Rockfall/rockslide in 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 Google Earth.
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 page.)
picture above (via Google Earth) gives a close-up view of the toe of the
rockfall/rockslide up against the edge of the undamaged forest.
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.
NASA photos 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
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
USFS estimates that the rockfall/landslide occurred in the spring of
2004. The following description is included in the USFS report:
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 rockfall area.
(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
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.
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
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 rockfall.
Geology of the Rockfall Area
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 “Landslide deposits”.
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