Originally posted on Mount Baker Volcano Research Center subscription website:
View a 2 minute video on YouTube I made at the Clay Banks landslide toe Thursday. The vid is annotated and narrated.
The video is here: http://youtu.be/Hxwu-cf3bFo
Thursday I visited the toe of the February 21 Clay Banks landslide on the Nooksack River, in company with Scott Linneman (landslide geomorphologist at WWU Geology) and John Thompson, a geologist with Whatcom County Natural Resources. If you haven’t been following this story, please visit Dan McShane’s blog post for the background to the story including some fine maps and photos, and my original post from February 22, the day after the landslide. Dan has also posted a great set of old maps and LiDAR showing changes at the Clay Banks as the river changes position. The landslide occurred just at midnight Friday February 21; you can see the sudden radical, short-lived drop in the river discharge graph below (recorded at the river gage a mile downstream at Cedarville). Click link to annotated You Tube video.
The Clay Banks are a steep bluff of Pleistocene glacial clay around 100 feet high on the south side of the river about 1.8 river miles upstream of the Nugents Corner bridge. The landslide bit deeply into only the upper half of the bluff, leaving a prominent bench hanging above the river. There is landslide debris on the bench, but most of it appears to have flowed over the bench, down the bottom of the bluff and into the river.
We crossed a shaky foot log across a narrow side channel to get to the big gravel bar on the north side of the river, and then walked 350 yards to the river side directly opposite the landslide toe. The landslide began in a layer of glacial clay a few meters above a prominent alluvial bed called the Deming Sand. If there is a stratigraphic layer at this level in the Clay Banks that contributed to this landslide it was not apparent using a high power spotting scope.
The river is still ponded against the landslide toe which protrudes into the river for 200 feet or so from the south bank. The river has cut through the outermost edge of the landslide toe. The river drains the partial impoundment by racing through a 60-foot-wide (19 m, measured with a laser range finder) gap. We couldn’t measure the river depth, but know it was 5 feet right up against the bank we were standing on, and 10 feet or more may be a reasonable guess. The landslide toe consists of car-sized, or maybe even garage-sized, clay blocks, and rests on the surface of a gravel bar; the bar gravel is exposed beneath the debris. At the time of the landslide, the river was closely beneath the high bluff, and the gravel bar we now see beneath the toe was part of the one we were on. The river then cut a new channel around the outer edge of the deposit, isolating the bar gravel beneath the toe from the gravel bar on the north side of the new channel. We found a single, angular clay block lying on the north side side of the channel, and a number of rounded ones. We figure the angular one (see photo below) was tossed laterally out of the landslide toe. It hadn’t been rounded by river flow, so is almost certainly right where it landed at around 12AM Friday 2-21-2014. You can see the same block in Dan McShane’s post, though when he visited the river bank was a few feet furthe away. This block is direct evidence that the river is continuing to cut outward and remove the bar on the north shore. This block will probably tumble into the river in the next few days.
The river makes an end run around the toe, and then cuts sharply south and runs directly into the base of the bluff below the landslide. This is certainly undercutting the base of the Clay Bank, setting the stage for another collapse, perhaps involving the entire bluff height rather than the upper half.
Among our observations is that there has been a second, much smaller landslide in the same place. We don’t know when this occurred, but it sent darker material over the bank and down onto the southern margin of the Feb 21 toe. Compare the photo taken by Dan McShane on the morning of Feb 22 with the one I took today (Feb 27). The center of the 12-hour old scarp in his photos is now deeper where the 2-?? landslide collapsed. Also in the photos, note the toppled trees that rode the slide over bank.
The Clay Banks extend another 1/4 mile or so upriver and 1/2 mile downriver from the 2-21 landslide. Earth movements have been happening here at various scales for decades, and are likely to continue long into the future.
