At least three holidaymakers drowned in the fast-flowing Kallar River during the Mahanavami holiday on Tuesday.
Police identified the deceased as Special Armed Police (SAP) Constable Firoz, 30, his brother, Javed, 35, and their nephew Sahwan, 16.
Kallar is a prime tourist spot at the foot of the Western Ghats ranges. It is about 60 km from the city of Thiruvananthapuram.
Police say the deceased were part of an eight-member family tour group from the Beemapally Coast.
The group was heading towards Ponmudi, a scenic and misty holiday destination at least 13km upriver from Kallar.
However, part of the uphill switchback road to the hill station had collapsed, forcing holidaymakers to turn around and picnic at Kallar. Ponmudi remained isolated.
Police believe the accident happened when visitors ignored warning signs and scaled a barricade to enter the stream to bathe. The tragedy is said to have unfolded when the current carried away a 20-year-old woman from the group.
Her relatives reportedly drowned as they battled the flowing waters to save her. Residents familiar with the river in Vattakayam carried six people ashore, but three died on the way to hospital. The other three were in a medically precarious condition.
The tourism department had posted warning signs following similar drowning deaths in Vattakayam. Over the years, several city vacationers have died in flash floods and creeks in the locality of Kallar-Ponmudi. Slippery rocks, rapids and plunge pools characterize the forested locality.