A surge of floodwater carrying rocks, tree limbs and mud swept through the main street of Bannack, washing away one historic building, damaging others and causing the abrupt evacuation of one of southwestern Montana’s top tourist attractions.

The flash flood arrived suddenly Wednesday evening while about 20 people were touring Montana’s first territorial capital, which now is a preserved ghost town and a state park featuring attractions from Montana’s gold-rush past.

“I’ve seen flash floods before, but never to this extent,” Bannack assistant manager Tom Lowe said Thursday.

The visitors were safely evacuated, with one person suffering a knee laceration that did not require medical attention, Montana State Parks spokeswoman Jennifer Lawson said.

The park has been closed and this weekend’s Bannack Days canceled. The annual celebration draws about 5,000 people and is one of the park system’s largest events.

Gold was discovered in Bannack in 1862, making it the first major strike in Montana. In 1864, it became the Montana territory’s first capital but soon lost the designation to Virginia City, which was having its own gold rush.

The old Assay Office, which had been used as a general store, was destroyed in the flooding. Most of the boardwalk along the main street was torn out, and more than 80 percent of the buildings were damaged from hail, mud and water, Lowe said.

Park officials were still assessing the extent of the damage and Lowe estimated the park will be closed for at least two weeks.

Shortly before the flood, the Pioneer Mountains to the north of Bannack received a sudden downpour of rain from storm cells passing through southwestern Montana Wednesday afternoon, said National Weather Service forecaster Jim Brusda.

That caused the sudden surge of water to cascade into town, where three-quarters of an inch of rain fell in a half-hour, Brusda said.

The same storm system caused a mud slide that shut down one of the main roads into Yellowstone National Park, dumped 1 1/4-inch hailstones east of Helena and caused minor flooding and mud runoff north of Helena where a fire burned last year.

U.S. 89 eight miles north of Gardiner was buried in mud and rock, up to 10 feet in some places. The highway from Livingston leads to Yellowstone’s northern entrance and is typically busy with park visitors this time of year.

Highway workers scrambled to remove the debris, and the highway was reopened to traffic at 11:30 p.m. Wednesday, Department of Transportation spokeswoman Lori Ryan said.

The cleanup is expected to last another two days, with traffic delays expected, before workers can examine the roadbed for damage, she said.

The brief, intense storm likely did not do much to help drought conditions in southwestern Montana, which is the driest part of the state. It happened too fast to affect the soil conditions and it packed lightning that may have left new fires smoldering, Brusda said. 

Conditions in the area are expected to be hot with low humidity over the next few days, ideal conditions for wildfires, he said. 

But the wind is expected to be light, which may help firefighters extinguish new starts before they spread, he said.

In the Flathead Valley, people were assessing damage from a violent hail- and rainstorm that blew through the area Wednesday afternoon.