Landslides:
Landslides are caused when the stability of a slope changes from a
stable to an unstable condition. A change in the stability of a
slope can be caused by a number of factors, acting together or
alone: Landslides occur throughout the world, some are recorded,
while others happen in remote areas unseen by the human eye and may
not be discovered for months or years. This landslide was not
witnessed by human eye, and is the most recent major landslide in
British Columbia.
Sometime during the period of December
2,2007 and December 5,2007 Mother Nature again demonstrated her
awesome power when a massive wall of rock, earth and debris slid
down the side of Mt Orrock into Chehalis Lake. The resulting
tsunami washed across the lake in a wall of water which is
estimated to have been 10 meters or more high. As this wall of
water struck the shoreline on the opposite side of the lake, it
destroyed all vegetation in it's path, ripping out huge trees by
their roots, drawing them back into the lake as it ricochets back
across the lake to continue its devastation of the shoreline on the
side of the lake where the slide occurred. How many times this wave
went back and forth across the less than one km wide lake (measured
directly across from the slide) can only be guessed. This wave also
traveled the 7 km length of the lake to it's southern end, ravaging
the shoreline along the way. As you can see in the photos, the
shoreline was stripped of trees where they once went right to the
waters edge. A huge mass of debris was left floating in the lake
after the event, causing great concern to First Nations Villagers
living downstream. A salvage effort of the floating debris is
currently underway to minimize the possibility of downstream
damage.
The Chehalis slide is thought to be second
only in size and scope to the Hope Slide (The largest ever recorded
in British Columbia) which occurred in the early hours of the 9th
of January 1965 and took the lives of 4 people. While no loss of
life occurred in the Chehalis slide, it could have been devastating
as the resulting tsunami completely destroyed two campgrounds,
Chelalis Lake North campground at the North end of the lake and
Skwellepil Creek campground which is down lake from the slide
approx 1.5 - 2 km. The slide also sent an estimated 4 meter high
wall of water over a third campground, Chehalis Lake South
campground at the southern end of the lake, approximately 7 km
away, this would have had devastating results for anyone camping in
the area. These campgrounds are used extensively in the spring to
fall months by those wishing to camp in more remote areas, they are
often only accessible with a 4x4 vehicle.
The cause of this slide, as with many
others, is difficult to determine, there was heavy rainfall in the
area at the time of the event, which was most likely a contributing
factor in this portion of the mountainside slumping away. Whatever
triggered the event, the mountainside must have been unstable for a
period of time prior to the slide actually occurring.
While the scars of this slide are very
much visible and will remain visible for a very long time, Mother
Nature has a way of healing these scars and creating areas of new
growth and beauty. This healing is demonstrated by the current land
created at Seton Portage where eons ago a massive slide created two
lakes out of one, and over time has created a rich and fertile
division between the two lakes. (see Seton Portage
Earthcache - GC16E8E)
The posted co-ordinates for this
Earthcache are at the point that the road crossed Cohoe Creek, now
buried in many meters of rock, earth and debris
In order to log this Earthcache you
must:
(a) Post a picture of yourself and
your GPS showing the landslide in the background.
(b) E-mail the owner what you think is the
largest diameter rock visible on the surface of the slide, and how
far across you think the slide is where it covers the
roadway.
(c) E-mail the owner with 3 natural and 3
human causes of landslides, and what type of landslide you
determine this to be.
Logs will be deleted that do not meet the
logging requirements of this earthcache.
WARNING: The area around the slide may be
unstable, do not go onto the slide itself, and be very careful
anywhere near this Earthcache. You can fulfill the logging
requirements without actually going on the slide. As with all back
country travel, go prepared and ideally not alone, the road into
this cache from the North is not accessible in a 2 wheel drive
vehicle, at this time there may be no access to this cache via
either the south or north, make sure you check on conditions of any
roads into this area before venturing out.
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