam's eroding spillway further proof that northern Calif. drought is over

But the FEMA program doesn’t provide enough money for states to enforce the regulations or inspections.

“A lot of states are behind on that schedule. That is a huge challenge right now across the country, having enough inspectors doing their jobs,” Spragens said.

People who want to learn more about dams in their own communities will have a difficult time tracking down critical data. Security concerns following the Sept. 11, 2001, terrorist attacks prompted restrictions on the availability of hazard ratings and conditions of specific dams.

“That is not going to be easy to find,” Spragens said. “You obviously don’t want people panicking, but it is very understandable: We want people to know.”

Contributing: Mark Nichols, USA TODAY. Benjamin Spillman reports for the Reno (Nev.) Gazette-Journal; Jill Castellano, The (Palm Springs, Calif.) Desert Sun; and Tracy Loew, (Salem, Ore.) Statesman Journal. Follow them on Twitter: @ByBenSpillman, @Jill_Castellano, @Tracy_Loew

Related:

► Alabama: Alabama has the biggest dam problem in the U.S.

► California: Environmental groups warned of dangers at California dam more than a decade ago

► California: Beyond Oroville: Dam inspections in California are behind schedule

► Colorado: How safe are northern Colorado dams?

► Florida: Lake Okeechobee dike safer, but work remains

► Kentucky: Dozens of risky Kentucky dams in poor condition

► Michigan: No pending Oroville, but Michigan's aging, crumbling dams pose a risk

► Nevada: High hazard Nevada dams lack emergency plans

► New Jersey: A third of N.J.'s most critical dams need repairs

► New York: After Oroville, attention turns to New York dam safety

► Ohio: Officials: Dam 'hazard creep' a local problem

► Oregon: 7 Oregon dams in 'unsatisfactory' condition

► Pennsylvania: Big dam failures unlikely in Central PA

► Pennsylvania: What's the status of Indian Rock Dam?

► South Carolina: Risky dams face long inspection gaps

► Tennessee: Threats found in major dams in Tennessee