Erosional retreat of a river-damming debris flow on the Rio Grande, Taos County, New Mexico
— P. W. Bauer

The 22 km (14 mi) stretch of NM-68 along the Rio Grande, between Velarde and Pilar, skirts the base of notoriously unstable slopes of Quaternary landslides and fractured Proterozoic bedrock and talus. Rockfalls, rockslides, and debris flows are common hazards to motorists, especially along the Pilar cliffs. On July 25, 1991, an intense rainfall event spawned a series of mass wasting events that closed the highway for 19 hours. A 3600 m³ (4700 yd³) debris flow at Sleeping Beauty rapid temporarily dammed the Rio Grande and changed the fluvial geomorphology. A 12-year photographic record of the erosional retreat of the debris flow documents a relatively rapid two-year modification of the debris flow, followed by 10 years of minor adjustments to the surface and slope of the flow. Apparently, the peak flows of the highly constricted river had the carrying capacity to move cobble-sized bed load out of the channel until approximately half the channel was clear of the debris flow. At that point, even a peak flow of 7000 cubic feet per second (cfs) could do little to modify the remainder of the debris flow.