The civil protection department said 23 people were rescued, including a 10-year-old boy, after surviving the flash rush of water in the narrow gorge in Italy‘s Calabria region, near Civita.
Six people have been taken to hospital.
Officials said at least five people ar missing but the total could be more because not all had entered the gorge with official guides and registered.
Rescue squads worked through the night with spotlights, and helicopters took off a first light to survey the area.
The nationalities of the dead and injured are not immediately known, but most tourists and trekkers who visit the area, in the country’s deep south, are Italian.
Six of the victims were women and five were men.
In some places the Raganello creek, part of the Pollino National Park, is at the bottom of a narrow, one-kilometre(0.8-mile)-deep gorge in the mountain.
Rescue teams used ropes to descend the sides of the mountain to reach the site.
Carlo Tansi, head of the civil protection department in Calabria, said: “The Raganello Gorge is narrow and tall, it can get up to 1km (3,200ft) high.
“The gorge filled up with water in a really short time.”
Mr Tansi said the gorge was only about four metres (13 ft) wide in some places, increasing the speed of the water and making the rescue more difficult.
Eugenio Facciolla, the chief prosecutor of the provincial capital, Cosenza, said: “It is really difficult terrain, filled with obstacles because of the geological formation of the area.”
He said rescuers working under spotlights were trying to locate areas where some people may have survived by ending up on small patches of shore or tiny islands in the creek.
Helmeted mountain rescue squads rushed from the nearest town of Civita to reach the gorge, a popular tourist attraction in summer.
The most seriously injured were taken by helicopter to hospitals in Cosenza and Castrovillari.
It comes after a motorway bridge collapsed in the northern Italian port city of Genoa, killing 43 people.