U.S. Geological
Survey
Open-File Report 01-424
By
INTRODUCTION The surficial
geologic map of lower Comb Wash was produced as part of a masters
thesis for Northern Arizona University Quaternary Sciences program. The
map area includes the portion of the Comb Wash alluvial valley between
Highway 163 and Highway 95 on the Colorado Plateau in southeastern Utah.
The late Quaternary geology of this part of the Colorado Plateau had not
previously been mapped in adequate detail. The geologic information in
this report will be useful for biological studies, land management and
range management for federal, state and private industries. Comb Wash
is a south flowing ephemeral tributary of the San Juan River, flanked
to the east by Comb Ridge and to the west by Cedar Mesa (Figure 1). The
nearest settlement is Bluff, about 7 km to the east of the area. Elevations
range from 1951 m where Highway 95 crosses Comb Wash to 1291 m at the
confluence with the San Juan River. Primary vehicle access to lower Comb
Wash is provided by a well-maintained dirt road that parallels the active
channel of Comb Wash between Highway 163 and Highway 95. For much of the
year this road can be traversed without the aid of four-wheel drive. However,
during inclement weather such as rain or snow the road becomes treacherous
even with four-wheel drive. The Comb Wash watershed is public land managed
by the Bureau of Land management (BLM) office in Monticello, Utah. The semi-arid
climate of Comb Wash and the surrounding area is typical of the Great
Basin Desert. Temperature in Bluff, Utah ranges from a minimum of 8°
C in January to a maximum of 35° C in July with a mean annual temperature
of 9.8° C (U.S. Department of Commerce, 1999). The difference between
day and nighttime temperatures is as great as 20° C. Between 1928
and 1998, annual rainfall in Bluff averaged 178 mm per year (U.S. Department
of Commerce, 1999). Annual rainfall in Comb Wash averaged 240 mm per year
from 1991 to 1999 while Bluff received an average of 193 mm for the same
8 year period. Most precipitation is monsoonal, convective storms that
bring moisture from the Gulf of Mexico beginning in early July and ending
by October. Large frontal storms during December and January are responsible
for most winter precipitation (Figure 2). The record from U.S. Geological
Survey gauging station number 09379000 operated by the BLM from 1959 through
1968 indicates that Comb Wash flows in direct response to precipitation
events. Most daily discharge and peak events occur in late July through
September, coinciding with high intensity monsoon thunderstorms. Comb Wash
supports a variety of vegetation typical of the Great Basin Desert and
the northern desert shrub zone as described by Fowler and Koch (1982).
On the lower alluvial terraces, bushes and shrubs dominate the vegetation,
including: sagebrush (Artemesia tridentata), rabbitbrush (Chrysothamnus
nauseosus), fourwing saltbush (Atriplex canescens), winterfat (Eurotia
lanata), greasewood (Sarcobatus vermiculatus), and shadscale (Atriplex
concertifolia). Juniper trees (Juniperus osteosperma) can be found on
the rocky colluvial slopes near Comb Ridge and on the higher terrace near
Cedar Mesa. The floodplain contains an abundance of riparian vegetation
including cottonwood (Populus fremontii), willow (Salix exigua), and tamarisk
(Tamarix ramosissima). Tamarisk is one of 7 non-native species present
in the lower Comb Wash watershed. At least seven known species of noxious weeds have invaded the watershed, including Bermuda grass (Cynodon dactylon), field bindweed (Convolvulus avensis), Canada thistle (Cirsium arvense), Russian knapweed (Centaurea repens), tamarisk and camel thorn (Alhagi pseudalhagi). Of these, tamarisk or salt-cedar has most aggressively colonized the southwestern United States, including the San Juan watershed. Graf (1978) estimates that since the late 19th century, tamarisk has spread at a rate of 20 km per year. Tamarisk first appeared in Comb Wash during the mid to early 20th century based on photographs taken by Gregory in the early 1900s (Gregory, 1938). |
File
Name
|
File
Type and Description
|
File
Size
|
README's
and METADATA
|
||
ASCII
text file of FGDC-compliant metadata
|
24
KB
|
|
ASCII version
of readme file that explains how to use the digital database |
||
Portable
Document Format (PDF) version of the readme file that explains how to
use the digital database.
|
||
DATA
|
||
Unix tar file containing the digital database |
1.6
MB
|
|
FILES
FOR VIEWING AND PLOTTING
|
||
Encapsulated
PostScript file of the Comb Wash map for plotting. The map is approximately
35 x 52 inches.
|
23.5
MB
|
|
Portable
Document Format file of the Comb Wash map for viewing
|
724
KB
|
|
PDF file of the explanatory pamphlet that accompanies the geologic map |
Download a free copy of Adobe Acrobat Reader version 5.0 For questions about the content of this report, contact Claire Longpré |
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| Department of the Interior | U.S. Geological Survey | Geologic Division | Earth Surface Processes |
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is only available on the web