Workers clean up the aftermath of the oil spill on the Yenisey river
Chemical pollution from Monday's explosion at Russia's largest hydro-electric power station has killed fish and spread down a major Siberian river.
Russian officials say booms are being deployed on the Yenisei river to trap the transformer oil. Absorbents are being thrown from helicopters.
In a stormy meeting with officials, relatives of the missing said they feared not being told the whole truth.
The blast killed as many as 76 people at the Sayano-Shushenskaya plant.
The search for dozens of missing workers is continuing.
The number of confirmed dead stands at 17.
The mayor of Abakan, Nikolay Bulakin, said drinking water was unaffected in the town of Abakan, because it was drawn from the Abakan river.
But he said said that environmental damage had already been done, and he had heard reports that many trout at fish farms had been poisoned.
At least 40 tonnes of transformer oil spilled into the Yenisei, which flows north to the Arctic.
In a tumultuous meeting with local officials, relatives of the missing men demanded to know what had gone wrong.
"They do not care what has happened to our men," shouted one, according to Reuters news agency. "We want to know the truth."
"The state and the owners just cared about profit," another relative said.
Others said they wanted to know if their relatives had died, but feared they were not being told.
The power plant, one of the world's biggest, lies some 3,000 km (1,875 miles) east of Moscow. It has now stopped working.
The dam is 245m (800ft) high and stretches 1km (0.6 miles) across the Yenisei river.
The explosion flooded a turbine hall and destroyed three generating units.
Divers are examining the debris in the hall and searching for bodies, in icy water.
Emergencies Minister Sergei Shoigu said it would take at least a week to assess damage in the flooded hall.
The blast's cause is unknown but reports said investigators believed a transformer exploded during repairs.
The plant's owner said the flooding had occurred due to a pressure surge in water pipes.
Wednesday is an official day of mourning in the remote mountain region of Khakassia where the plant is located.
RusHydro said the damage would run into "billions of roubles" and take several months to repair.
Mr Shoigu said repairing the turbine hall alone could cost 40bn roubles ($1.3bn; £762m).
But he said it would still be worth doing the repairs, because the dam - undamaged by the blast - had accounted for 80% of the construction cost.
Opened in 1978, the station provides a quarter of RusHydro output and is a major power supplier to at least two smelters owned by United Company RUSAL, the world's largest aluminium producer.