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

On July 28, 1999, about 480 debris flows were triggered by an
afternoon thunderstorm along the Continental Divide in Clear
Creek and Summit counties in the central Front Range of
Colorado. This cover shows the initiation locations of the
debris flows. Jonathan Godt and Jeffrey Coe mapped the debris
flows from 1:12,000 scale color aerial photography using 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 1:12,000
scale. The mapped landslides were digitized manually in
ArcInfo. The final scale of the data are 1:24,000. 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.