Map showing alpine debris flows triggered by a July 28, 1999 thunderstorm in the central Front Range of Colorado: Debris-flow initiation points

The debris-flow source areas, travel paths, and deposits were mapped using 1:12,000-scale color aerial photographs and a Kern PG-2 photogrammetric plotter. The photographs were scaled and oriented to the topographic base map using prominent topographic landmarks and plotted on a transparent polyester overlay registered to the topographic base maps at a scale of 1:12,000. The mapped debris flows were digitized manually in ArcInfo. Debris-flow initiation locations were defined as the points farthest upslope for each mapped debris-flow. In many cases, single mapped debris flows have multiple initiation locations, some of which are not visible at 1:24,000 scale. For soil slips, the initiation point was easily identified as the center of the headscarp of the landslide failure. However, for the non-soil slip category, the initiation locations in the database may represent multiple small rills. The initiation locations of the fire- hose flows are the highest point in the bedrock channel that appeared freshly scoured in the aerial photographs. See Open- File Report 03-050 for description of debris-flow initiation processes.