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.
Two videos are published on YouTube:
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 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.
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
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.