Residents in a US state capital have been told to evacuate as a huge basin of rainwater and snowmelt behind a glacier starts to leak, bringing warnings of a record flood surge.

On Tuesday, officials in the Alaskan capital of Juneau confirmed water had started escaping the ice dam on the Mendenhall Glacier.

Flooding is expected by Thursday (AEST), with about 1000 people in the city’s flood zone advised to leave.

“There’s no need to rush out of neighbourhoods, but we encourage folks to take a moment, take a deep breath, remember that they have a plan,” Juneau emergency manager Ryan O’Shaughnessy said at a media briefing interrupted regularly by the sound of emergency alert tones.

“This is something that we’ve been planning for as a community: Grab their go-bag, let their family and friends know where they’re going, and then execute that plan.”

The glacier is about 19 kilometres from Juneau and is a popular tourist attraction due to its proximity to Alaska’s capital city and easy access on walking trails. It acts as a dam for Suicide Basin, which fills each spring and summer with rainwater and snowmelt.

Homes on Juneau’s outskirts are within kilometres of Mendenhall Lake, which sits below the glacier. Many front the Mendenhall River.

The water being released in the glacial outburst is flowing into the river, putting homes that are closest to the river at risk. The US National Weather Service said it expected flooding to peak on Thursday morning.

“This will be a new record, based on all of the information that we have,” NWS meteorologist Nicole Ferrin said.

Flooding from the basin has become an annual concern, and in recent years has swept away houses and swamped hundreds of homes.

This year, US government agencies installed temporary barriers to try to protect several hundred homes in the inundation area from widespread damage. A temporary levee covers about four kilometres of the riverbank.

This week, Capital City Fire/Rescue Assistant Chief Sam Russell warned people not to climb onto barriers or watch the rising river from nearby bridges.

“As the flood goes up, our ability to navigate the water goes down due to the debris that flows down through,” he said.

“It makes navigating the water with a boat very, very difficult.”

Parts of the Alaskan capital have faced the threat of so-called glacier outburst flooding since 2011. Last year, about 300 houses were damaged.

In some years, there has been limited flooding of streets or properties near the lake or river. But this year’s flood is predicted to peak at up to 5.12 metres.

A large outburst can release some 56 billion litres of water, according to the University of Alaska Southeast and Alaska Climate Adaptation Science Centre. That’s the equivalent of nearly 23,000 Olympic-size swimming pools.

The Mendenhall Glacier is shrinking. But in the mean time, it acts as an ice dam – as risk that researchers say could last another 60 years.

-with AAP