HISTORIC LANDSLIDES IN THE SAN JUAN MOUNTAINS, SOUTHWESTERN COLORADO

Historic, non-forest-fire related landslides (debris and earth flows, and rock, debris, and earth slides and slumps) in the San Juan Mountains have occurred in various geologic terrains and have had at least one of three triggers: summer rainstorms, winter or spring snowmelt, or the cutting of slopes by natural or human activity.

Debris flows in the San Juans have originated on steep slopes or in channels that were mantled by loose, easily erodible materials. More than 90% of debris flows were triggered by rainstorms in July and August, storms that are commonly associated with moisture from the North American monsoon. Rainfall data indicate that activity was triggered when rainfall exceeded about 13 mm/hour. Areas subject to multiple historic debris flows include drainage basins above Ouray, along Highway 550 near Red Mountain Pass (including some snow-avalanche basins), along Highway 145 in and near Telluride, and along Highway 160 west of Wolf Creek Pass.

Non-debris-flow types of landslides in the San Juans have occurred on moderate to steep slopes mantled by colluvium, glacial, or landslide deposits, and underlain by Tertiary volcanic rocks or Jurassic and Cretaceous sedimentary rocks. The causes of these landslides were about evenly divided (25% each) between prolonged rainfall, snowmelt, cutting of slopes, and other or unknown means. Examples of these landslides include: two earth flows and one debris slide south of Cimarron, the Ames earth flow, the West Lost Creek Trail rock slide/debris avalanche, the Slumgullion earth flow, which moves continuously and dumps acidic water (pH of 2-4) into Lake San Cristobal, a catastrophic failure of a rock glacier in Barlow Creek near Rico, slides and slumps in Mesa Verde National Park, a large rock slide north of Durango that was used as a firebreak during the 2002 Missionary Ridge fire, the Moving Mountain landslide that may have been triggered by an underground coal fire, and the Jackson Mountain, Carbon Mountain, Animas City Mountain, and Rymen Creek landslides.

Productive areas of landslide research in the San Juans include defining rainfall thresholds for triggering landslides, quantitative analyses of landslide hazards, modeling the influence of climatic variations and stream incision on landslide occurrence, and studies on the environmental impacts of landslides.