Posted by: magmatist | June 14, 2013

No increased hazard foreseen from second landslide into Middle Fork Nooksack

Google Earth image showing points of interest in upper MF Nooksack. Click to enlarge any image.

Google Earth image showing points of interest in upper MF Nooksack. New pond is probably much smaller than shown in this estimate. Click to enlarge any image.

A second landslide has been discovered in the Middle Fork Nooksack. This one is smaller than the one that occurred on May 31 (see preceeding posts if you aren’t familiar with this issue). The smaller landslide is located about 2 km (1.2 miles) upstream of the Ridley Creek trail ford. The landslide entered the river bed and has partially blocked the river to form a pond a couple hundred meters long (that’s an educated guess). Google Earth images clearly show that this area has been sloughing and slumping for many years, and landslides may even have blocked the river in the past. The river is cutting down through the margin of the landslide deposit so the river is not blocked. We don’t know when this smaller slide happened. It shows in an aerial photo taken by Steph Abegg on the afternoon of June 6. On Monday June 10 Mike Savatgy of Deming made an arduous traverse through the woods above the trimline from the debris flows down in the river bed and provided photos of the small pond. It is possible that the May 31 debris flow undercut the sidehill and triggered the smaller landslide, but we can only speculate on that.

Mike Savatgy's photo of the pond, taken June 10 from above the landslide scarp. Brush on deposit surface appears to have been moved downward intact.

Mike Savatgy’s photo of the pond, taken June 10 from above the landslide scarp. Brush on deposit surface appears to have been moved downward intact.

Steph Abegg's June 6 photo taken from John Scurlock's ariplane shows the muddy little pond in the river. Used with permission.

Steph Abegg’s June 6 photo taken from John Scurlock’s airplane shows the muddy little pond in the river. Used with permission.

This pond probably does not present any greater downstream hazard than the earlierĀ  debris flows did, and none to the closest residents, who are more than 15 miles downstream. The water level is lower than the trimline from the May 31 flow, so any flood would be confined to a lower level in the river. Also, the pond lies at a lower elevation than the May 31 landslide/debris source (~3300 ‘ vs. 4650’), so has lower potential energy than that event. According to consultations with USGS hazards scientists, failure of the landslide dam would release a volume of water and cause a localized flood or debris flow the next time the river rises, which would endanger hikers at the Ridley Creek or Elbow Lake trail crossings. But the level of water in the pond is at or below the level of the 5/31 debris flow. So, the pond is unlikely to spawn a catastrophic debris flow or flood that would travel many miles downstream.

An on-site examination of the smaller landslide and the pond is planned for Saturday, June 15. Stay tuned for more information.

Click here to visit Mike Savatgy’s Picasa album for more photos of his traverse up the right bank of the Middle Fork to the pond.

Looking down at the landslide deposit in the river. Courtesy Mike Savatgy.

Looking down at the landslide deposit in the river. Courtesy Mike Savatgy.

Signs similar to this will be posted at Elbow Lake and Ridley Creek trailheads by the USFS.

Signs similar to this will be posted at Elbow Lake and Ridley Creek trail heads by the USFS.

The US Forest Service will continue to warn hikers around Ridley Creek Crossing and Elbow Lake Trail about the increased threat of debris flows. Warning signs are being made, and will be showing up at the trail heads soon. Debris flows are ALWAYS a hazard in the upper Middle Fork, either from landslides, heavy rain, landslides, glacial outburst floods, or volcanic events, so travel in the river bed should NEVER be taken likely. Residents along the Middle Fork and downstream in the Deming area should always be aware of the inherent risks of living along a river (floods happens when it rains a lot) and from living downstream of a volcano (eruptions and large lahars are always a possibility).


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

%d bloggers like this: