Fig. 1. This aerial shot shows the debris flows that line the Dakota hogback in the foreground and Mount Sanitas behind it (Photo courtesy of Nate Rock, dated October 3, 2016).

Fig. 2. This is a complex of shallow landslides that is located to the west and upslope of Wonderland Lake in north Boulder. These slides evolved into mudflows allowing long distance downslope transport of rocks as large as 30 cm in diameter.  In the foreground, one can see that clumps of grass remain in the path of the flow, meaning that the flow did not scour to the bed here. The person in the distance (center of the photo, in blue) is standing above a site where two slides/flows merged.  The small scale wrinkle in the landscape below him served as a ponding site for the material, only a small fraction of which spilled over the top to go further downhill. (Photo by Robert S. Anderson).

Fig. 3. This shaded relief map shows the locations of fresh landslides (green dots) along the eastern side of Dakota Ridge in the foothills to the west of Boulder.  Note how the landslides in this area occurred consistently along a particular bedrock boundary and topographic setting. In a reconnaissance survey by the USGS Landslide Hazards Group found that failures were triggered in a wide range of environments, from the High Plains to alpine slopes, and in soils developed in rocks ranging from Paleozoic shales and sandstones to PreCambrian granites and gneisses (see full report).

Fig. 4.  This bedrock geology map shows the variety of rock types or lithologies in the Boulder area.  New landslides are shown as green dots.  The landslides consistently orginated at along a shale/sandstone boundary.

Map Legend:

Yellow (Qls): Landslide deposits, Holocene/Upper Pleistocene
Orange: Colluvium, Holocene/Upper Pleistocene
Turquoise/light blue: Benton shale
Green: Dakota Group or sandstone
Ocean Blue: Morrison Sandstone

Fig. 5. This is a large headscarp of a landslide on the slope to the west of Forrest St. in Boulder. The headscarp is at a sandstone/shale contact. This headscarp is 1.5m high, 9m long, and 8m wide.  The bottom of the headscarp sits about 10 m below the level where sandstone outrcops.  Sand was prevalent and organic material was missing in the cut exposed by the headscarp. There were signs of overland flow (flowing water on the surface) on the sides of headscarp. (Photo by Eric Winchell, dated Sept. 20, 2013).

Fig. 6. This photo from Sept. 16, 2013, shows two landslides that occurred in close proximity to each other. (Photo by Eric Winchell).

Fig. 7. The largest landslide flowed ~150 m downslope and hit a house near Forest Ave in north Boulder.  The landslide had quite a uniform width along its length.  At the time of this photo, Sept. 16, 2013, water was seeping out of the headscarp and for another ~25 downslope (see photo). The headscarp exposes sandstone, which is likely the Dakota Sandstone. (Photo by Eric Winchell).

Fig. 8. Looking up at the scar of the debris flow also shown in Figure 7. Note that the grass is still present in the path of the flow (Photo by Eric Winchell). Slope saturation by water is a primary cause of landslides. In this area, the source of the water comes in the form of intense rainfall, snowmelt, changes in ground-water levels, or a combination of those. Landsliding and flooding are closely connected because both are related to precipitation, runoff, and the saturation of ground by water.

Fig. 9. The headscarp of one landslide, which revealed the roots of this tree. (Photo by Eric Winchell).

Fig. 10.  These two landslides are separated only by a ~1 m wall of sediment. (Photo by Eric Winchell, Sept. 16, 2013).

Fig. 11.  Aerial view of debris flows on Porphyry Mountain west of Jamestown, Colorado.  Porphyry Mountain is a quartz monzonite porplyry intrusion into heavily faulted PreCambrian rocks  (Photo by Nate Rock, Oct. 3, 2013).