Hydrobiologist Adriano Bordalo e Sá, a professor at the Abel Salazar Institute of Biomedical Sciences (ICBAS), points out that “despite all this tragedy, there is a positive aspect”: the release of natural sediments, which act as fertilisers and provide essential nutrients for rivers, farmland, and the sea, these releases carry more than just water—they also transport “sand and sediments that act as natural fertilisers.” When the water “floods agricultural fields” and is later absorbed, it leaves “a very fine layer of mud on the surface” that improves soil fertility.

According to CNN Portugal, several dams in Portugal were close to full capacity, prompting controlled water releases. The Alqueva Dam was a standout, opening its gates for the first time since 2013. Over just two days, about 500 million cubic metres of water were released, carrying along sand and fine sediments that had built up over the years.

“The sediments are light and very fine, and they settle on the fields, serving as a natural fertiliser,” he explains, highlighting that despite the damage to infrastructure, farmers “may spend less on fertilisers this year,” particularly in areas such as the rice fields of Baixo Mondego and the tomato and corn crops in Ribatejo.

The impact extends to the sea as well. Bordalo e Sá points out that “the ocean desperately needs this fresh water,” pushing back against the idea that it’s wasted. He explains that the water released from the dams “is not lost at sea.” As it carries sediments, the water “turns brown” and ultimately enriches the marine environment, supplying phosphorus and nitrogen to phytoplankton, the foundation of the marine food chain. These microscopic algae feed many species, including sardines, meaning that “based on studies of previous flood events, we can expect fat, healthy sardines for the upcoming São João and Santo António festivals.”

The professor also notes that dams—built mainly for hydroelectric power—disrupt the natural transport of sediments and the migration of species, contributing to “a loss of biodiversity.” For example, before the 1950s, the Douro River carried approximately 2 million cubic metres of sediment to the sea each year, a figure that today is only about 200,000 cubic metres. Replenishing these materials is crucial for maintaining beaches and supporting the biological productivity of the ocean, underscoring the importance of these natural water-release events.