DEBRIS FLOW DISTRIBUTION AND HISTORY IN COLORADO

Colorado's mountain and plateau regions contain numerous alluvial fans upon which debris flows have occurred during the Holocene Epoch. Relatively small-sized alluvial fans are found throughout the area at the mouths of first- and second-order drainages. The largest alluvial fans are found along the margins of mountain blocks that have experienced significant tectonic uplift since Neogene time; an example is the Mt. Blanca Massif along the eastern edge of the San Luis Valley, which is flanked by coalescing alluvial fan complexes having over 600 m of topographic relief. These fans have been modified by Pleistocene glaciation and outwash, and may date back to the post-Oligocene initiation of normal faulting along the Rio Grande Rift. Holocene streams have largely bypassed these larger fans through incised channels, and the recent fan deposits are found in localized areas near either the apex or distal portions of the complexes. Climatically, most of Colorado's alluvial fans are alpine-type fans, and may be influenced by snowmelt as well as rainfall.

As a geologic hazard, debris flows have affected Coloradoans throughout historical time. There have been two major periods of time in which debris flows have particularly affected the state's occupants. The first was during the mining era, from about 1859-1890, when a sizeable percentage of the state's population lived in the mountains. The second wave of debris flow and human interactions began in the 1960s and continues to this day, as more people move to the mountain and plateau regions to work, recreate, or retire. Many Colorado towns have a long history of debris flow episodes. Glenwood Springs, for example, has had over 17 episodes since 1900. The town of Marble was nearly destroyed by two debris flow episodes during the early 1940s. Other towns that have been affected by debris flows through time include Aspen, Vail, Ouray, Telluride, Colorado Springs, Manitou Springs, Idaho Springs, and Georgetown.

Debris flows create hazardous conditions along Colorado's highways as well. Two large debris flows near the Eisenhower Tunnel closed Interstate 70 for 25 hours in 1999. Numerous other highways in the mountain and plateau regions of the state have been closed at times by recurrent debris flows. Post-wildfire debris flows have become a recognized concern in Colorado since the 1990s.