The planned dam, designed to hold back hundreds of millions of tons of mining waste, will be similar in structure and height—soaring 250 feet above Mr. Ehman’s century-old log cabin—to one in Brazil that burst in January, killing 270 in a tsunami of sludge.

That disaster, the deadliest of its type in half a century, has upended the global mining industry. The world’s biggest mining giants have spent months and millions of dollars re-evaluating their dams. Institutional investors are scrubbing their portfolios, looking for companies with risky structures—and helping to publicize potential stability issues. And environmentalists are getting new support from residents, some of whom are learning for the first time about the potential dangers of the dams in their communities.

The U.S. remains one of the countries where the type of dam used in the Brazil disaster, known as an upstream design, is still being built. They are effectively banned in parts of Canada, in many situations in the European Union and now, in Brazil itself. The planned dam outside of Embarrass uses the design. Last month a court suspended permits for the mine until the builder makes clear how it has assessed the Brazil disaster.

“We are allowing dams in the U.S. that countries in the developing world do not accept,” said

Steve Emerman,

the owner of Utah-based mining and groundwater consultants Malach Consulting.

In the Brazil disaster, a dam holding tons of iron ore tailings near the town of Brumadinho crumbled, sending water, rock and mud flooding over a square mile. Inspectors had worried about the dam’s integrity for months.

Upstream design dams are built up with the tailings in a stair-step fashion. The design is one of the simplest and least expensive. Critics say it is also one of the most dangerous. The U.S., unlike some other mining nations, doesn’t have an accessible database of upstream dams, though experts estimate there are more than 500.

The Army Corps of Engineers monitors some, but not all, of the tailings dams in the U.S. The corps monitors more than 1,300 tailings dams of all designs, and roughly a quarter of these are classed as having a “high” or more severe hazard potential, meaning a failure could cause the loss of human life.

In reaction to the Brazil disaster, a group of large institutional investors led by the Church of England Pensions Board asked big miners to disclose details about their mines globally—listing potential hazards they posed and whether they had ever experienced any stability issues.

At U.S. and Canadian mines operated by 34 companies that disclosed information, about 11% of the roughly 560 tailings dams reported stability issues. According to company filings, about 8% didn’t have full engineering records, the plans which experts say are necessary to ensure a proper audit of their safety.

In the U.S. alone, stability issues were reported for 18 dams, and 14 of those were classed as having a “high” or more severe hazard potential. Of the 18 that reported stability issues, 16 were upstream dams and one was a partial upstream dam.

Vale SA,

VALE -1.95%

which operated the failed Brumadinho dam, disclosed stability issues at a complex of six of its dams near the city of Thompson, in the Canadian province of Manitoba.

Niki Ashton,

a Canadian legislator representing the area, said when she learned about the disclosure, she asked local officials if any of them had been told anything before by Vale. No one had, she said.

“We were all shaken up by Brumadinho,” she said.

A spokeswoman for the government of Manitoba said the provincial regulator, which inspects the dams, hadn’t been informed of stability issues. The regulator made an inspection in August and saw no imminent risk, she said.

The disclosures were the first warning for many residents that they were living near structures with problems.

Hilda Fitzner,

who lives in Thompson, said she heard about the stability issues from a friend’s

Facebook

post. “It caught me completely off guard,” she said.

A Vale spokesman said a routine company inspection after the Brazil disaster revealed the Manitoba dams had a safety factor—an engineering term that refers to the factor of strength greater than what is needed for an intended load—below the level prescribed by the Canadian Dam Association, a trade group. He said there is no imminent threat of failure.

Barrick Gold Corp.

, one of the world’s largest gold miners, disclosed that the upper section of a dam on a disused mine near Hope, British Columbia, didn’t have a full set of engineering records. It said later, after questions from The Wall Street Journal, that engineers commissioned by the company had raised concerns in the past about the dam’s upper and lower sections. These concerns weren’t disclosed in its public filing.

Both sections have a “very high” hazard rating, meaning a collapse could result in up to 100 deaths and significant environmental damage.

