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Environmentalists say one of Europe's most important wetland areas is under threat as Spain and Catalonia argue about the future of the Ebro river.
Campaigners say Spanish government plans to restrict water flow could destroy the fragile landscape.
They are worried that ultimately these waters could be transferred to other, drier regions of Spain.
Tens of thousands of demonstrators took to the streets of Amposta in Catalonia on Sunday to protest against the plans.
Running for 930km (578 miles) the Ebro is the third longest river flowing into the Mediterranean.
Its journey begins in Cantabria on the north coast of Spain but it flows through nine autonomous communities before joining the sea in a Catalan delta that stretches over 32,000 hectares (79,000 acres).
The delta is a national park and it is recognised internationally as being a critical resource for birds and wildlife. In addition the rich waters help produce huge amounts of foodstuffs including rice, fruit and vegetables.
The Spanish government has been trying for decades to put in place a plan to regulate the river and put the rights to use the waters on a firm legal footing. In January it finally agreed on what is termed the Ebro River Basin Management Plan.
But the approach has drawn huge criticism from local campaigners who believe they have prioritised the interests of farmers and developers over the environment.
"Under EU law you have to set the minimum environmental flow to conserve the river and the delta and then you can start to talk about any excess water and what to do with it," said Brian Cutts, who lives in the region and is a long time member of PDE (Plataforma en Defensa de l'Ebre).
Read complete article: http://www.bbc.com/news/science-environment-35502084