SEATTLE — A massive landslide of dirt, trees and rocks in rural Washington killed three people on Saturday, injured at least eight others and destroyed six houses, authorities said.
The slide — at least 135 feet wide and 180 feet deep — hit just before 11 a.m., Snohomish County authorities said.
The landslide completely blocked State Route 530 near the town of Oso, about 55 miles north of Seattle. Also blocked was the North Fork of the Stillaguamish River, and there were concerns about possible downstream flooding affecting more homes.
Residents both upstream and downstream from the slide were being asked to prepare to leave at a moment’s notice, Snohomish County spokeswoman Bronlea Mishler said.
‘‘We are not issuing an evacuation order,’’ she said. ‘‘However, we need residents living along the river to be prepared. Conditions are changing very rapidly.’’
The Snohomish County Sheriff’s Department initially reported that two people had been killed. Authorities said later that one of the people who was rescued died at a hospital.
The injured included a 6-month-old infant, Snohomish County spokeswoman Shari Ireton said. People were transported to a nearby hospital, as well as medical facilities to the north in Skagit County and to the south in Seattle.
Five of the injured were brought to Cascade Valley Hospital in Arlington, but one has already been treated and released, said hospital spokeswoman Jennifer Egger.
The facility was expecting more injured people, but Egger said she didn’t know how many and couldn’t comment on the condition of those already at the hospital because they were still being evaluated.
‘‘We’re on standby waiting to see what happens next,’’ Egger said.
The American Red Cross has set up at the hospital and is seeking donations of food, water, blankets and clothing, Egger said. An evacuation shelter has been set up at Post Middle School in Arlington.
Search-and-rescue help came from around the region, plus the Washington State Patrol, and the Army Corps of Engineers.
Authorities believe the slide was caused by ground water saturation from recent heavy rainfall.
The National Weather Service has issued a flash flood watch for Snohomish County through Sunday afternoon.
If the blockage in the river gives way, flooding downstream toward the larger community of Arlington was possible, forecasters said.
Spokesman Bart Treece of the Washington State Department of Transportation said he didn’t know how long the two-lane rural road will be closed. Drivers were advised to find another way to get between Darrington and Arlington, he said.
‘‘We’re standing by ready to help out where we can,’’ Treece said.