the Rio Chama Headwaters
A large rockfall occurred in a remote area of the
upper Rio Chama headwaters sometime between Sept. 1998 and June
2005. There appears to be no available information about the
rockfall, and the author happened to find it while browsing with
The two Google Earth pictures below show the area
before and after the rockfall. The rockfall itself is in
Colorado’s southern San Juan Mountains about 14 miles north of
Chama, NM. (Rockfall is at Lat. 37.11 N Long. 106.59 W) The
rockfall/landslide fell about 1,400 vertical feet, and the runout
covered a little over half a mile.
The picture above is a Google Earth view that looks north over the
West Fork of the Rio Chama as of Sept. 26, 1998.
The picture above shows the same area, but seven
years later in 2005. In the interim, a large block of rock broke
loose from the cliffs and avalanched down through the forest
below. It appears that the rockfall took place sometime between
Sept. 29, 2003 and June 3, 2004. (See NASA photos further down the
The picture above (via Google
Earth) gives a close-up view of the toe of the
rockfall/rockslide up against the edge of the undamaged
As the rockslide plowed into the forest, it
snapped off trees at their base. The momentum of the rockslide
would shove the base of a tree forward while the rest of the tree
would topple backward onto the rock rubble. Some of the broken
trees appear to have been carried several hundred feet from their
The picture above is another Google Earth view that
looks straight down at the rockfall/rockslide. The green line is
the boundary between Archuleta County to the left (west) and
Conejos County to the right (east). The east to west field of view
is about 6 miles.
confine time of the Rockfall
The two photos below are from NASA’s Visible Earth
website and confine the timing of the rockfall to sometime between
Sept. 29, 2003 and June 3, 2004.
For location purposes, in the above Google Earth
picture, note that the rockfall is on the north side of the valley
of the West Fork of the Rio Chama. The mountains surrounding the
valley have a horseshoe shape – open to the southeast. This same
dark green, horseshoe shaped valley appears in the center of both
of the NASA pictures below. (The arrows point toward the dark
green valley.) The east to west field-of-view in both pictures is
about 106 miles.
The first NASA picture (below) has pretty good
resolution with no sign of the rockfall as of Sept. 29, 2003. If
the rockfall had taken place by Sept. 2003, the picture would show
a gray blotch in the western end of the valley.
Original picture accessible at: http://visibleearth.nasa.gov/view_rec.php?id=5885
High resolution at: http://eoimages.gsfc.nasa.gov/ve/5885/Utah.A2003271.1810.250m.jpg
The resolution in this 2nd NASA picture is not as
good, but if you look closely, there is a faint light gray streak
in the western (left) end of the valley. By itself, the picture
isn’t clear enough for definite confirmation. However, the picture
by “nmnc” as of August 2004 (see below) does indicate that the
rockfall occurred before Aug, 2004.
NASA picture date: June 3, 2004
Original picture accessible at: http://visibleearth.nasa.gov/view_rec.php?id=6889
High resolution at: http://eoimages.gsfc.nasa.gov/ve/6889/UnitedStates.A2004152.1740.250m.jpg
Note: the horizontal line in both NASA pictures is the
Colorado/New Mexico border.
A third NASA picture as of Nov. 19, 2003 is
accessible at http://visibleearth.nasa.gov/view_rec.php?id=6104
(high resolution at http://eoimages.gsfc.nasa.gov/ve/6104/UnitedStates.A2003322.2020.250m.jpg
), but it is difficult to differentiate between winter snows and
the possible rockfall/rockslide.
The photograph above was taken by John Rawinski, and published in
the U. S. Forest Service’s:
Monitoring and Evaluation Report
Rio Grande National Forest, Colorado
The USFS estimates that the rockfall/landslide occurred in the
spring of 2004. The following description is included in the USFS
In June 2004, a trip was made into
the West Fork Rio Chama to investigate a new 40 acre landslide,
that originated under natural conditions. A huge block of
Conejos Formation conglomerate separated from a ledge and plowed
a ¼ strip in its path below. It cause a fish kill in the stream
killing Cuttthroat and Rainbow trout.
Topo USA Pictures
The next two pictures were generated by Delorme’s ( http://www.delorme.com/
Topo USA program. The first picture is a topographic map of the
(Click on the map for a larger, more detailed view)
The picture above is a topographic map of the
rockfall area. The county line (mentioned previously) is on the
right side of the picture while the red dashed line traces the
Continental Divide Trail. The rockfall started when part of the
cliff in the center of the picture broke loose with the resulting
landslide wiping out the trees below the cliff. (The topo map
shows the area before the rockfall – there’s no swath through the
trees.) If you check the larger detailed map view, the lower end
of the rockslide covers the “Rio Chama” portion of the “West Fork
Rio Chama” label.
The picture above was also generated by Delorme’s
Topo USA program and shows a 3D view of the area. The rockfall
occurred on the steep cliff near the center of the picture.
Google Earth can also display photographs that have
been uploaded. A photo taken by “nmnc” about Aug. 1, 2004 appears
to show the rockfall. (See http://www.panoramio.com/photo/16408766
The photograph appears to have been taken some 3.2
miles SE of the rockfall, and looks toward the NW. The
light gray streak on the mountain in the center distance is the
Geology of the
The picture below is an excerpt from Ogden Tweto’s
Geologic Map of Colorado. The printed version of the map is
clearer, but a free online version can be downloaded from
The rockfall covers the “T” of the “Tial” label just below the
center of the picture.
Light purple areas (Tpl) on the map are 30 to 35
million-year-old “Pre ash flow andesitic lavas, breccias, tuffs
and conglomerates”. In the Google Earth pictures, this layer
includes the cliff. The “Tpl” layer is capped by the younger, red
“Taf” layer which is more ash-flow tuff, but only 26 to 30 million
years old. The “Tial” label points to “intra-ash flow andesitic
lavas” which cap the ridge to the southwest of the valley. (Not
visible in the first two Google Earth pictures.)
After the volcanic layers were uplifted to
approximately their present elevation, the area was heavily
glaciated over the last 2 million years. Glaciers typically carve
out “U” shaped valleys with steep sides. Glacial erosion at the
base of these steep sides can over-steepen them to the point where
they are unstable.
As the glacial ice melted away, it left rock rubble
in its wake. Also, the unstable, overly steepened sides
occasionally collapse – as seen in the Google Earth pictures.
Other nearby Google Earth views show the remains of other
rockfalls/landslides beside the one highlighted in the pictures.
The last geological unit of interest (as depicted in
the geological map) is the stippled light blue “Ql” zone that
appears in the valley underneath the “Tial” label. The “Q” stands
for Quaternary (recent) age. The “l’ as you might guess, indicates
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