Posted by: magmatist | July 1, 2013

Repeat visit to the lower Middle Fork Nooksack landslide

Cross country to the lower landslide in Middle Fork Nooksack River on  June 29, 2013

By Dave Tucker

Google Earth image showing location of the June 6th landslide and small pond.

Google Earth image showing location of the June 6th landslide and small pond. Click any image to enlarge.

A repeat visit was made to the lower landslide in the Middle Fork Nooksack River (MFN). This landslide is believed to have occurred on June 6th, 2013 when turbidity spiked at the Nooksack Tribe’s monitoring site down river in the mid-morning. This landslide is believed to be the source of a second, smaller debris flow that reached nearly to the Ridley Creek Trail crossing of the MFN channel 1.9 km (1.2 mi) up river from the Ridley Creek trail crossing.The landslide partially blocked the river. A small muddy pond was visible in photos taken by Steph Abegg from John Scurlock’s airplane on the afternoon of June 6th. This landslide was initially investigated by geologists on June 15. Read the report of that trip here.

Chart showing spikes in Nooksack turbidity on May 31, June 1, and June 6. Courtesy Nooksack Tribal Fisheries.

Chart showing spikes in Nooksack turbidity on May 31, June 1, and June 6. Courtesy Nooksack Tribal Fisheries.

Trip Report

The trip was made by Dave Tucker, Michael Savatgy, and brothers Peter and Ben Scherrer. The landslide was approached by bushwhacking up the right bank of the river from the Ridley Creek trail access. Brush was only moderately god-awful; the trip is mostly in old growth but the slopes are generally steep. A harrowing ascent of a 75-foot-high, hard-packed slope of glacial till studded with big boulders was made using a hand line set up after the first person climbed the nasty slope. Right after, we reached an overview of the landslide deposit, just upstream from our perch on the edge of the forest.

Aerial view of the landslide into the Nooksack River at 3400'. Courtesy John Scurlock.

Aerial view of the landslide into the Nooksack River at 3400′. Courtesy John Scurlock.

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.

It was immediately apparent that major changes have occurred in the two weeks between visits. The small pond is even smaller, mostly filled by gravel bars and silt. The river has enlarged the channel it is cutting through the deposit’s toe, along river left (south side of the channel). A second channel is now being cut down the center of the landslide toe’s steep down-river slope; on June 15th, there was no channel there, though water could be seen springing out of the base of the deposit. No further landslides have come down the unstable 400-foot-high slope that was descended on June 15th.

Lower landslide on June 29th from a similar perspective. Click to enlarge. Gravel bars are visible in the upstream end of the pond. Note enlarged river channel and new channel in center of the landslide toe.

Lower landslide on June 29th from a similar perspective. Click to enlarge. Gravel bars are visible in the upstream end of the pond. Note enlarged river channel and new channel in center of the landslide toe.

Telephoto view showing Ben Scherrer climbing the nasty 75 foot till slope. Peter is in the tree at upper left, out of reach of falling rocks.

Telephoto view showing Ben Scherrer climbing the nasty 75 foot till slope. Peter is in the tree at upper left, out of reach of falling rocks.

Conclusions

Any fears of a small outburst flood and attendant debris flows from a lake breakout are reduced to very low levels of concern. There is less water in the ponded river. The river continues to entrench into the unconsolidated clay, boulders and rocks of the deposit. It is likely that within a year or so, there will be little trace of this landslide deposit. The slope that spawned this avalanche has clearly slid multiple times in the past, and the river has likewise cleared all traces of landslide toes in the channel.

Other observations

The lower waterfall, in argillite of the Nooksack Formation.

The lower waterfall, in argillite of the Nooksack Formation.

The river crashes over two waterfalls between the Ridley Creek trail crossing and the landslide slopes. The first of these is a narrow bedrock constriction less than a km up river from the access point. The second is another 200 m upstream and is a vertical fall of tens of meters. It is only visible after considerable effort in brush and on very steep slopes. The river channel in between is thoroughly scoured of brush by the debris flows of 2013. The wall of mud and boulders of the large May 31 debris flow pitched over the upper falls in the dark morning hours and must have been an imposing and awesome sight.

The upper waterfall. The May 31 debris flow filled the channel to at least 10 meters here.

The upper waterfall. The May 31 debris flow filled the channel to at least 5 meters depth here.

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Responses

  1. Thanks for the hard efforts to document what is happening and the risk levels.

  2. Really a rare set of observations – Excellent work and great photos. Thanks for taking the time to keep us updated.

  3. I have to echo the previous comments. I am glad to get this information without the sweat, tears (and possible blood) involved.

    • Bob,
      Well, there was a little blood, from crashing through brush, climbing a nsty steep slope, and smashing a skeeter or two.
      dt


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