Geologic And Hydrologic Factors Governing ... - Gunnison County

Geologic And Hydrologic Factors Governing Impacts Of Development
On The Crystal River Near Marble, Colorado
Gunnison County, Colorado
the slide began to move rapidly and erratically (as much as several feet of movement were observed
in one day). Townspeople immediately grasped the potential of a major block of Mancos Shale fal-
ling into the stream, temporarily creating a dam. The dam could quickly overtop and plunge a mud
and debris flood into the Marble town site. Unpalatable choices faced the decision-makers in pre-
paring for the possible event. The lower channel flow might be diverted to either the east or the
west, thus condemning that part of the remaining town site, or the channel could be left alone in
hopes the flow would remain in the present channel and harmlessly cover that portion of the town
site previously destroyed in the 1941 and 1945 mudflows. Fortunately, the wet period ended before
total collapse of the slide mass, and it returned to slow, periodic movement. Hopefully, Carbonate
Creek’s continued erosion of the toe may periodically remove small portions of much of the slide
mass and avoid a possible catastrophic release. One must be aware that the risk still exists. Some-
time in the future a large slide could dam the canyon of Carbonate Creek. Overtopping of that dam
could cause a catastrophic debris flow into the town site. A large debris flow could easily plug the
present channel and divert itself either to the east or west.
3.2.2 Potentially Unstable Slopes
A large typical area of unstable slopes (Appendix A, Figure 4, Location 1f) has been mapped on the
slopes northeast of the town site. Although we saw little evidence of past failures during the site vis-
its (Fall 1995), the steep slopes, weak, severely-jointed Mancos Shale bedrock and spring snow melt
saturation indicate serious potential slope stability problems. The prediction of Rogers and Rold
(1972),
“Most slopes in this part of Mt. Daly range form 30% to 60%. Excavation of any cut
slopes which will have the effect of steepening existing slopes and daylighting weak
surficial layers will pose serious long-range stability problems.”
came to pass May 14, 1975. A section of new road approximately 150 feet long and part of two
condominium sites released as a wet landslide. Incorporation of additional runoff water quickly
converted the material to a debris flow which poured rapidly down the mountainside into Beaver
Lake. Observers in the valley reported hearing a grinding, rumbling sound and then being treated to
a dramatic display of violent geologic processes that lasted only a few minutes. The lesson was not
951-110.000 Wright Water Engineers, Inc. Page 15