Snohomish County is encouraging residents who live in the floodplain downstream of milepost 41 to the city limits of Arlington to “find another place to stay tonight.”
Saturday’s mudslide has blocked the north fork of the Stillaguamish River near Oso, leading to concerns about flooding upstream of the landslide and flash flooding downstream, according to the National Weather Service (NWS).
A flood warning is in effect for the north fork area and a flood watch has been issued for Snohomish County. However, the NWS said “catastrophic flash flooding” is unlikely.
Hydrologists estimate the water is rising 10-12 inches per 30 minutes upstream of the landslide dam, Snohomish County said.
Even if all the water backed up behind the landslide were to rapidly drain, the river at Arlington would not come close to flood stage. The area around the slide could see flooding.
A river gauge immediately downstream of the landslide showed the river level drop from 3.1 feet to .9 feet an hour after the landslide occurred, confirming it has blocked the river.
However, during a news briefing Saturday night, Snohomish County Executive John Lovick said officials were still recommending evacuation for those below the mudslide.