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Report of a site visit on Sunday, June 9, by Dave Tucker (MBVRC) and Chris Magirl (USGS-Tacoma). Both of us contributed to this report. More trips will be made. Links to other posts and videos are in red.
Debris flows continue to descend the Middle Fork Nooksack River. Profound erosional changes occurred in the time since the preceding visit on June 5. More on the May 31 flow is posted here on this blog. The initial debris flow activity was caused by a landslide from a glacial moraine into the river at the terminus of the Deming Glacier. We found a deposit post-dating the big one of May 31. This one was probably on June 6 when another big spike in river turbidity occurred. Perhaps the continued activity is from more slides, or from high-water mobilization of May 31 deposits during high snow melt.
We made preliminary measurements to determine the volume and extent of the
main flow, and walked much of its length. Among the most important or startling findings:
- Debris flow hazard remains. People should stay out of the river channel, or be ready to RUN up the bank into the forest to escape a new one. Always be alert for unusual noises coming suddenly from farther up the valley!
- Much of the May 31 deposit is already eroded away by high water due to snow melt.
- The river has cut a 10-m-deep canyon through the debris flow deposit upstream of the Ridley Creek trail crossing.
- Because of this rapid erosion, the geologic record of these type of events is probably very incomplete. There may have been many of these events over the decades, but their deposits are now entirely eroded.
- The May 31 debris flow extends 5.6 km (3.4 mi), nearly to the Elbow Lake trail crossing.
- Beyond 5.6 km, hyperconcentrated flow continued for an unknown distance (more on this below).
- The June 6th debris flow was forceful enough to topple and move a 14-foot-high boulder photographed on June 5th.
- Mud pits at least 1-meter-deep remain in the river channel hidden by seeming solid gravel and cobbles. These can be hazardous, especially to children, but hard for anyone to get out of.
Based on Nooksack River turbidity data recorded by the Nooksack Tribe near Nugent’s Corner, three distinct debris flows occurred: 5/31 was the biggest; a second, smaller debris flow occurred on 6/1; another occurred on 6/6 that was similar in size to the 6/1 event. This 6/6 event occurred after Dave’s first visit to the site (on 6/5). Here are the turbidity data from the Nooksack Tribe (thanks to Jean Snyder and colleagues there). Since measuring turbidity starting in 2009, the largest spike in turbidity before this event was in October 2009.
We had a laser range finder and measured simple cross sections. The largest cross-section of the 5/31 event was taken 250 m upstream of the Ridley Creek trail ford, where the cross-sectional area was 220 m2, the width of the debris flow across its top was 35 m, and the mean depth was about 9 m. The super-elevation here was 4-5 m (meaning it ‘banked’ off one valley wall at a curve, and the left trimline is higher than right trimline).
What is ‘hyperconcentrated flow’? Once a debris flow has lost sufficient momentum and thickness, the boulders drop out. The fine-grained sediment continues to surge downstream at a lower velocity. This type of flow deposits gravel, sand, silt and clay as it loses energy. Deep mud pits remain until the deposit has dewatered.