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Aspen HighiillUU 51i Ar"" - Draft Environme",a} 1mpa<:/5/01"""'''' undermined several large trees. The water flow on the steep (-80 percent or 400) slopes above and below the catwalk entrained enough debris to be classified as a debris flow (pierson and Costa 1987). and resulted in debris deposition for a distance of at least 480 feet on a gently-sloping bencb about 100 feet downslope from the catwalk. On the steep, sparsely vegetated slopes east of Maroon Creek and ex.tending 1.800 feet downstream from the channel associated with June 15 failure. several colluvium slab failures lI'ansformed into small debris flows. and deposited small cones of debris at the edge of the floodplain. Given the steep. bare. unstable nature of this hilIslope, rapid infllll'ation from local snowpack melting may have triggered these small slides. although on the upper parts of the slopes saturation could have been increased by infiltration from the June 15 debris flow. Another recent landslide can be observed 1,200 to 2.100 feet upstream of the June 15 erosion gully. The depth of the thin colluvium, compared to the slide thickness. suggests that more than superficial debris may have been involved with this slide. The possibility of bedrock landsliding on this oversteepened valley wall is also supported by atypical topography. northeast of the head of this slide. Immediately downslope from the northwest margin of the lower Moment of Truth lI'ail, numerous bedrock ridges and swales nearly parallel contours. These atypicallandfarms. whicb cannot be ex.plained by normal erosion along the strike of nonresistant beds. occur at the head of an unfailed portion of slOpe between the large landslide described above. and a similar zone of failures below the Moment of Truth Catwalk. Based on these landforms. and the presence of two flanking areas that have failed in recent geological time. this area may contain potential slope instability. Golden Barrel Slides Two small landslides occurred in lune of 1995 just east of the Golden Barrel Trail. between elevations of 8.900 to 9.000 feet. The smaller slide is a 9 to 13-foot-high slump along an abandoned road. The slump deposit is about 10 feet wide. about 80 feet long. and instead of crossing the road it turned to flow down it. The bulk of this deposit is contained in a relatively inlacl mass immediately below the failed roadcut. while the rest of the slide deposit is contained in a long tongue of debris that flowed down the old road in a northeasterly directio,n. Downslope of the old road is a classic colluvium debris slide that mobilized into a debris flow. Debris slides of this type are generally attributed to the buildup of a perched water table in the permeable colluvium or regolith overlying less permeable bedrock, whicb results from rapid infi1ll'ation of rainfall or snowmelt. The debris slide scar is 35 feet wide. 40 feet long. 4.5 to 5.5 feet deep and occurred on a 75 percent (370) slope. Downslope (northwest), the slide scar narrows to an 8-foot-wide "throat" aod then widens into a debris fan deposited on the relatively flat ski run. Beyond this primary toe. a more liquid debris flow extends northeast for about 125 feet. The proximate cause of both the Golden Barrel slides appears to be a ditch consll'Ucted in 1994. whicb collects water from a large area above the base of the Exhibition n Lift and discharges it into the forest east of the Golden Barrel Trail. The flow path of this ditcb water can be traced downslope to just east of the site of the roadcut slump, and approximately upslope of the debris slide. Adjacent unfailed slopes were able to maintain stability, despite the rapid melt of snowpack, because there was no concentrated source of surface water to cause complete regolith saturation. In the Leadville family of soils found in this area. higher clay content presumably limited permeability and inhibited lI'ansport of the high volumes of inflltrating ditch water. whicb then resulted in high water pore pressures and slab failure. DEBRIS FLOWS The existence of continuous alluvial fans and debris fans at the base of the upper Aspen Highlands ridge proves that these fans. and the slopes above them. have been subject to debris flows and debris avalanches. In Figure 3.1. all alluvial and debris fans have been depicted as debris-flow prone and debris-avalanche prone slopes. This underestimates the areal extent of the debris-flow and debris-avalanche processes in the source zones and in intermediary channels. Under extreme precipitation. almost aoy colluvium mapped in a drainage basin fronted by an alluvial fan could be mobilized into a debris flow. 3-6 Geology aM Soil. Gt!ologic HozOTtU