What’s happening? The world will lose about a quarter of its original dam storage capacity by mid-century due to sediment build-up, a new study by the UNU Institute for Water, Environment and Health has found. Researchers analysed more than 47,000 of the world’s 60,000 large dams in 150 countries. (Reuters)

Why does this matter? Large dams and reservoirs play an important role in the provision of drinking and irrigation water, hydroelectricity production and flood control. Thus, storage losses will impact many parts of the economy, from farming to power production and water supply.

What did the study say? The study, conducted by the UN’s water think tank, analysed data from 47,403 dams for which information on initial storage capacity and date of construction was available. It found that the world’s dams have already lost between 13-19% of their capacity due to trapped sediment. By 2050, countries will lose an average of 23-28% – a capacity equivalent to the combined annual water consumption of China, India, Indonesia, France and Canada.

The Asia-Pacific region is the most heavily dammed part of the world, with China containing more than 24,000 dams. Around 23% of the region’s initial storage capacity will be lost by mid-century. Meanwhile, the Americas and Europe are forecasted to see average losses of 28%, while Africa’s capacity reduction will reach 24%.

What is sedimentation? Sediment accumulation is a common yet underestimated problem. In naturally flowing rivers, sediment is washed downstream to wetlands and coasts. Dams interrupt this flow, leading to silt build-up in reservoirs, which reduces their capacity and damages hydroelectric turbines and other components. Disrupted sediment flows also harm downstream communities and habitats as well as upstream regions by making them more prone to flooding.

What can be done? To address the issue, the UN researchers stressed the importance of long-term sediment management strategies. While dredging is expensive and only a temporary solution, sediment flushing is cheaper but comes with substantial impacts on downstream areas. Another common method is bypassing, by which downstream flows are diverted through a separate channel, reducing sedimentation by up to 80-90%.

There are other risks – Last year, the UN also warned that many mega-dams are approaching the end of their lifetime and require increasing maintenance to avoid dam failures, overtopping and leaking. Climate change-induced extreme weather events are further exacerbating the problem. More intense and frequent flooding can accelerate a dam’s ageing process and raise the risk of collapse. Moreover, intense rainfall contributes to upstream erosion and increases sedimentation.

At the same time as dams are facing substantial capacity losses, they are also coming under greater scrutiny for their environmental and social impacts. Interrupting the natural flow of rivers and nutrient-rich sediments reduces plant growth, diminishes food sources for aquatic animals and leads to habitat fragmentation. Research has found that the Itaipu dam along the border of Paraguay and Brazil, for instance, has caused a 70% reduction in biodiversity. The subsequent erosion of riverbeds and reduced fertility of land has knock-on effects for farmers and local communities, as the example of the Mekong river shows.

As a result, some regions have decided to turn to other forms of power production and return rivers to their natural state. US regulators, for example, have recently approved the removal of four dams on California’s second-largest river to restore important salmon habitat.