Silver Creek: sedimentary structures and flood erosion in the Chuckanuts

By Dave Tucker

January 29, 2010

Keith Kemplin walks the pipeline road in Silver Creek Gorge, January 2009. Click any photo to enlarge.

Silver Creek is a hidden gem, close to Bellingham on public property. It flows through steeply dipping Chuckanut Formation rocks southwestward from Lo0kout Mountain, crossing under Interstate 5 near the North Lake Samish interchange ( Exit 246). The narrow gorge of Silver Creek was scoured of sediment during the same pineapple express rain-on-snow floods (early January, 2009) that caused the Racehorse Creek landslide, which you can also visit on this website. The floods removed vegetation from the  channel of Silver Creek, exposing wonderful bedrock exposures in the Chuckanut Formation sand- and mudstones. It is now a great place to see crossbedded alluvial sandstone, large concretions, and the most extensive pebble-cobble conglomerate I know of in the entire Chuckanut. The flood also stripped brush and years of accumulated soil from a 100-foot-high conglomerate wall, exposing a lovely silver waterfall cascading down the wall in several threads. This is certainly among the loveliest waterfalls, and perhaps the highest,  in the Chuckanuts. A large logjam was left in the creek by the flood; fiberoptic cables, phone lines, and a gas pipeline were threatened. A  private road was cut, and a debris flow filled yards on the shore of Lake Samish at the creek’s mouth, taking out portions of a paved road. Here , and here, see video footage of the flood at the stream’s mouth near Lake Samish (it is called ‘Finney Creek’ at that point). Scouring by the flood has now opened up a really fun short hike on public land in a dramatic geologic landscape. You can do this trip in a few hours.

Getting there: Although there is no trail up the gorge of Silver Creek, it is relatively easy to hike. Begin at the east side of I-5 at the North Lake Samish interchange; this is a fun bicycle ride from Bellingham via either Lake Samish or Old Samish roads. The trip begins at a paved road paralleling the east side of the freeway; it may be gated, although as of this writing, the gate is actually missing. If the gate is up, and open, park here still. It is not worth getting locked in behind the gate! The roundtrip distance to the  waterfall, our turnaround point, is about 2 miles (3.2 km) from here.

Afoot, or astride your two-wheeled steed, head downhill on  the paved road next to the roaring river of I-5. Fear not, you will soon find silence. This road is the abandoned route of Old Highway 99 that reached Bellingham from the rest of the world to the south, supplanted by the interstate in 1965. In about 3/10 of a mile, leave the pavement on a well worn track branching to the left (east). UTM here is E0544680 N5392490; as always, I give this in NAD 1927, with precision ± ~10 m. But first, walk another couple hundred feet, crossing some barriers, to the wash out from January 7, 2009. A funky looking concrete cocoon covers a fiberoptic cable suspended from a log across the deep gully. Since the flood, a rough track has been made by wheeled users to cross Silver Creek. The narrow corridor along I-5 is used by several utilities.

The Silver Creek logjam

Go back to the obvious trail and turn away from I-5. This is a popular mountain bike route to roads and trails on Lookout and Galbraith Mountains, among the clearcuts. Cross a buried gas pipeline (the same one that blew up at Whatcom Falls Park June 10, 1999).   Note that this pipeline crosses under the creek on you right. The mountain bike trail branches left just after this- go straight and walk a few feet further until you find yourself against the jumble of the January 2009 logjam, stacked up against the trees. This is the high water mark, and will be all the more impressive once you see how small the stream is just beyond. Leave your bicycle here. Carefully climb over the logs; be careful, they can be pretty slick. Stay towards the left, and head for the gravel flats you see ahead of you. Note that the logjam extends downstream from here, filling the stream channel. It served as a dam, and gravel filled  the channel to the east, at the mouth of a narrow gorge. The prominent gravel flat is what you are heading for. Silver Creek has begun to downcut into this gravel fill. It will be interesting to monitor how fast the downcutting proceeds.

1930s City of Bellingham water pipe in Silver Creek

At the gorge, keep an eye out for bedding in the Chuckanut Formation rocks, which are primarily sandstone. The beds are tilted- they dip down a little north of west at about a 50 degree angle. The creek has downcut its gorge along a weakness between beds, or along the strike of the beds. Walk up the stream bank, or hop rocks and slabs to go upstream. Sometimes you can just step across the stream. There is a small coal outcrop on the left just after you enter the gorge. Lengths of wooden water pipe are strewn about in the creek bed. These are part of the water diversion project built by the City of Bellingham in 1931. Water was impounded behind a small dam closely upstream and carried in this wooden pipe to Lake Padden, which served as Bellingham’s water supply at the time. The pipeline is buried under the bank on your right heading upstream. The bank is an old access road, and can be walked much of its length. The fact that this pipeline survived intact until January 7, 2009 shows how unusual and dramatic this flood was. Wait yet, you’ll see more evidence just ahead.

