Who would want to live downstream of the 125-year-old Mullaperiyar Dam, nestled in a seismic zone of the Western Ghats mountains in India? The 176-foot-high relic of British imperial engineering cracked during minor earthquakes in 1979 and 2011. According to a 2009 study by seismic engineers at the Indian Institute of Technology, it might not withstand a strong earthquake larger than 6.5 on the Richter scale.

Three million people live downriver of the dam. But their demands for it to be emptied are held up by a long-running legal case in the nation’s Supreme Court between Kerala, the state under threat, and Tamil Nadu, the state upstream that operates the dam to obtain irrigation water and hydropower.

Or how about living below the Kariba Dam, built by the British on the Zambezi River in Southern Africa 62 years ago? Back then, it was seen as Africa’s equivalent of the Hoover Dam. But in 2015, engineers found that water released through its floodgates had gouged a hole more than 260 feet deep in the river bed, causing cracks and threatening to topple the concrete dam, which is 420 feet high and holds back the world’s largest artificial lake.

Downstream are some 3.5 million people, as well as another giant dam, the Cahora Bassa in Mozambique, that engineers fear would probably break if hit by floodwater from a Kariba failure. Despite the urgency, the $300-million repair work won’t be finished until 2023 at the earliest.

Both dams exemplify the potentially dangerous mix of structural decay, escalating risk, and bureaucratic inertia highlighted in a pioneering new study into the growing risks from the world’s aging dams, published in January by the United Nations University (UNU), the academic and research arm of the UN. It warns that a growing legacy of crumbling dams past their design lives is causing a dramatic increase in dam failures, leaks, and emergency water releases that threaten hundreds of millions of people living downstream. Meanwhile, safety inspectors cannot keep up with the workload.

The 20th century was a boom time for dam builders. The peak, particularly in Asia, was from the mid-1950s to mid-1980s, when dams were in vogue to generate hydroelectricity and store water to irrigate crops and keep taps flowing, as well as to smooth out river flow to prevent flooding and improve navigation.

But the boom is over. “A few decades ago, a thousand large dams were being built each year; now it is down to a hundred or so,” report co-author Vladimir Smakhtin of the UNU’s Institute of Water, Environment, and Health in Hamilton, Canada told Yale Environment 360. Most sites sought by dam engineers, such as in narrow valleys, have been plugged. Dams now barricade the majority of the world’s rivers, and can store the equivalent of a sixth of their total annual flow. Meanwhile, environmental and social concerns about flooding land and wrecking river ecosystems have grown, and there are many alternatives for generating low-carbon energy, says Smakhtin.

So the world’s stock of large dams, defined as those higher than 15 meters (49 feet), is aging fast. The World Bank estimated last year that there are already 19,000 large dams more than 50 years old, which the UNU study concludes is the typical lifespan before it needs major repairs or removal.

Britain and Japan have the oldest dams, averaging 106 and 111 years old respectively. U.S. dams average 65 years. But China and India, the epicenters of the mid-20th century dam-building craze, are not so far behind, with average ages of their 28,000 large dams now 46 and 42 years respectively. “By 2050, most of humanity will live downstream of large dams built in the 20th century” that are “at increasing risk of failure,” the UNU report says.

This burgeoning legacy of aging dams poses ever-growing safety risks, as their structures become more fragile and climate change increases stresses on them by increasing extreme river flows, says Smakhtin. The report finds a steep increase in the rate of dam failures since 2005. There is no global database, says co-author Duminda Perera, also of UNU. But he found reports of more than 170 failures between 2015 and 2019, whereas prior to 2005 the average was below four per year.

Just last month, Zambia’s Kandesha Dam, built in the 1950s, collapsed in heavy rains, displacing thousands of people. Last June, a 55-year-old irrigation dam in China’s Guangxi region gave way, after its 492-foot wall was swamped in heavy rain. A month earlier, two old dams in Michigan collapsed during heavy rain — the 96-year-old Edenville Dam on the River Tittabawassee unleashed a flood that demolished the 94-year-old Sanford Dam downstream.

In August 2019, one of Britain’s oldest dams almost failed. Around 1,500 inhabitants of the town of Whaley Bridge were ordered out of their homes after the flood spillway on the 188-year-old Toddbrook Dam, built to supply water to a canal, collapsed in heavy rains, spilling water that began to eat away at the dam itself, raising fears that the structure would collapse and engulf the town.

In 2017, a spillway collapsed at the 50-year-old Oroville Dam in California’s Sierra Nevada foothills. It caused the evacuation of around 180,000 people. The 770-foot dam is the highest in the U.S. and, after repairs to the spillway, remains critical to the state’s water supply.

Dam engineers say the greatest threats for the coming decades are probably in China and India. Both countries have in the past suffered dam failures that killed tens of thousands. In 1979, the disintegration of the Machchhu Dam in Gujarat, India, during a flood, killed as many as 25,000 people.

