By Greg Botelho and Janet DiGiacomo
(CNN) -- A devastating landslide Saturday killed three people, cut off a small town and a river and prompted an evacuation notice for fear of a potentially "catastrophic flood event," authorities said.
The Snohomish County Sheriff's Office said, in addition to those dead, seven adults and a 6-month-old child were rescued and sent to local hospitals.
At least six houses were destroyed, though the sheriff's office noted they are still assessing exactly how many structures have been affected.
The first reports of the landslide came in around 10:45 a.m. (1:45 p.m. ET) along State Road 530, the sheriff's office said.
Photos provided by the Washington State Patrol show floodwaters and sprawling debris covering a rural patch of that two-lane road, framed by woodlands and snow-capped mountains.
CNN first learned of the landslide via Twitter.
Groundwater saturation tied to heavy rainfall in the area over the past month was blamed for the landslide, which authorities say measured at least 45 yards wide.
Because it blocked SR 530, the landslide cut off Darrington, a town of about 1,350 people located 75 miles northeast of Seattle and within close proximity to Round Mountain, Whitehorse Mountain and White Chuck Mountain. Part of the Stillaguamish River also was blocked.
Residents got reverse 911 calls warning them of "flooding upstream from the slide, as well as the possibility of a downstream flooding should there be a catastrophic breach by the river," said Shari Ireton, a sheriff's spokeswoman.
The county later said "we strongly recommend" that those living in the north fork of the Stillaguamish River flood plain, from Oso to Stanwood, to "evacuate your home immediately."
"We are working on establishing shelters for those who have nowhere to go," county spokeswoman Rebecca Hover said in a statement. "Until then, people should get to higher ground as soon as possible.
"Nightfall is approaching, and we do not want to take any chances."
Gov. Jay Inslee expressed dismay later Saturday about the "tragedy in Oso," the remote community of about 180 people 15 miles west of Darrington.
"On behalf of all Washingtonians, my condolences to the families who lost loved ones in (the) mudslide today," Inslee tweeted.
A number of agencies have responded, including the state transportation and emergency management departments, the U.S. Navy and fire departments across Snohomish County.
Upon arriving at the scene, firefighters and state troopers heard calls for help, trooper Mark Francis said.
The Snohomish sheriff warned people to stay clear of trestles or bridges or anywhere near the Stillaguamish River downstream of the slide.
"Water could break through at any moment," the sheriff's office tweeted.
CNN's Michael Martinez and AnneClaire Stapleton contributed to this report.
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