by Terry Spurgeon, Coquitlam, BC
November 24, 2010
This is an update to the 8 October 2010 virtual field trip to the Capricorn debris flow in British Columbia. That page in Northwest Geology Field Trips described the effects of a very large volcaniclastic debris flow on August 6 down Capricorn Creek, on the east flank of the Mount Meager volcanic complex, into Meager Creek and then down the Lillooet River. This area is 60 km west of Lillooet, BC, and 218 km north of Vancouver. Refer to that post for a map and information about some locations mentioned in this update.
On October 30th, I was again accompanied by Paul Adam. The following is based on our observations and discussions. We took many photographs, clay ball samples were collected, field notes recorded and the previously published mileages (km) were verified. Our first observation was essentially to remark that “wow, it really looks different”.
It is safe to say the environment, as all would anticipate, is rapidly changing. The Lillooet River has now cut a new and less wandering channel almost directly through the debris field, essentially re-establishing in its old channel. This has led to at least one flood event which overtopped the banks in the immediate area of the B.C. Forest Service campground, leaving new deposits in the northernmost camping platform. The overall impression is that things continue to change quickly. We couldn’t access previously visited areas and assume more changes will occur over the winter and spring wet season.
The deep pond covering the Meager Hot Spring access road is nearly gone, down 2 to 3 metres depth, except beavers seem busy now trying to establish new containment. The east bridge abutment is now visible and the bridge deck seems to be rotated such that it lies parallel to the west bank. This is best seen from out on the debris deposits.
In addition to bridge remnants (not useable and the road no longer exists anyway) it appears the famous warning sign (see previous article) and the green attendant’s shack may have survived. The debris deposits along the rapidly downcutting Lillooet River are collapsing almost while watching and are very unstable – BEWARE! Some areas accessed on 25 Sept. are simply gone. There is a boulder field where the flow has carried away the matrix. A side note is that there were chum salmon in Sampson Creek again (at 35.7 km), so the turbid waters of the Lillooet have not deterred them from their annual migration!
The rate of change in this environment is rapid in the extreme and with winter coming on much of the deposited flow sediments will be disrupted, eroded, remobilized and redeposited elsewhere. The area and the primary debris flow deposits will likely be very different by the time spring arrives. If this holds true for the upper reaches along the deposits of the Meager and Capricorn, then researchers are going to get an erosionally distorted picture at the surface. We are doing photo comparisons at the moment and see big changes. The combined Meager/Capricorn Creek flows now occupy the southwestern reaches of the debris deposits. This is visible best from the air. Across the Lillooet there are visible abandoned hanging channels that Meager Creek occupied after the debris flow as it re-established a channel in the thick sediment deposit. They show interesting sections but are not accessible from our vantage point (good pictures though). Meager Creek appears to have migrated west and south towards the SE valley wall, so more collapses and channel cutting have occurred. The view up the flow valley shows an ever deepening set of deposits that emanated from the Capricorn Creek area.
Hummocky debris deposits are wearing down quickly and the clay balls I keep mentioning are rapidly deflating in the rain and high water events. We collected four samples of clay ball material (bagged, position recorded, notes, standard Munsell colors noted etc.) to analyze, especially since they will all have disappeared in another few weeks. The clay balls are ubiquitous, randomly scattered on the surface, not hard to locate and comprise something less than 0.5 % of the surficial deposits. When deflated they leave a pile of clasts which eventually degrade into the matrix, the clay being long gone. The remaining pea gravel sized deposits will disappear in subsequent high water events.
The surficial deposits are split about 50-50 between rounded to sub-rounded granite cobbles and boulders, and sub-angular to angular, frequently shattered volcaniclastics. The slide debris is generally matrix supported in sands and muds where useful sections can be seen. Naturally there are some organics embedded in the deposits. The volcanic clasts include andesites, welded tuffs and even examples of prismatically jointed blocks. There are long streaks of grayish sediment and smashed clasts at points along the surface. There are potholes with
volcaniclastic linings in spots, where the clasts seem the result of powerful shattering events (collisions). The flow carried, entrained, re-entrained and cut through numerous strata on its journey to final deposition making for a very complex depositional field.
I will be examining the clay ball samples and we will be comparing photos of the ongoing changes in the future. This is just a glimpse at a very dynamic and still potentially a rather dangerous area, in the short term at least. I would not be surprised that further small debris flows will occur overwinter and next spring and that by our next visit things will have changed dramatically. Certainly gaining access to the west side of the Lillooet River channel and up the debris deposits and closer to the origin area should prove most revealing.
So, what’s the big deal about the clay balls? Clay is sediment comprised of extremely fine mineral fragments that potentially have many roles to play in lahars or debris flows. Some clays are formed in hydrothermal environments and this is a situation present at Meager. Clay can consist of weathered volcanic products such as ash and pumice. It is so fine that normal high power microscopy fails to properly define it. Suffice it to say that I have been looking into clay in recent days and am becoming ever more frustrated; I expect it would take the study of a lifetime to become wholly conversant with clay. Couple this with the somewhat unusual clay ball form found on the flow debris deposits and . . . well, who knows? Clay is a common constituent of flows and the percentage contained frequently determines the type of flow. Clay or other fine particles may rise to the surface in some flows, the result of buoyancy. Clay, which has an affinity for water to twice its volume, may be a causal factor for slide initiation, especially where there is an abundance of water and appropriate sediment available in addition to certain other conditions. The Capricorn flow probably changed character several times over the course of its 6.2
mile (10 km) 4,900 feet (1500m) descent: bulking with additional sediment it picked up along the way, debulking by dropping some of that load, slowing in lower gradients and going around corners, speeding up in steeper areas, wetting and drying and even being dammed. Big flood anddebris outbursts were likely in the aftermath, each event having individual impacts, including affecting clay ball formation. Since subsequent researchers are unlikely to identify the dissolved clay balls, plus their unusual nature being uncommon, they are an interesting, ephemeral curiosity worth studying now.
Pierre A. Friele and John J. Clague, 2004. Large Holocene landslides from Pylon Peak, southwestern British Columbia. Canadian Journal of Earth Science, 41: 165–182.
Vallance, James W., 2000. Lahars. In: Sigurdsson et al (Eds.), Encyclopedia of Volcanoes, Academic Press, 601-616.