Water Resources of the United States

PROJECT ALERT NOTICE (WA) Update: River in NW Wash. partially blocked by landslide

Date: Mon, 24 Mar 2014 22:15:50 EDT

Summary: Water has started to flow across the top of the natural dam that formed in the North Fork Stillaguamish River, Washington, as a result of a large landslide. A lake has formed behind the dam and downstream flows have returned to normal.

A large landslide occurred in northwest Washington on Saturday morning, March 22, 2014, that partially blocked flow in the North Fork Stillaguamish River. Flows were reduced by about 1,200 cubic feet per second for more than a day, while water accumulated behind the newly formed natural dam. By mid-afternoon Sunday, March 23, 2014, water had started to cut a channel across the top of the natural dam and downstream flows returned to normal.

A lake has formed behind the natural dam. Water continues to flow across the natural dam and is gradually eroding a larger channel. Under current conditions, the chance for a catastrophic dam break is considered unlikely. The newly developed channel is being monitored closely.

The USGS operates a streamgage (North Fork Stillaguamish River at Arlington, station no. 12167000) about 12 miles downstream from the landslide. For near-real-time gage data and to view the drop and rise in downstream water levels and discharge, see http://waterdata.usgs.gov/wa/nwis/uv/?site_no=12167000.

Fourteen people have died as a result of the landslide. Additional people remain missing.

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