Results of Investigation of Erosion at the Petroglyph National Monument, New Mexico

Gellis, Allen

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 IT, 6 to Class ill, and 1 to Class IV. 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 channelize 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 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 the 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. Although the gullies may not be a direct threat to petroglyphs, increased runoff from dirt roads could increase erosion on rocks containing petro glyphs. The effects of gullying may have a greater effect on the aesthetics of the monument and the residences located down gradient from a gully.