Wednesday, February 9, 2022 The Nugget Newspaper, Sisters, Oregon
BULGE: Rate of uplift is
increasing but <there9s
no imminent threat=
Continued from page 1
making its way to the sur-
face and poised for an erup-
tion,= he elaborated, adding,
<We9re not seeing any of that
right now.=
The last eruption on any of
the Three Sisters was a mere
2,200 years ago 3 yesterday
in geologic time. The strato-
volcanoes were constructed
between 150,000 and 50,000
years ago for North Sister
and 50,000 to 15,000 years
ago for Middle and South.
What USGS is seeing
with the aid of satellite imag-
ery is an upward shift in the
ground of less than an inch,
across a 12-mile area west of
South Sister. One might ask
if it9s even worth mention-
ing. Maybe. Maybe not. But
if USGS finds it interesting,
then by extension so do we,
yet in a wider context.
There are moraine-
damned lakes on the Sisters,
some of which hold a lot of
water. These freshwater lakes
are formed behind a moraine
or ice dam. One in particu-
lar, Carver Lake on the east
side of South Sister about
12 miles from town, could
be breached in the event of a
major seismic event. Should
we worry?
Not seriously, according
to scientist David George,
who studies in detail things
most would find tedious.
George notes that the outlet
channel for Carver is a small
tributary of Whychus Creek.
Models, however, illustrate
that a big chunk of town
could be under anywhere
from .05 to .25 meters (1.95
to 9.75 inches) of water, and
in areas close by Whychus
and smaller creeks, double
that.
That would seem fairly
catastrophic to the untrained
o b s e r v e r. B u t G e o rg e
explained in a seven-page
technical report: <Compared
to earlier studies employing
1-D equations and coarse
topography, our modeling
suggests a strikingly different
result for our test case involv-
ing inundation near Sisters.
Owing in part to the use of
2-D equations as well as
high-resolution lidar topogra-
phy, our results suggest that
flow avulsion and diversion
on the alluvial fan surround-
ing Sisters would lead to a
less severe flood hazard to
the community.= (Emphasis
added.)
Seismic activity like this
also gets the attention of the
Forest Service, who main-
tains two earthen dams at
Upper and Lower Three
Creeks Lake. Sisters District
Manager Ian Reid tells The
Nugget that it is only coin-
cidental that these dams are
under review in 2022.
<It9s part of a routine
inspection and analysis that
we undertake every so many
years,= he said. <These are
old dams.=
The larger of the lakes is
28 acres and is 28 feet deep
in the center. That equates to
about 230 million gallons of
water. Reid says that water,
if breached, would not find
its way to the city. And while
old and under observation,
there is nothing to suggest an
imminent break.
Officials regularly warn
of a potentially devastating
earthquake at the Cascadia
Subduction Zone just off
the Oregon coast. Central
Oregon is affected by the
same subduction zone, which
created the Cascades millions
of years ago. At the Cascadia
Subduction Zone, the Juan
de Fuca Plate is being sub-
ducted under the North
American Plate, creating the
potential for both quakes and
eruptions.
When that happens, the
plate goes down and cre-
ates lava, magma. Below the
Cascades, there are magma
chambers. It9s simple: the
Cascade Mountain Range is
an active volcanic area, and
activity occasionally reminds
us that we live among
volcanoes.
21
Base from U.S. Dept. Agriculture orthophototgraph, 2014. Cultural features modified from
U.S. Geological Survey and State of Oregon digital data, various scales.
Universal Transverse Mercator Zone 10 projection. North American Datum of 1983.
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