Four years before, the Banqiao Dam in Henan, China, burst, sending a wave of water 7 miles wide and 20 feet high downriver at 30 miles per hour. It killed an estimated 26,000 people directly, including the entire population of the town of Daowencheng. As many as 170,000 more died during an ensuing famine and epidemics. The disaster has been called the deadliest structural failure in history. It was kept a state secret for many years.

Both these disasters involved young dams, aged 20 and 23 years respectively. Still, their demise suggests there may be many more ticking timebombs from that era.

China has around 24,000 large dams. Many date from the days of the Cultural Revolution, when Maoist ideology trumped engineering prowess in the dash for economic development. A third of the country’s dams were “considered to be of high-level risk because of structural obsolescence and/or lack of proper maintenance,” a 2011 analysis by Meng Yang, now of the Huazhong University of Science and Technology, found.

In India, the director of the Central Water Commission, Jade Harsha, warned in 2019 that the country would have more than 4,000 large dams above the age of 50 by 2050. More than 600 are already half a century old. Dams that India’s first Prime Minister Jawaharlal Nehru in 1954 called “the temples of India, where I worship,” are now aging edifices whose safety Harsha now sees as “blind spots in India’s water policies.”

The World Bank agrees. Last December, it announced a $250-million loan to India for an ongoing project to “strengthen dam safety,” with better inspections and management of the country’s large dams, which hold back 240 million acre-feet of water — starting with 120 of the country’s aging dam fleet.

Martin Wieland, chair of the committee on seismic aspects of dam design at the Paris-based International Commission on Large Dams, the leading body of dam professionals, told Yale e360 that “many dams could last much longer than 50 or 100 years if well designed, well-constructed, and well-maintained and monitored. The oldest concrete dam in Europe, the Maigrauge Dam [in Switzerland] was completed in 1872 and is expected to reach 200 years.” But, he said, “the safety of a dam may deteriorate very fast.” He suggested a large part of the growing risk was “not aging, but the increased number of people downstream.”

Dams are mostly made of earth, masonry, or concrete. They can fail because of decaying concrete, cracking, seepage, hidden fissures in surrounding rocks, or buckling under their own weight. They can suffer lining failures, earthquakes, sabotage, or being washed away when floods breach their crests. Regular inspections are vital, says Wieland.

But there is growing concern worldwide about a lack of inspectors capable of assessing the risk from aging dams, leading to backlogs of inspections and hazards that are missed. An investigation after the failure of the Oroville Dam in the United States found that past inspections had failed to spot structural flaws. As Wieland puts it: “Not everything is visible or measurable.”

Many old dams are now being abandoned as their reservoirs fill with sediment dropped by the rivers they barricade. An international study in 2014 headed by G. Mathias Kondolf of the University of California, Berkeley, estimated that more than a quarter of the total sediment flow of the world’s rivers is being trapped behind dams.

On the Yellow River in China, the world’s siltiest river, the Sanmenxia Dam filled in just two years. India’s reservoirs are losing almost 1.6 million acre-feet of water-storage capacity each year due to sediment build-up, according to officials.

The accumulation of sediment makes dams less useful, but sometimes also makes them more dangerous. This is because with less reservoir space, the dams are at greater risk of being overwhelmed during heavy rains. To save their structures, operators are more likely to make abrupt emergency releases down spillways at the height of floods.

After Hurricane Mitch ripped through Central America in 1998, several hundred people died in their beds in the Honduran capital of Tegucigalpa when a “wall of water” rushed through the city’s poor riverside communities. Investigators from the U.S. Geological Survey concluded that the “wall” appeared when operators of the city’s two main dams made emergency releases at the height of the flood. The two dams were built only in the 1970s, but had lost much of their capacity to siltation.

Meanwhile, climate change, which is bringing more extreme floods to many places, and aging dams threaten to be a lethal combination. “Old dams were designed and built on the basis of hydrological records in a pre-climate change era,” says Smakhtin. “Now things are different, and this is worrying.”

What should be done? Some aging dams remain safe, but all will require much more rigorous inspection as they grow older, experts say, often followed by expensive repairs. Many more will need to be reengineered to cope with extreme river flows different from those envisaged when they were first built.

But the UNU report points to a growing legacy of dams that cease to serve much purpose — either because of siltation or because there are alternative sources of electricity — and are retained mostly because removing them is expensive and technically difficult. This is both a safety threat and a tragedy for river ecosystems that could be restored by their removal.

The U.S. is the world leader in decommissioning dams, with more than a thousand removed over the past 30 years. But even so, its dams removed to date have mostly been small, usually less than 16 feet in height. An exception was the 87-year-old Glines Canyon Dam in Olympic Park, Washington, removed in 2014. At 210 feet, the dam was the largest ever taken down. The task took two decades to plan and execute. But thousands more such removals are likely to be necessary to prevent an upsurge of dam disasters, says Smakhtin.

“Some dams are so big it is difficult to imagine how to approach the problem,” he says. “Look at the Kariba dam. It is absolutely huge, and by mid-century, it will be a hundred years old. Hopefully there will by then be technology to decommission it. But right now we don’t know how to do it.”