Over half of the planet’s freshwater reservoirs will be “functionally dead” by 2060 due to sediment build-up, a study has predicted.

Dams block silt, sand and gravel from flowing downstream, so over time this material accumulates in reservoirs, shrinking the space for water. The trapping of sediment can also compromise dam safety and have damaging impacts on downstream ecosystems.

Kai Liu at the Chinese Academy of Sciences, Nanjing, China, and his colleagues used satellite imagery, sedimentation data and machine learning to analyse the capacity of over 550,000 reservoirs globally.

They found that the amount of water being lost annually to sedimentation is more than 36 cubic kilometres – comparable to the volume of China’s massive Three Gorges Reservoir.

According to the researchers, a reservoir is considered “functionally dead” once it is over half full of sediment.

Australia and Spain are projected to be the worst-affected countries. Nearly 85 per cent of Australian and three-quarters of Spanish reservoirs are predicted to pass their functional lifespans by 2060.

In arid regions, nearly three-quarters of reservoirs may become functionally dead by 2060, compared with half of those in humid zones. In Namibia, over 99 per cent of dams are in danger, and along the Western Australian coast, the figure is nearly 96 per cent.

Liu and his colleagues estimate that each decade the world is losing over 7 per cent of its freshwater storage capacity, meaning that water supplies to over 2 billion people are threatened, along with more than a quarter of global irrigated land.

To address the problem, the team recommends strategies such as including upstream reforestation, land stabilisation and erosion control, which would reduce sediment flow into reservoirs. Engineering solutions such as dredging and bypass tunnels will also be required, but the cost of these measures has been estimated at up to $100 billion.

Ian Wright at Western Sydney University, Australia, who wasn’t part of the study, says there are some “very confronting” findings in the study, especially as climate change is predicted to increase the rates of sedimentation due to greater rainfall. The problem is further exacerbated, he says, because many of the world’s storages are required to supply water to a rising population.

“Sedimentation is like a cancer that is slowly reducing reservoirs’ capacity,” says Wright.

He also agrees that Australia is a sedimentation hotspot. “Our soils are very fragile, and the clearing of catchment vegetation exposes them to accelerated erosion – and thus provides an ongoing source of sediment into storage dams,” he says.