Snohomish County authorities are urging people who live near the North Fork of the Stillaguamish River near Oso to evacuate the area before nightfall.
Spokeswoman Rebecca Hover says the deadly landslide that has blocked the river has authorities worried about the potential of a catastrophic flood between Oso and Stanwood.
The National Weather Service says that if the blockage caused by Saturday's landslide gives way, flash flooding downstream toward Arlington is possible.
Shelters are being set up for people who have nowhere to go.
Snohomish County authorities say the landslide killed three, injured at least eight others and destroyed six houses.
The weather service says roads, some houses and many fields could flood if flash flooding happens.
A flash flood watch is in effect through Sunday afternoon.