FACTORS AFFECTING THE DEGREE OF HAZARD POSED BY LAHARS AT MOUNT SHASTA, CALIFORNIA

Lahar frequency at Mount Shasta, California, varies with the origin of the lahar, resulting in different degrees of hazard for communities around the volcano. The most frequently occurring lahars are induced by climatic fluctuations. Relatively small lahars may occur within decades of a regional climatic shift, while larger lahars may occur during glacial and interglacial thresholds. The town of McCloud faces a greater risk of lahar inundation due to its proximity to Mud Creek, a glaciated, active drainage that produced lahars in the 1920's and 30's. During the 1990's, Whitney Creek, on the north side of Mount Shasta, produced lahars that damaged a major highway, though the area remains sparsely populated. Unglaciated drainages on Mount Shasta have not experienced any significant lahars since the end of the Pleistocene, and pose a lower threat to nearby communities. Eruptions on Mount Shasta occur about every 600 years (Miller, 1980); lahars resulting from eruptions would have the same frequency, with the location dependent on the site of eruptive activity. The last eruptive activity originated from the Hotlum cone, which would likely produce syneruptive lahars in Whitney Creek. A lack of recent eruptive deposits on the southwest side of the volcano, near Mount Shasta City, indicates a lower frequency for eruption-induced lahars on this side of the mountain. Lahars originating from debris avalanches have only occurred once in Mount Shasta's 500,000+ year old history. Extensive deposits have been recognized on the north side of Shasta, and recent findings suggest the existence of correlative deposits on the south side near Mount Shasta City. While lahars from such an event would be extensive and potentially devastating, the infrequency of such as event concludes these lahars as a very low hazard. Lahar inundation can also be approximated using DEM-based software. Recently, lahar inundation modeling has been performed for the Mud Creek, Whitney Creek, and Big Canyon Creek drainages. Resulting maps may be used as broad guidelines for hazard delineation, as the modeling still cannot predict exact inundation paths.