Several massive avalanches have damaged hydroelectric power lines near Juneau, Alaska, knocking out 85 per cent of the city's electricity and likely causing utility prices to soar.

The series of avalanches came early Wednesday, wiping out transmission lines and support towers along the 45-kilometre power line extending from the Snettisham hydroelectric dam to Juneau.  

The damage is extensive and the danger of more avalanches is still high, said Gail Wood, spokeswoman for Alaska Electric Light and Power.

She said it may be three weeks before crews can get to the downed lines and it will probably take at least two months before the lines can be repaired. 

Wood added power rates in Juneau could triple because the city will have to rely on diesel generators. 

She estimated Juneau could burn 400,000 litres of diesel fuel a day to make up for the loss of the hydro power.

One in five households in Juneau is heated with electricity, city officials said.

The financial impact on public, private and commercial properties will total about $25 million US, according to the city, which issued an emergency declaration asking for state assistance. It included a request to seek federal help.

With files from the Associated Press