Visitors evacuate as unprecedented flooding tears through northern half of America's oldest national park, washing out bridges and roads, officials say.

No one has been reported killed or injured in the flooding.
No one has been reported killed or injured in the flooding. (AP)

Over 10,000 visitors — all but one group of backpackers — have evacuated the Yellowstone National Park, officials said as they evaluated the damage from massive flooding in the popular US park.

Superintendent Cam Sholly said on Tuesday the visitors were asked to leave after roads and bridges washed out and power was knocked out from heavy rains and snow melt.

The flooding hit historic levels in the Yellowstone River, where it washed out several sections of the main highway from the park’s north entrance.

The torrent undercut the river bank and toppled a house where the families of six park employees had dived into the raging waters. The building, which had been evacuated, floated eight kilometres downstream before sinking.

Sholly said one group of campers was still in the backcountry. They had been contacted and crews were prepared to evacuate them by helicopter, but that hasn’t been needed yet, he said.

Sholly added he didn't believe the park had ever shut down from flooding. He said the north entrance is expected to be closed all summer.

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'Scariest river ever'

The flooding across parts of southern Montana and northern Wyoming from days of rain and a rapidly melting snowpack indefinitely closed one of the nation's most iconic parks just as a summer tourist season that draws millions of visitors was ramping up.

Instead of marveling at the site of grizzlies and bison, burbling thermal pools, and the regular blast of Old Faithful's geyser, tourists found themselves witnessing nature at its most unpredictable as the Yellowstone River crested in a chocolate brown torrent that washed away anything in its path.

"It is just the scariest river ever," Kate Gomez of Santa Fe, New Mexico, said on Tuesday. "Anything that falls into that river is gone. The swells are huge, and it's just mud and silt."

While no one has been reported killed or injured, waters were only starting to recede on Tuesday and the full extent of the destruction wasn't yet known.

While the flooding can't directly be attributed to the climate crisis, it came as the Midwest and East Coast sizzle from a heat wave and other parts of the West burn from an early wildfire season amid a persistent drought that has increased the frequency and intensity of fires that are having broader impacts. Smoke from a fire in the mountains of Flagstaff, Arizona, could be seen in Colorado.

READ MORE: US Southwest region hit by widespread wildfires

Northern part badly affected 

Officials in Yellowstone and in several southern Montana counties were assessing damage from the storms, which also triggered mudslides and rockslides. Montana Governor Greg Gianforte declared a statewide disaster.

Some of the worst damage happened in the northern part of the park and Yellowstone’s gateway communities in southern Montana. 

On Monday, Yellowstone officials evacuated the northern part of the park, where roads may remain impassable for a substantial length of time, park Superintendent Cam Sholly said in a statement.

But the flooding affected the rest of the park, too, with park officials warning of yet higher flooding and potential problems with water supplies and wastewater systems at developed areas.

Source: AP