"Fast-moving, severe floods have required the evacuation of residences and shut down critical roads in northeastern Oregon," Xinhua news agency quoted Governor Kate Brown as saying on Friday.
The state of emergency will remain in effect for 30 days in the three counties of Umatilla, Union and Wallowa in northeastern Oregon.
Local newspaper The Oregonian said Friday that Pendleton, the county seat of Umatilla county, was one of the hardest hit by the flooding of the Umatilla River and its tributaries, which have caused widespread damage.
Westbound traffic on an about 320 km-long stretch of Interstate 84 Highway was closed between Hermiston and Ontario due to the floods.
An estimated 45 residents, including all living at two mobile home parks in the riverside area, were evacuated, while about 200 structures were submerged or soaked in flood water, said the newspaper.
It quoted Roger Cloutier, a meteorologist for the National Weather Service in Pendleton, as saying that the flooding was the worst in history for the eastern Oregon city.
Governor Brown pledged to mobilize the Oregon National Guard, state police forces and other government agencies to help local authorities to combat and aid in the recovery from the flood.