Areas of the Petroglyph National Monument, specifically those
located along the West Mesa escarpment, are being affected by erosion
and gullying. A reconnaissance along the 17-mile-long escarpment
identified 50 gullies. The gullies were given a qualitative ranking of
Class I, least erosion, to Class IV, highest erosion. Of the 50 gullies
identified, 21 were assigned Class I, 22 to Class II, 6 to Class III,
and 1 to Class IV. Although the gullies may not be a direct threat to
petroglyphs, the effects of gullying may have a greater effect on the
aesthetics of the monument and the residences located downgradient from
a gully.
Most of the gullies were found along the northern part of the
escarpment. This area, which is more developed than the southern areas
of the escarpment, contains many dirt roads and nonpaved foot and
bicycle paths. These features channel surface runoff and increase
erosion. Thirty of the 50 gullies were noted as being connected to the
runoff from dirt roads.
High-intensity storms during the summer of 1991 may have caused or
increased gullying. Analyses of these storms indicate recurrence
intervals of rainfall of no more than 2 years. Indirect measurements of
peak discharge in La Boca Negra Arroyo after the August 22, 1991, storm
indicate that this runoff event may have a frequency of no more than 10
years. Regional frequency reports on rainfall and data collected at the
rain gages indicate that gullying and erosion that occurred during the
summer of 1991 were not a result of infrequent rainfall or runoff
events.
