Posted by: magmatist | June 16, 2013

Middle Fork Nooksack: Visit to the lower landslide

Gogle 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.

The landslide that partially blocked the Middle Fork Nooksack was closely examined on Saturday, June 15th by Scott Linneman (WWU Geology), Dave Tucker (MBVRC), Keith Kemplin, and Mike Savatgy. Mike took the original ground-based photos when he approached the site on June 10. It is located 1.9 km (1.2 mi) up river from the Ridley Creek trail crossing. The approach involves a nasty descent on a landslide-prone slope. For safety, do not attempt to repeat this trip. We were wary of this slope and did not tarry long.

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.

Two videos are published on YouTube:

At the top of the slope, still in the woods. The river bottom is 400 feet below. Click to enlarge any image.

At the top of the slope, 400 feet above the river. Click to enlarge any image.

http://youtu.be/duW7coTHNPE and                                      http://youtu.be/_YRTLHK0n60

This living tree has been split vertically by a fracture 20 feet from the top of the unstable slope.

This living tree at the top of the slope has been split from the bottom up by a fracture in the ground.

The obviously very fresh landslide is larger than we had expected. The downstream face is around 25-30 meters high. A small muddy lake, 80-100 m long, and probably shallow (5 m?) is impounded against the upriver margin of the landslide deposit. The landslide toe rests against the opposite (south) valley wall. The river is cutting a narrow and steep channel through the debris along this south wall. A subequal volume of water is passing under the landslide toe and rejoining the river downstream. The source of the landslide is part of a very unstable south-facing slope on the north side of the valley. This slope has clearly slid in the past. There is a vertical scarp at the top of the slope, 120 m (400 feet) above the river, where active slip is splitting living trees in half lengthwise and left fractures in the forest floor. Below this active scarp the slope is very steep, at the angle of repose, and consists of talus, scree and a few small trees. The scree slope is cut by numerous fractures running across it, indicating frequent movement.

The landslide is deposit is at right; trees were carried with it. It ran down the slope at center.

The landslide deposit lies in the river at right; trees were carried down, too. The slide ran down the rubble-covered slope at center. Note water issuing from the top of an impermeable layer part way up the slip face.

The head scarp of the fresh landslide is about 60 m (200 feet) above the river. Immediately above the scarp are exposures of andesite lava that are about 500,000 years old from the Black Buttes volcano. The slope below this scarp is covered in mud and fractured rock. A layer of dark clay is exposed near the base of the slope. Water pours out of the slope exposed by the landslide above this clay layer.

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.

The toe of the landslide. Scott Linneman is circled for scale. Water gushes out of the debris just to his left and  beyond. Large block at upper left is 15-20' high.

The toe of the landslide. Scott Linneman is circled for scale. Water gushes out of the debris just to his left and beyond. Large block at upper left is 15-20′ high.

We speculate that this landslide occurred on June 6th and was the source for the small debris flow recorded on turbidity meters at Nugent’s Corner on that day. The mass of mud and boulders ground to a halt rest just above the Ridley Creek trail crossing but sent turbid water down the river all the way to the monitoring equipment at Nugent’s Corner (about 25 miles by river). The slope may have been undercut just above the river level by the passage of the large May 31st debris flow spawned by the much larger landslide near the toe of the Deming Glacier.

The lake appears no larger than it was last week. Two deltas are building into it: one from the river on the east, and another from the north, built by the stream springing out of the landslide scarp. There is lots of sediment eroding from the larger May 31 landslide and resultant debris flow up the valley, and the pond could conceivably fill with mud quickly. The landslide in the river valley appears relatively stable. More landslides can be expected at this site, and these would almost certainly generate debris flows that would sweep down the river.

Boulder-choked debris flows and

The landslide scarp.

The landslide scarp.

landslides are a fact of life in this active valley. The USFS has posted signs at the Elbow Lake and Ridley Creek trail heads warning of sudden debris flows, and these should be heeded. If you enter the river bed, always keep an ear cocked for an approaching rumbling roar. Do not tarry in the stream bed. If you hear that sound, a debris flow is approaching RIGHT NOW and you’d better get out of the channel and into the forest FAST!  Many of  your potential escape routes cannot be done in a sprint due to the very unstable nature of the material, or due to luxurious North Cascades vegetation.  You’ll have very little time to reach safety; the alternative is grim.


Responses

  1. Thank you for these excellent videos and explanations about what is happening at the slide locations.

    Judith Sult

  2. I can sure understand why you four chose the more arduous route in there, rather than worry your way up the river bottom. And besides, you would have had to climb out and around that 100 ft. or so waterfall, “Tucker’s Tumble’, anyway. A sterling job!

    Bob Mooers

  3. Having hiked up this valley wall to reach the glacier in 2002, and deciding it was too steep to be safe or stable, your observations make complete sense.

  4. Debris flows below Boulder Glacier are quite common as well, though smaller as there are not the steep valley walls to supply material.

  5. Reblogged this on PalaeoMud and commented:
    Nice info and video of the Middle Fork Noosack landslide.

  6. […] 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. […]


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