Crossbeds in Chuckanut Formation sandstone

As you hike up the stream, watch for water-washed exposures of cross bedded sandstone, and a few knife-sharp contacts where sandstone sits directly on fine-grained darker mudstone. The conditions of the ancient streams that deposited the Chuckanut sediments were sometime pretty darned dynamic for a flood plain. More on that ahead, too.

The gorge is quite narrow. Prior to the flood, it was so choked with fallen trees and debris that it was almost impassable; the stream was, and is, normally just a small trickle a few feet across. Keep your eyes peeled for contorted black streaks of coal in the sandstone. Within a couple of hundred yards of entering the gorge, note the rounded concretions protruding from the rock wall on your left (west). These are formed by concentrations of silica cement within the sandstone, which bind the

Bob Mooers marvels at the log marking high water in January 2009. Note the bedrock is stripped to that height.

sand grains a little more strongly, and resist erosion just a touch more than less cemented adjacent rock. The ones along this wall are splendid examples, particularly large and numerous. Just beyond the concretions, be prepared to be wowed. You will note that the wall on the west side of the creek has been scoured by the floods. How high was the water? Well, note the peeled log with its root ball, wedged between trees about 20 feet above the channel. THAT’S how high the water was! The road on top of the old pipeline was also covered with flood water- sand and cobbles were deposited on it. This surface is already beginning to revegetate, and already no longer shows much of the flood’s effects. But that peeled log wedged in those trees is evidence enough. Aren’t you glad you weren’t thrashing your way up the creek on January 7, 2009! Still, it would have been a sight to behold.

The 1931 Silver Creek dam. Climb over the left side.

The old Silver Creek dam is close (UTM E545120 N539300). Climb over the 10-foot-high concrete slab on the left side; the catchment pool above it is completely filled in. A large boulder on the left has well-developed slickensides on its surface, including shmeared out pebbles. ‘Slicks’ form on either side of a shear plane in a fault zone, from the friction and pressure of earth movement. I suspect most of this took place during folding and uplift of the Chuckanut rocks. In the case of this boulder, we are seeing only one side of the shear zone. The shearing happened along a weakness between the pebbly conglomerate layer and whatever sort of sedimentary rock lay in contact against it, now removed by the shearing motion along this fault.

Slickensides above the dam. Pebbles on the right are sheared by the pressure of the faulting that deformed the rock. Blue pencil for scale.

Silver Falls is another couple of hundred yards ahead, and is the turnaround for this field trip. Between the dam and the falls it is necessary to climb over some logs, and cross the stream a few times. Just before the waterfall, you will start to notice that the rock face on the east (right) side of the creek is now a pebble and cobble conglomerate. Notice how many different types of clasts make up the conglomerate, a veritable petrology course’s worth. Most of the clasts are pretty well rounded. What alluvial environments deposit pebbles like these? Didn’t the Chuckanut come all the way across what would become Washington State from the Rocky Mountain highlands some 50 million years ago? How would those lazy rivers carry such large stones so far? Perhaps there were highlands near here, and tributaries were dumping their loads into the Chuckanut flood plains. This conglomerate bed is not very thick, only a few meters. The entire 100 foot rock face at the falls is the same conglomerate bed, tipped to a 50 degree angle during folding of these rocks. Note that Silver Creek upstream from the waterfall tributary is not scoured out. This debris-choked channel is what the drainage looked like prior to the flood and debris flow.

Silver Falls. Note Keith Kemplin at right center.

What caused the flood in Silver Creek? It occurred during a heavy, warm rain storm that caused flooding and landslides throughout the region. There was quite a bit of snow on the ground in the Chuckanut hills, even in Bellingham, and the ground was frozen. All that snow stored up the initial rain fall. The frozen ground was unable to absorb the runoff, so it was all concentrated into small streams. There is no single source for this flood, no blown out culvert  or broken debris dams in Silver Creek. However, about 1/4 mile feet up the steep tributary above the falls, there is a new, steep, rubbly gully that has shed a lot of rocky colluvium into the narrow stream channel. There are no blown out culverts at the high road that crosses this tributary, either.

I have climbed up along the water fall into the upper part of the stream, which is well worth doing- the Chuckanut geology is even more interesting up there. The stream runs at right angles to the strike of the beds, and make some steep ascents up dip slopes in the Chuckanut. Explore that with a comrade.