Barrick said that in 2016 it determined the upper section of the dam needed bolstering, and that in 2002, an engineer raised concerns about the lower section of the dam because of a pond of water that formed on top of it. Water collecting on a tailings dam can weaken the structure.

“My kids swam in that,” said

Bruce Glowienka,

who owns property near the dam. “Wow, wow, wow.”

Barrick said it spent 20 million Canadian dollars, or about US$15 million, addressing the issues. The work took the possibility of a failure from a “relatively low likelihood to a very, very low likelihood,” said

Patrick Malone,

Barrick’s head of legal and regulatory affairs in North America.

After the Brumadinho collapse, investor

Robert Crayfourd,

a fund manager at London-based CQS New City Investment Managers, sent an email to all the miners in his portfolio asking them for more information on their dams. The company manages about $182 million in the resource sector. Post Brumadinho, any upstream dam “would be a major red flag,” Mr. Crayfourd said.

Joe Foster,

a New York-based fund manager at VanEck, also dug deep into the tailings-dam exposures of the miners whose shares he owns. Though the former geologist didn’t find anything of concern in his own portfolio, Brumadinho brought home the risks that tailings dams pose, he said.

Miners have made changes since Brumadinho. Canadian-based gold miner Agnico Eagle Mines Ltd. began bringing a specialist in tailings dams to its meetings with funds for the first time, CEO

Sean Boyd

said.

After reading that Vale’s outside safety auditors had known about risks at the Brumadinho dam, Richard Adkerson, chief executive of

Freeport-McMoRan Inc.,

called his company’s senior managers into a room. “If anyone in our organization, or any of our external experts, has any indication of problems, it is not acceptable to not pass that on,” he said he told them.

The exact causes of the Brumadinho disaster aren’t yet known. While mine dams of all designs have failed, the upstream type has produced more disasters.

In 2015, another upstream dam operated in Brazil by a joint venture between Vale and

BHP Group Ltd.

collapsed, killing 19 people. A year earlier, one at Mount Polley, owned by

Imperial Metals Corp.

, in British Columbia burst, sending some 25 million cubic meters of gold and copper-mining waste pouring into a pair of glacial lakes. No one died in the sparsely populated region, but the accident is classified as one of Canada’s worst environmental catastrophes.

In 1972, an upstream dam holding back coal mining waste collapsed in Buffalo Creek, West Virginia, releasing 130 million gallons of slurry that killed 125 people, wiped out entire towns and left more than 4,000 people homeless—the worst tailings dam failure in U.S. history.

After Brumadinho, Brazil banned upstream dams completely. Earthquake prone Chile and Peru have prohibited them for some time. In Ontario, where

PolyMet Mining Corp.

PLM 0.55%

, the company building the dam outside Embarrass, is headquartered, upstream dams are effectively banned because they are no longer considered part of the mining industry’s “best practices,” a requirement before local authorities grant a permit.

The European Union also bans them in certain situations, such as in places where there is a risk of seismic activity.

“Why is an upstream construction method good enough for Minnesota, when Brazil has found this design so unacceptable that old upstream dams must be decommissioned as well as new ones prohibited?” said

Paula Maccabee,

an attorney with Water Legacy, an environmental group focused on protecting water quality in Minnesota.

Water Legacy cited Brazilian academic research that found that 66% of mining dam failures world-wide involved upstream tailings dams, and it pointed to a United Nations report that listed eight such failures between 2014 and October 2017 in Canada, Mexico, Brazil, China, Israel and the U.S.

Last month, the Minnesota Court of Appeals suspended permits for the new PolyMet mine, preventing construction until the state regulator advises the court on its evaluation of the Brazil dam failure, among other things.

In response to the ruling, a spokesman for PolyMet, which is majority owned by London-listed mining giant

Glencore

GLNCY -3.20%

PLC, pointed to an analysis from the Minnesota Department of Natural Resources, the state regulator that approved the plan for the dam, which concluded there were critical differences between the Brazil dam and the planned U.S. dam, including the design, topography and local seismic activity.

“Nothing in the [Brazil] dam failure changed what we’ve evaluated and the results of our stability analyses,”

Christie Kearney,

PolyMet’s environmental site director, said before the ruling. Both PolyMet and the Minnesota Department of Natural Resources said the dam was safe.

