An international study of the river’s sediment deposits also showed that the Danube has become up to 40 per cent narrower since the mid-19th century, Bavaria’s environmental office said.

The report found the changes were caused by interventions to straighten the river’s flow, as well as flood protection measures and dam construction.

These human interventions meant sediment could no longer flow through the Danube delta and out to the Black Sea. Instead, deposits built up on the banks of the river, permanently changing its shape.

At a length of 2,850 kilometres, the Danube is the second-longest river in Europe, after the Volta in Russia.