Description:
This report summarizes the geology and geomorphology of 12 small watersheds in the
Peloncillo Mountains, southwestern New Mexico. The study site is located in the central
portion of the Malpai Borderlands project area, on the eastern piedmont of the southern
Peloncillo Mountains (Figure 1). It consists of an eastward trending ridge with small
dissected basins on either side, and is bordered on the north and south by Walnut and
Whitmire creeks, respectively. These creeks flow eastward into southern Animas Valley.
The U. S. Forest Service has selected 12 watersheds along the ridge (Figure 2) for a
paired watershed experiment to compare effects of two burning seasons, spring and
summer, on native vegetation, wildlife, surface water flow, slope stability and erosion (G.
Gottfried, US Forest Service, pers. comm., October 2000). As part of this experiment,
the Forest Service is installing a sediment trap and two different-sized flumes on each
study watershed to record and compare sedimentation and surface flows before and after
burning. One flume will be located about 50 feet downstream from the sediment trap and
will record higher volume flows of up to 57 cubic feet per second (cfs). Another flume
will be located 18 feet further downstream and will record lower volume flows of up to 4
cfs (G. Gottfried, US Forest Service, pers. comm., October 2000). A major concern with
this study area is that local geology may provide pathways for surface water to infiltrate
and flow between study watersheds, causing erroneous runoff results, or to
circumnavigate the flumes altogether. Another concern is a potential increase in erosion
rates, or decrease of slope stability, after burning. The purpose of this report is to
describe the geologic and geomorphic framework of the study area, to delineate surficial
deposits to help define potential erosion, and to provide information concerning potential
for sub-surface flow between watersheds or under the flumes. Field investigations and
mapping were done in November, 2000, and January, 2001. A 1:12,000-scale map
showing bedrock and surficial geology is included with this report.