The U.S. Mine Safety and Health Administration, a Federal agency that enforces compliance in mining with health and safety standards, said it has met with Brazil’s National Mining Agency to discuss Brumadinho. “Once the cause of the dam collapse is determined, MSHA will evaluate the findings and how they relate to practices in the United States,” a spokeswoman said.

The Association of State Dam Safety Officials, a trade body that tries to improve U.S. dam safety, is considering recommending the addition of specific information about tailings facilities for the first time as it helps update the U.S. Federal Emergency Management Agency’s guidance on dam safety regulation. It was last updated in 2007 and is tentatively planned to be updated in late 2020.

For now, though, tailings dams in most U.S. states are regulated in the same way as water dams, despite the fundamental differences in their materials, operations, construction and purpose, according to dam experts.

“There can be potential issues if the regulatory authority doesn’t have sufficient expertise and experience with tailings dams,” said

Mark Ogden,

a technical specialist at the Association of State Dam Safety Officials.

Environmental groups have used the powerful video of the collapse of the dam in Brumadinho, which was recorded by a camera on site, to capture the attention of the public.

In Queen Valley, Ariz., population 829, a March meeting to discuss a potential dam for a nearby mining project including a viewing of the video. About 300 people attended the event, sponsored by two local environmental groups, compared with about 100 who attended such meetings before the disaster, according to

John Krieg,

a retired electrician who lives in Queen Valley and attended the meeting.

“We’ll be looking up at a 500-foot dam, containing 1.6 billion tons of toxic waste and wondering when it is going to collapse and bury the community,” Mr. Krieg said.

A spokesman for Resolution Copper, a joint venture between BHP and

Rio Tinto

PLC that is proposing the mine, said the dam it plans won’t have the upstream design of the one in Brazil. He said tailings experts, government agencies and the public are all involved in consultations for the project.

Environmentalists have for years opposed the plan for the new dam outside Embarrass. The project will build a new upstream mining-waste dam on top of an existing upstream dam—which is already visible from Mr. Ehman’s land. The project will almost double the dam’s capacity to 525 million tons to hold waste PolyMet expects from a planned copper and nickel mine. Many people living close to the mine have embraced the plan, eager for the return of jobs to the region.

Mr. Ehman’s property and 34 others sit inside what is called the “inundation zone” of the project: A full-blown failure would send a wave of waste reaching as high as 9 feet and traveling at speeds of up to 25 feet per second through the zone, according to a study by PolyMet.

No one told Mr. Ehman, a 61-year-old professional landscaper.

“Oh, jeez. What are you saying? That, possibly, could happen?” Mr. Ehman asked.

The inundation-zone report was published on the Minnesota Department of Natural Resources website but not flagged to Mr. Ehman and other residents interviewed in the zone, who didn’t see it.

A spokesman for PolyMet said the company planned to inform residents in the inundation zone after they had concluded a second report on what would happen in the event of a collapse. The company said it has an evacuation plan in case of emergency, and that there are a series of lakes and water dams that would act as barriers to any serious spread of tailings in a breach.

The Minnesota Department of Natural Resources said the chances of a catastrophic breach are “very unlikely,” based on the dam-break report that PolyMet completed. “No other project in the history of Minnesota had been more thoroughly evaluated,” the department spokesman said.

Tony Licari,

whose barn is visible from the north end of the existing dam, isn’t worried. “We’re a mining region,” said Mr. Licari, who works as a pit supervisor at a nearby taconite mine. “It’s what we do up here.”

A Native American tribe that hunts and fishes close to the mine has expressed opposition, said

Nancy Schuldt,

water-protection coordinator with the Fond du Lac Band of the Lake Superior Chippewa. Brumadinho “played into the sense of dread we have about the likelihood of that sort of catastrophic failure,” she said.

—Angela Calderon contributed to this article.

Write to Alistair MacDonald at alistair.macdonald@wsj.com, Kris Maher at kris.maher@wsj.com and Kim Mackrael at kim.mackrael@wsj.com