PERMFILE70463 - Laserfiche WebLink

Home Browse Search

2004 Geologic Hazard Field Observations South of Divide Mining Area • Figure 4 (the linear, light-colored zone), 3) a tongue-like debris flow/mudflow deposit that fills the bottom of Dry Fork for a distance of some 3,000 feet (Map 1). Based on a stereoscopic observations of the 1963 and 2004 aerial photos, the shear zone was most likely caused by differential landslide movement due to the buttressing affect of the Minnesota Reservoir earth-fill dam (Monument Dam). On the west of the shear zone, the northward landslide movement was reduced by the lateral support provided by the dam, whereas, on the eastward side of the shear zone, the landslide could move more freely towazds Minnesota Reservoir. 2. Two smaller landslides occur on the north side of the reservoir: 1) one in the abutment area of the dam (Figure 2) and 2) the other west of the mouth of Horse Gulch (the south- flowing stream that flows into the approximate middle of the reservoir shown in Figures 4 and 5). • The lower altitude vertical aerial photograph of the Minnesota Reservoir area (Figure 5), shows these significant differences that have occurred over the last 41 years; 1. Renewed landslide activity occurred in 1987 west of the reservoir, and removed a section of the Dry Fork road (the fresh-looking slide at the sharp double bend in the road in Figure 5). 2. The debris flow/mudflow in the Dry Fork valley now extends an additional 600 feet further downstream towards the mouth of Pond Gulch, compared to its end position in 1963. 3. The shear zone, and adjacent landslide to the west, located south of the dam of Minnesota Reservoir appeazs to be less active, and more vegetated, than it was in 1963. 4. Although the slide on the north dam abutment shows little or no change, renewed activity occurred in parts of the slide to the west of the mouth of Horse Gulch in the mid 1990s. 831-032.621 November 2004 Wright Water Engineers, Inc. Page 24