According to UN researchers, many of the world's large dams are so clogged with sediment that they could lose more than 25% of their storage capacity by 2050, posing a threat to water security.
A new study from the UN University's Institute for Water, Environment, and Health estimated that by mid-century, dams and reservoirs will lose about 1.65tr cubic metres of water storage capacity to sediment.
The number is very similar to the combined yearly water use of China, France, Canada, Indonesia, and India.
"Global water storage is going to diminish - it is diminishing now - and that needs to be seriously taken into account," the study's co-author and institute director, Vladimir Smakhtin, said.
Researchers examined roughly 50,000 major dams in 150 countries and discovered that they have already lost approximately 16% of their water storage capacity.
They calculated that if current accumulation rates continue, that figure will rise to almost 26% by mid-century.
This is significant, according to the researchers, because large dams are a major source of hydroelectricity, flood control, irrigation, and drinking water around the world.
Rivers naturally wash sediment downstream to marshes and coasts, but dams impede this flow, and the accumulation of these muddy deposits steadily reduces the room for water over time.
According to Smakhtin, this "endangers the sustainability of future water supplies for many" and poses concerns about irrigation and power generation.
Sediment buildup can affect coastal communities and wildlife habitats downstream in addition to causing flooding upstream.
Sedimentation is a small component of a bigger problem: by 2050, tens of thousands of huge dams will have reached or exceeded their design lives.
The majority of the 60,000 large dams in the world, built between 1930 and 1970, were intended to endure 50 to 100 years. After that time, however, they run the risk of failing, which would harm more than half of the world's population who live downstream.
The authors of the article suggest a number of ways to solve the impending problems of aging dams and reservoir sedimentation.
Water flow downstream can be diverted through a separate river channel via bypass or sediment diversion. Another method is to remove, or "decommission," a dam in order to restore the natural flow of silt in a river.
However, solving water storage issues is extremely difficult because there is no one-size-fits-all solution, according to Smakhtin.
"The loss of water storage is inevitable for different reasons," Smakhtin said. "So the question we should be asking is what are the alternatives?"
Longtime critics have cautioned that the benefits of large dams are greatly outweighed by the long-term social and environmental costs.
