Great Cascadia Earthquake Penrose Conference - The Oregonian Article
The Oregonian: Science - Landslide Sleuths (May 15,2002)
RICHARD L. HILL
The waterlogged stumps sticking
out of the Columbia River fascinated
Lewis and Clark as they made
their way to the Pacific and
the first to write about the
curious scene in the Columbia
River Gorge, accurately described
the river as obstructed by large
rocks, "which seem to have
fallen promiscuously from the
mountains into the bed of the
river" and with "stumps
of pine trees scattered for some
distance in the river, which
has the appearance of being dammed
What they saw
is now the Northwest's most famous
and studied avalanche -- the
Bonneville Landslide -- an event
near present-day Cascade Locks
that probably led to Native American
stories about the "Bridge
of the Gods" linking Oregon
after Lewis and Clark's observations,
the ancient landslide and the
questions it raises continue
to intrigue scientists. Did an
earthquake cause the huge slide,
possibly the powerful magnitude
9 quake that is thought to have
rocked the Northwest in 1700?
Could a similar landslide occur
in the gorge today?
making some progress in determining
when the slide occurred,"
said Patrick T. Pringle, a Washington
Department of Natural Resources
geologist who became interested
in the Bonneville Landslide about
15 years ago. "But we still
have a ways to go. The work is
both challenging and can be frustrating
at times, but it's important
to find out when and why it occurred."
recent findings about the landslide
Tuesday at a regional meeting
in Corvallis of the Geological
Society of America. He is one
of nearly 1,000 scientists at
the meeting through today on
the Oregon State University campus.
The scientists are exploring
the landslides, volcanoes, earthquakes
and other geologic mayhem that
are part of the active Northwest
the date of the slide is hard
to come by. Many clues were destroyed
during construction in the 1930s
of Bonneville Dam. The dam sits
on part of the slide and inundated
the information-rich tree stumps
and other scientists are making
headway with advances in radiocarbon
dating, tree-ring dating -- called
dendrochronology -- and a technique
called lichenometry, which can
estimate a landslide's age by
measuring the growth rate of
certain lichen on the rock surfaces.
A legend it
born The massive Bonneville Landslide
crashed southward from Washington's
Table Mountain about 30 miles
east of Portland, covering more
than 5.5 square miles and creating
a 200-foot-high earthen dam across
the Columbia River.
might have been able to cross
the river on the dam, giving
rise to oral histories about
a bridge. The slide covered a
3.5-mile stretch of the river,
drowning a narrow forest of trees
for 35 miles.
the rising lake broke through
the dam. No one knows how long
that took, perhaps two years,
but it unleashed a flood that
eroded much of the slide and
submerged present-day Troutdale
under 100 feet of water.
Lewis and Clark
estimated that the slide had
occurred only a couple of decades
before their arrival, but the
looks of the slowly decomposing
trees were deceiving. Recent
investigators have placed the
slide as old as 900 years and
as young as 250 years.
dates taken in 1958 from drowned
trees indicated that the slide
occurred between 1250 and 1280.
A quarter-century later, a radiocarbon
date of wood samples taken from
within and below the landslide
deposit put the date at about
Four years ago,
Pringle and Robert L. Schuster
of the U.S. Geological Survey
had radiocarbon dates taken of
a buried Douglas fir that indicated
the tree died between 1500 and
1760. That would place the slide
close to the earthquake in 1700
that devastated the Northwest
tree rings, with each ring representing
one year of the tree's life,
they estimated that the tree
died in about 1699.
New twists to
mystery But the mystery of the
slide's date has taken new twists
Last year, Nathaniel
D. Reynolds, then a graduate
student at Washington State University
in Vancouver, used a technique
called lichenometry to estimate
the age of the Bonneville Landslide.
The dating method uses the growth
rate of specific lichen species
as an indicator of the age of
the surface the lichen is growing
organisms formed from an association
between a fungus and an alga,
can be used for dating earthquakes
and landslides because they quickly
colonize fresh rockfalls that
occur in the wake of a quake.
Once established, they form at
a constant rate if left undisturbed.
with the U.S. Forest Service,
said his study indicates the
landslide probably happened between
1670 and 1760. "These results
demonstrate that the Bonneville
Landslide may have occurred more
recently than previously believed,"
Reynolds said in a recent article
in Washington Geology.
The dates "provocatively
bracket" the powerful offshore
Cascadia Subduction Zone earthquake
of 1700, he said, but he cautioned
that the study doesn't prove
that the quake caused the landslide.
plot thickened with the recent
surprising discovery of tree
samples cut from the landslide
site in 1934 by the late Donald
B. Hamilton of Johns Hopkins
University and the University
of Minnesota before Bonneville
Dam was completed.
Reynolds, along with colleagues
Jim E. O'Connor, a hydrologist
with the Geological Survey in
Portland, and Alex C. Bourdeau,
an archaeologist with the U.S.
Fish and Wildlife Service in
Sherwood, had wondered where
the samples were. The Western
Forestry Center, where some of
the samples had been kept, burned
to the ground in 1964.
But when Reynolds called the
what is now the World Forestry
Center in Portland's Washington
Park, he learned that a crate
of the samples had been stored
in the center's basement. They
apparently had been in a scientist's
lab at OSU when the fire occurred
and were returned when the center
four of us went over there and
they drag these slabs out,"
Pringle said. "We couldn't
believe it. They had four slabs
-- two were from living old trees
and two were from this submerged
forest of the Columbia. We were
Bourdeau took samples for radiocarbon
testing, and Pringle took samples
for tree-ring dating.
that the sample from a submerged
forest tree appears to have died
the same year -- 1699 -- as the
buried tree that he and Schuster
amazed when I found that these
two trees from different sites
had died the same year,"
Pringle said. "It was a
victorious moment. And that 1699
date, almost the same as the
1700 earthquake date, just stopped
finding, however, the mystery
of the landslide date remains
tree-ring and lichen studies
point to a slide date around
1700, the radiocarbon dates O'Connor
obtained from the tree samples
found at the World Forestry Center
indicate that the trees died
we have conflicting evidence,"
Pringle said. "We have our
work cut out for us in trying
to resolve these ambiguities
from the different dating techniques."
he hopes additional research
will help resolve the conflicting
dates. The M.J. Murdock Charitable
Trust recently gave the Washington
Department of Natural Resources
a $14,000 grant that will allow
Rusty Weaver, a science teacher
at Vancouver's Heritage High
School, to work with Pringle
in taking tree-ring samples from
trees that have grown on top
of the landslide.
excited about getting to do this
upcoming work, because it's going
to allow us to collect more evidence
about the date of this slide,"
Pringle said. "It's a question
that befuddles us. So I think
this work will help a lot in
helping us find an answer."
You can reach Richard L. Hill
at 503-221-8238 or by e-mail
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