Which of the world’s hundreds of thousands of ageing dams will be the next to burst?

The collapse of two dams in Libya, unleashing torrential floodwaters that left at least 3,000 people dead and over 4,200 still missing, was both predicted and preventable. And they won’t be the last big dams to collapse unless we remove and repair some of the aging and obsolete structures that are long past their expiration date.

Like many dams around the world, the Wadi Derna dams in Libya were built in the 1970s during the era of peak global dam construction, when 1,000 large dams were installed each year. Now most of these dams are reaching the end of their life spans.

Details are still emerging, but the Libya dam collapses appear to have been caused by poor maintenance, and by poor monitoring of reservoirs that were overwhelmed by a huge rainstorm. Critical warnings were issued last year about the dams’ deteriorated state and the repairs needed to avert such a scenario, yet no action was taken.

Similar disasters are waiting to happen around the world. The biggest danger is in India and China, where the 28,000 large dams built in the mid-20th century are now nearing obsolescence. Mullaperiyar Dam in Kerala, India, is over 100 years old, visibly damaged and located in a region prone to earthquakes. Its collapse would harm 3.5 million people downstream.

In the United States, the second most prolific dam-builder after China, the average age of dams is 65 years old and an estimated 2,200 structures are at high risk of collapse. The recent infrastructure law directs $3 billion into maintaining some of them, but there are still thousands of dams the federal government is not responsible for, and it will cost an estimated $76 billion to fix them.

The risks from ageing dams are of particular concern in the face of climate change. Dams are designed to withstand worst-case conditions as they can be imagined at the time of construction. But what were once considered once-in-a-century weather events have begun occurring with increasing regularity, putting dams at great risk of either failure or a significant weakening of their integrity.

Before the disaster in Libya, extreme weather worsened by climate change was already taking its toll on these structures. Heavy rainfall damaged California’s ageing Oroville Dam in 2017, prompting mass evacuations amid fears of major uncontrolled releases. A piece of a Himalayan glacier destroyed one dam and damaged another in northern India in 2021, killing dozens of people. Glaciers melting rapidly as a result of warming are now a major safety hazard to dams and communities living downstream.

The default approach has been to repair ageing dams where needed, monitor reservoir levels and try to anticipate rainfall and increased flows from upstream. Take the Kariba Dam on the Zambezi River in southern Africa, which is undergoing extensive repairs to prevent its collapse after the riverbed below it was found to be severely weakened. At a cost of $300 million, these repairs are required simply to keep the dam standing on a river where hydroelectricity production has already plummeted from drought. Projects like these essentially paper over the cracks, and are often significantly more expensive in the long run than removing dams that are already obsolete.

While some ageing dams still supply drinking water and help farmers irrigate their fields, many that were built for hydropower only generate a fraction of the electricity they once did as sediment accumulates behind their walls. Deepening drought linked to climate change has also crippled hydropower generation all over the world, leading to energy rationing and blackouts in the United States, China and Brazil.

The fact that it’s increasingly difficult to justify many dams’ existence is one reason there is a growing movement, often led by Indigenous peoples and other marginalised populations, to remove them. Most notably, the removal of four dams on the Klamath River along the Oregon-California border, set to be completed next year, will be the largest such effort in history.