6 Responses

  1. Lori Jo and I hiked this today. As you said, Silver Creek IS a gem. We loved it. Did you notice the very green boulder near the base of the falls, just downstream? What is that? Is there a contact of Shuksan Green Schist further upstream? We loved the concretions, especially the lovely grouping near the flowing water in the sculpted rock bottom.

    Thanks for sharing this field trip. We’re going to explore the section above the waterfall next time.

  2. Randy and Lori Jo,
    Very glad you enjoyed the hike; I do encourage you to make the steep climb to the top of the waterfall. Do that by staying in the trees just to the left of the falls. Careful climbing the slabs further up!
    I noticed some of those almost mint green boulders, too. There are some at the gully that cuts the road, and more up at the gas pipeline crossing of the creek. These must be riprap, brought in to shore up the stream banks. They are a chlorite-rich greenschist, perhaps from the quarry above Nooksack Falls.
    How convenient of you to mention them, as those old volcanic rocks are likely to be the next field trip.
    dt

  3. Dave, my Geology 211 class and I walked Silver Creek today. The initial logjam can be quite easily passed by staying toward the right side as one ascends.
    I have a different take on the slickensides you describe at least for one large boulder we saw with excellent slickensides. This rock had clearly fallen down recently (perhaps during or shortly after the Jan. 2009 flood event) from a steep outcrop of bedrock above. I believe it is clear that in its case, the slickensides were produced not by ancient faulting but by slippage of the large (about 2 – 3 meters per side) against its bedrock base.
    This is a wonderful trip, and thanks for listing it. It is truly a hidden gem.
    We ventured to above the magnificent waterfall this morning, turning around perhaps 400 yards upstream from the upper brink. The steep slope to climber’s left is beginning to get a boot track , but adventurers are cautioned that there is some loose soil and some exposure, especially right near the top. It is no place for small children, too-frisky dogs, or anyone with vertigo.
    The falls will be a great place for ice climbing in the rare winter cold spells that last at least a week. I’ll be back for that! It is short enough it could be toproped , handy for us older climbers.
    We were curious about the dam. The date says 1831, but no way it was built then. Perhaps someone played a prank with a chisel and changed 1931 to 1831.

    • Glad you liked it, Doug, and hope your students did, too. I don’t know about the slickensides, we’ll have to go there together to discuss this. As to the dam, I just looked at my photos from February 2009, the date inscribed on the dam also says ;1831′, so clearly a prank. Too bad.
      The dam is a City of Bellingham water project, the wooden pipes used to move water to Lake Padden when the lake was the city’s water supply. I have an old map showing the pipeline route, now mostly beneath I-5.
      Dave

  4. I took another trip up Silver Creek this weekend. Good weather and this time I wanted to meet your challenge of going above the falls. The climb above the falls got to be a lot like my typical work but I was determined to find the debris flow trigger – again a lot like my geohazard work.

    There was a fairly decent size slide on the north side of the creek in some very steep nasty convergent topography with thin soils over weathered bedrock. I suspect that this slide was just enough to create debris flow conditions in the narrow gorge and based on your descriptions of the area prior to the debris flow and observed in the tributary at the base of the falls there was plenty of debris to build up on the way down. My conclusion is that this debris flow was not logging related unless one argues that cutting 50 plus years ago played some sort of role.

    I found the scramble up repellent enough that I opted to loop north out of the drainage for a relaxing ridge top/logging road stroll with views of the San Juans, Bellingham Bay, Vancouver Island and the Olympics. Came back down along a newer mountain bike route. Some of the jumps are about as crazy as the heights of the debris flow.

    I have seen slickenslides with the Chuckanut and suspect it is primarily slippage along weaker planes due to spacial issues posed by the folding of the formations. No hard data; but I see this more frequently in the lower portions of the formation.

    Johnson (1982) talks about fault bound basins within the Chuckanut depositional environment and Dragovich and others (1997) have alluvial fan facies in the Huntingdon Formation.

    • Dan,
      Thanks for this note. I scrambled above the waterfall shortly after the Silver Creek debris flow last winter, with Keith Kemplin. We found the same place you describe- I was sceptical that this was the ultimate source at the time, but Keith later told me there were no smoking guns any higher. We climbed to the top of that nasty nasty gully, but descended the way we came in. There was certainly plenty of water during the January ’09 Pineapple Express to mobilize the material in that small slide, plus mobilize the sediment lower in the stream. These debris flows tend to bulk up with sediment and actually get larger and more viscous further downstream, in the same fashion as lahars.
      dt

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