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.