Romania's National Institute of Hydrology says the average river flow where the Danube enters the country is 4,700 cubic metres per second in July, whereas on Wednesday it was down to less than a third of that, at 1,600cm per second. The institute forecast it would continue to fall to 1,500cm per second by Tuesday.

The Danube runs 2,857km (1,775 miles) through 19 countries.

The Upper Danube goes from Germany to the Slovak capital Bratislava, the middle basin runs from Bratislava to the border between Serbia and Romania and the Lower Danube extends through Romania and Bulgaria into the three main branches of the Danube Delta that flow into the Black Sea.

The low waters of the river have resulted in several unexpected discoveries. The remains of a sunken Hungarian cargo ship resurfaced in Croatia, while the hulks of a German river flotilla was seen in Serbia.

And a resident of the northeastern Bulgarian city of Ruse found bones on the Danube river bed, which experts determined were the remains of a mammoth.

They were able to recognise a lower jaw, two tusks, and possibly a mammoth rib.