Don't panic, your beautiful alien-green river is totally safe, town mayor decrees

A mayor in Spain was forced to reassure locals everything was ok after the river that winds through their town turned into a frightening shade of fluorescent green.

Disarming and beautiful at the same time, the Valira River resembled a kind of thermonuclear catastrophe as it flowed down from Andorra and the Pyrennes Mountains into Spain.

The mayor of Seu d’Urgell, Albert Batalla, said the green dye had been deliberately added to the river, as investigators tried to pinpoint the source of gastroenteritis that had affected thousands of people.

Unlike the water-borne bug that had swept through Catalonia, the green water was "entirely harmless, non-toxic and biodegradable", Batalla told residents, according to The Local.

The investigation had zeroed in on the Arinsal water bottling plant.

It was believed contaminated water from the plant had been filled into office water coolers across the region, causing a mini epidemic.

Photos and videos of the electric green rapids cutting through ravines and under picturesque village bridges were shared heavily over social media.