Gullying at the Petroglyph National Monument, New Mexico

Gellis, Allen

Fifty gullies were mapped along a 32 km (20 mi) escarpment in the Petroglyph National Monument, New Mexico, of which 14 were located along dirt roads, 7 along foot and bicycle trails, 18 on undisturbed hillslopes, and 11 part of a natural channel network. The dirt roads and foot and bicycle trails channel surface runoff and increase erosion. Thirty of 50 gullies assessed received runoff from the dirt roads. Ten gullies mapped in 1991 were not evident on 198 7 aerial photographs and may have been formed during 198 7 to 1991. Natural factors, such as high-intensity rain storms during the intervening period, were thought to have initiated or increased erosion and gullying in the National Monument. Analyses of storm data during this period indicated that most storms had recurrence intervals of 2 years or less for rainfall intensity, measured in millimeters per 30-minute, 1-hour, and 2-hour durations. An exception was a storm on july 28, 198 7, that for a 1-hour period had a recurrence interval of 25 years. However, the rainfall for this >tonn was highly localized, falling in the southern portion of the monument, and not in the vicinity of the 10 new gullies. Results from this study show the sensitivity of semitirid landscapes to certain forms of development, mch that storms with recurrence intervals of 2 years or less might cause considerable erosion.

Gullying at the Petroglyph National Monument, New Mexico

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