THE WEST SALT CREEK ROCK AVALANCHE: A HIGHLY MOBILE, COMPLEX LANDSLIDE IN WESTERN COLORADO

On May 25, 2014, a 30 x 10⁶ m³ rock avalanche–rotational rock slide–debris flow occurred in the West Salt Creek drainage about 10 km southeast of the town of Collbran, Colorado. The avalanche occurred during snowmelt, about 3.25 hours after 0.5 hour of rainfall with an intensity of 21 mm/hr. The avalanche mobilized from a pre-existing rock slide on the northern flank of Grand Mesa and traveled about 5.4 km down the confined drainage. Two ridges were overtopped by the avalanche. Estimated velocities exceeded 80 km/hr. Three men working in the drainage were killed by the avalanche. According to the landslide mobility index H/L, where H is the maximum elevation traveled, and L is the maximum length traveled, the avalanche was highly mobile with H/L = 0.10. Early movement was documented between 0930 and 1400 MDT, when local residents noticed disrupted irrigation water and falling trees on the face of the old rock slide. Seismic data indicate that catastrophic movement began at about 1744 and lasted about 3 minutes. Currently, a large, back-rotated, rock-slide block remains at the top of the avalanche. This block has obstructed the flow of West Salt Creek, forming a lake between the block and the headscarp.

High-resolution imagery and digital-topographic data from Unmanned Aerial Vehicle (UAV) and LiDAR acquisitions are being used as base materials for ongoing, 1:1000-scale field mapping of landslide geology and structures. This mapping shows that most of the avalanche is composed of rotated, broken, and brecciated Eocene Green River Formation; and basalt-rich, Quaternary colluvium and/or landslide deposits. The basalt-rich deposits formed distinct flow bands on the surface of the avalanche. The chronological sequence of movement was complex, with at least 5 phases: a debris flow; the catastrophic, high-energy rock avalanche, including the rotational rock slide; movement of the hummock-rich, central core of the avalanche deposit; a second debris flow; and ongoing movement of the upper central core. Ongoing potential threats to downstream residents include: the catastrophic failure of the rock slide block, a large failure upslope from the headscarp, a rapid release of water from the lake, and rapid or slow movement of the avalanche deposit. Ongoing instrumental monitoring provides near real-time information on these threats.