ACCESS NOTE: All access is over private land, so you are dependent on reports from authorized visitors. Sorry. Don’t even try to get there yourself. It is difficult to see the Clay Banks scarp from anywhere on the north side of the river. You may get a glimpse from the Deming Road somewhere in the first half mile after it branches off Mount Baker Highway a bit east of Nugents Corner. However, you will be 7/10 of a mile away, and for sure can’t see the river or the landslide toe.
Filed under: geology, Glacial stratigraphy, landslides, natural hazards, North Cascades geology | Tagged: Clay Banks, Deming Washington, landslide, landslide dam, Nooksack River, western Washington geology | 4 Comments »
Whether you were caught up in the ballyhoo over last week’s mammoth tusk find in Seattle, or not, you may find a first-hand report by UW biology grad student Dave DeMar of interest. Dave was one of the small crew who excavated the tusk. Click to read Dave’s posting. Also, here is a link to a Burke Museum Facebook page with a number of photos of the excavation and rescue: including cute kids cheering and showing their fan club banners. Prominent in the photos are advertizing banners hung in the pit by the various contractors associated with the construction project. Thanks to David B. Williams for sending these links to me via his excellent GeologyWriter blog. His analysis of all the hoopla is great; heck, his blog is always a good read. I’ll repeat his kudos to the construction workers who knew to stop when they uncovered the first glimpse of the tusk in the excavation.
The rescue of the mammoth tusk brought back fond memories of the discovery and rescue of the fossil footprint of the giant bird Diatryma fromthe Chuckanut Formation near Kendall, Washington in 2010. Here is the page that links to all the stories on this blog about that fun adventure!
My friend Dan McShane visited the Nooksack shore immediately across from the Clay Banks landslide toe Saturday and posted a great set of photos on his Washington Landscapes blog, as well as further analysis of the changes to the river channel at the Clay Banks in the past years. Click this to read his report. Dan visited a different area than I did, and his report is well worth reading. His newest photos of the landslide deposit are toward the bottom of the post. Among his observations are blocks of clay across the river from the failing hillside of the Clay Banks. One of his photos is below, showing the landslide toe where the river has cut through it.
Filed under: Glacial stratigraphy, landslides | Tagged: Clay Banks, Dan McShane, Deming Washington, landslides, mass wasting, Nooksack River, northwest Washington geology, Washington Landscapes | 1 Comment »
A landslide about 1 AM February 21 diverted the Nooksack River. The slide occurred at the infamous ‘Clay Banks’ about 1.8 miles upstream of the Baker Highway bridge at Nugents Corner, near Deming. I’m passing along the essentials from Dan McShane’s Washington Landscapes blog. I visited the site this morning with geologists Scott Linneman (WWU) and Eric Grossman (USGS), and have posted a few photos and a new Google Earth map. The river has cut through the low-relief landslide toe, and there is now no impoundment of the river. According to residents we spoke with there was another large landslide a couple (?) of weeks ago. They said they could hear a deep rumbling when the February 21 slide released.
Access to all viewpoints is on various private properties, which are amply posted ‘No Trespassing’, etc, etc. You can’t see anything from anywhere without trespassing. Watch for possible updates. We’ll make and effort to get more information and better photos soon.
The Clay Banks consist of Pleistocene glacial till sitting on top of a very permeable sand layer, which in turn sits on top of older glacial till. It is very prone to sliding and has destroyed two houses that were built in the forest several hundred yards back of the top of the bluff in the late 1980s or early 1990s.
Nooksack River Temporarily Blocked by Landslide
At Lynden 11 miles downstream a less substantive, but still sharp drop and recovery took place. Lynden Public works reported a sharp increase in river turbidity.
USGS staff noted the dip in the discharge and Whatcom Flood Division confirmed a large landslide at the Clay Banks had blocked the current main channel and deflected the river back to an older channel.
…As can be seen the river has been up against the steep bluff in the past. However, the frequency and duration of the river being against the steep bluff has been enhanced by the construction of a high rip rap levee on the opposite bank preventing the natural meander of the river.
This new page directs readers to projects they can participate in, usually as volunteers with a minimum of geologic experience. Go to the Citizen Geology page.