Florida River, Colorado
Nearly two-dozen shallow landslides were active during spring 2005 on a hillside located along the east side of the Florida River about one kilometer downstream from Lemon Reservoir in La Plata County, southwestern Colorado. Most of the area where the landslides occurred was burned during the 2002 Missionary Ridge wildfire. Shallow landslides on the hillside appear to be much more likely following the Missionary Ridge fire because of the loss of tree root strength and evapotranspiration. The US Geological Survey Landslide Hazards Project has performed geologic mapping, subsurface exploration and sampling, radiocarbon dating, and shallow ground-water and ground-displacement monitoring to assess landslide activity. Active landslides during spring 2005 were as large as 35,000 m3 and confined to colluvium. Debris flows were mobilized from most of the landslides, were as large as 1,500 m3, and traveled as far as 250 m. Landslide activity was triggered by elevated ground-water pressures within the colluvium caused by infiltration of snowmelt. Landslide activity ceased as ground-water pressures dropped during the summer.
Location map of Florida River, CO landslide monitoring
Landslides triggered during spring 2005 occurred within a 1.97 x 107 m3 older landslide that extends, on average, about 40 m into bedrock. Radiocarbon dating of sediments at the head of the older landslide suggests that the landslide was active about 1,424-1,696 years ago. This landslide appears to have crossed the valley floor and been subsequently eroded from this area. We found no evidence that landslide debris across the valley floor formed an impoundment of the Florida River, although it is very likely. Erosion of buttressing landslide debris from the valley floor and the lower strength of the landslide basal shear zone relative to pre-slide strength created less stable conditions than were present prior to occurrence of the landslide. However, deep ground-water conditions largely control the stability of the slope and are unknown here; hence, the potential for future deep landsliding is unknown.
To reduce potential hazardous effects of landslides in this area, the U.S. Geological Survey Landslide Hazards Project has installed instruments to monitor rainfall, air temperature, ground-surface displacement, and orientation of the shallow subsurface. Instrument measurements are made every 10 minutes. Measurements are transmitted to office computers, and graphs are updated and posted to the Internet on a daily basis. Internet postings may be interrupted occasionally by computer or network malfunctions.
For more information
Contact Information
William Schulz
wschulz [at] usgs [dot] govU.S. Geological Survey
Geologic Hazards Team
Box 25046, MS 966
Denver, CO 80225
