The Debris Flow of September 20, 2014, in Mud Creek, Mount Shasta Volcano, Northern California

Abstract:

The debris flow in Mud Creek on September 20, 2014 occurred during a warm spell at the end of an unusually long and hot summer. No precipitation was recorded during or immediately before the event, and it appears to have resulted from rapid glacial melt. It initiated on the toe of the Konwakiton Glacier, and immediately below it. The flow track was small in the upper parts (40 feet wide), but between 8,000 and 10,000 feet in elevation, it entrained a large volume of debris from the walls and bed of the deeply incised gorge and transported it down to the apex of the Mud Creek alluvial fan (4,800’). At that point, it overflowed the channel and deposited debris on top of older (1924) debris flow deposits, and the debris plugged a road culvert 24 feet wide and 12 feet high. A small fraction of the flow was diverted to a pre-existing overflow channel which parallels Mud Creek, about 1,000 feet to the west. The main debris flow traveled down Mud Creek, confined to the pre-existing channel, but locally got to within a foot or so of overflowing the banks. At elevation 3920’, video was taken during the event by a private citizen and placed on YouTube. The video revealed that the flow matrix consisted of a slurry of water/clay/silt/sand/gravel, transporting boulders 1-6 feet in diameter along with the flow. Cobble-sized rock appears to be absent.

Sieve analysis of the debris flow matrix material revealed a fining of particles in a downstream direction, as expected. The thickness of deposits on the fan generally decreased in a downstream direction. Deposits were 5-6 feet deep above the Mud Creek dam, which is at 4,800’ elevation, and 4-5 feet deep at the dam itself. Further downstream, thicknesses decreased as follows: 3920’aqueduct crossing, 3-4 feet; 3620’ Pilgrim Creek Road crossing, 2-3 feet; 3,520’, 1-2 feet; 3,440’ abandoned railroad grade, 1 foot. 

This event damaged roads, and future events could threaten life and property. There is a need to better understand local glacial melt processes to predict such events. USGS LiDAR (2011) is available, and the US Forest Service will acquire LiDAR for Mud Creek later in 2015. These data sets will allow a quantitative measure of changes in the glaciers and channel network, as well as the amount of erosion and deposition which occurred in 2014. This information will facilitate a sound assessment of potential future hazards.