By Alasdair Pal and Neha Arora

TAPOVAN, India (Reuters) – Authorities in India warned on Thursday of rising water levels in a Himalayan river valley hit by a major avalanche as they scaled back a search for 35 construction workers trapped in a flooded tunnel.

Rescue workers have found the bodies of 36 people since Sunday’s avalanche that tore through dams and swept away bridges in the Dhauliganga river valley of Uttarakhand state.

Some 171 people remain unaccounted for, most of them workers at the state-run Tapovan Vishnugad hydroelectric project and at the smaller Rishiganga dam, which was swept away by the avalanche-driven torrent.

An official with the National Disaster Response Force (NDRF) said the number of rescue teams were limited at the tunnel site after river water levels began to surge.

“There was an input from a village that the river upstream was swelling so we sounded an alert. The rescue mission was stopped for about 30 minutes,” Swati Bhadoriya, Chamoli District Magistrate, told Reuters.

Relief workers have been drilling inside a 2.5-km-long tunnel connected to the Tapovan project, where slush and water has been so heavy that soldiers have made only halting progress in four days.

Experts have cautioned there could be still be huge amounts of rock, debris, ice and water that could get released due to changes in temperatures.

“Snow melt or rain could trigger a debris flow at any moment, probably not of the size of the event on Sunday, but critical for anybody and anything close to the river,” said Holger Frey, a senior scientist with the Glaciology and Geomorphodynamics Group (3G) in the geography faculty at the University of Zurich.

RESCUE EFFORTS, DISTRAUGHT FAMILIES

After clearing more than 100 meters of mud, rocks and debris, relief workers on Thursday sent water tankers and generators deep into the tunnel to assist in drilling.

They were searching for signs of life in smaller tunnels and rooms branching off from the main passage, officials said.

Relatives continued to arrive at the site, but five days after the disaster, frustration at the lack of progress mounted.

“They are not telling us anything,” said Praveen Saini, whose nephew, Ajay Kumar Saini, is trapped in the tunnel.

Another man was clinging to hope that his brother had survived after he was able to ring his mobile phone. “If his phone survived, maybe he survived,” Jugal Kishore said.

Originally thought to be a glacier breaking apart in India’s second highest mountain Nanda Devi and crashing into the river, some scientists now say the flood was more likely to have been caused by an avalanche.

“It appears that the event was caused by a very large rockfall from high up the mountainside which picked up lots of snow and ice on the way down and melted these because of the frictional heat created by the rock fall,” said Stephan Harrison, professor of Climate and Environmental Change at the University of Exeter in Britain.

(Additional reporting by Saurabh Sharma and Neha Arora; Writing by Neha Arora; Editing by Sanjeev Miglani and Mark Heinrich)