By Dave Tucker, with scientific input from John Thompson (Whatcom County Water Resources), Seth Moran and Steve Malone (Pacific Northwest Seismic Network).
Wednesday, June 5, 2013
This post was updated on June 7 with new information. Please visit http://mbvrc.wordpress.com/2013/06/07/update-nooksack-debris-flow-initiated-by-landslide-not-outburst-flood/
A sizable debris flow descended the Middle Fork Nooksack River early in the morning of Friday, May 31. The source is not known with certainty, but most likely the flow began with a landslide off a glacial moraine above the terminus of the Deming Glacier. The earlier hypothesis that it began as a glacial outburst flood (jokulhlaup is the commonly used Icelandic term) is probably incorrect. f (More on this process at the end of the post.) The flood rapidly bulked up with debris in the river channel and from glacial moraines lining the upper part of the valley. In particular, moraines are the likely source for the clay-rich matrix that dominates the deposit. The deposit was examined today by Bob Mooers and me where the Ridley Creek trail reaches the river bank, just beyond the end of the Middle Fork Road and 2.2 miles downstream from the Deming Glacier. Our observations and some photos follow.
This would have been a dramatic but probably terrifying event to witness in person. (See a YouTube video of an event that was probably very similar to this). People commonly visit the river bank for picnics after hiking 10 minutes through the forest. Fortunately, this event occurred in pre-dawn hours. It is unlikely anyone on the river bars would survive the wall of sediment and water (more sediment than water- this was flowing concrete, not a ‘flood’) that came roaring down the narrow valley floor. To get away would have required a rapid scramble up a steep loose bank into the forest.
The large boulders pitched 15 feet above the stream channel show that survivors would have had to move FAST for some way distance from the channel.
Bob and I found a new levee of boulders embedded in sticky clay-rich mud a full 15 feet above the river channel. This levee veneers the slope of a much more extensive terrace left by a June 1927 glacial outburst flood. Mud-coated andesite boulders up to 10 feet across are pitched onto the crest of the debris flow levee. Many trees were knocked over. Surviving trees were coated
with mud on the river-facing side for another 5-6 feet higher above the top of the levee. The channel of the Middle Fork is now a 100-foot-wide flat plain of mud and small boulders and cobbles. The opposite side of the new channel fill is banked against a Sumas-age glacial moraine. There is no terrace on that side, but a train of 5- to 10-foot boulders extends for several hundred yards along that margin of the channel. There is little to no wood in this debris. The river was very turbid, much more than
usually observed even in streams draining glaciers on a warm day (it was in the upper 60s when we visited). The river is rapidly cutting down through the sheet of sediment that fills the channel, and mud-free boulders are found immediately adjacent to the water. All else is coated with gray mud. Some of the mud is saturated, and it is easy to sink in over your boot tops.
We visited the river again at the Elbow Lake trail crossing, 1.8 miles downstream. Here, the layer of fresh mud coated terraces 5 feet above the river, but logs piled higher on the terraces were not coated. We saw no newly-deposited boulders. A view of the river from three miles further down the road revealed no new mud.
This event was first detected instrumentally. The MBW seismometer only 4 miles away detected the event at 2:53 AM. Two hours and 23 minutes later, a large spike in turbidity and discharge was noted on the Middle Fork stream gage) at 5:15 AM, 12.5 miles downstream from the terminus. Discharge (volume of water per unit time) increased from 865 cubic feet per second) at 5 AM to 976 cfs at 5:15 and back to 874 cfs by 5:30. Velocity of the big pulse of water to the Middle Fork gage was 5.3 mph. Discharge spikes were also detected on gages at Nugent’s Corner and Lynden. Ned Currence, a fish biologist with the Nooksack Tribe, went to the scene Tuesday afternoon and circulated an email with the first photos. He recognized that a glacial outburst was the most likely trigger for this large debris flow.
As the climate warms glacier termini will continue to thin. They may become more honeycombed, holding more water. If this is the case, we can expect more of these events in drainages below Mount Baker’s glaciers. Other glaciers at Baker are susceptible to glacial outburst floods. The Mazama in particularly has a long, stagnated, debris-covered tongue. It lies in the narrow, talus-walled gorge of Bar Creek which enters Wells Creek. An outburst flood from the Mazama would rapidly bulk up into a heavily laden debris flow, reaching the North Fork Nooksack just below Nooksack Falls.
About glacial outburst floodsStagnating glaciers can store a lot of water in cavities within their rotting termini. If the load of water gets too high, or if a hot day provides more water than the ice can store, the ice can give way and release a sudden flood of water. This surge can bulk up with sediment scoured out of the river channel, and erode steep walls of lateral moraines lining the channel. The latter provide plenty of clay and silt to the evolving flow, making it more viscous. Debris flows can grow if enough sediment is entrained. By definition, these are flows containing more sediment than water. You may be familiar with volcanic lahars, which are simply debris flows initiated on volcanoes , sometimes, but not always by eruptions. Some might call outburst floods from Baker’s glacier lahars, but since many associate the term with eruptions, I prefer not to. An event identical to the Middle Fork debris flow of May 31 could begin on any glaciated mountain with big moraines, volcanic or not.
Monitoring and avoidance
If you are in a river valley on a hot day downstream from a stagnating glacier, be alert to the rumble or vibration of an approaching outburst flood, and have an escape route in mind. You can never outrun one of these things. Just charge straight up hill.
Outburst floods can not be predicted
other than to say such-and-such a glacier has the right conditions to spawn them. We saw that the May 31 Deming Glacier outburst flood was detected on seismometers, which telemeter their data directly to the Pacific Northwest Seismic Network’s center at the University of Washington. However, a received signal would do no good for anyone closely downstream. If the shaking detected at PNSN was recognized as a really large outburst flood, it is conceivable that warning could be given to people well downstream. However, it is unlikely one of these events would be of the scale of a big lahar coming off of Baker.