RECENT OBSERVATIONS OF POST-FIRE DEBRIS FLOWS IN FIVE MEGAFIRES IN THE WESTERN CASCADES, OREGON

To better understand the influence of wildfire on debris-flow occurrence in the Pacific Northwest, we surveyed five burn areas in Oregon for debris-flow activity during the 2020-21 and 2021-22 winter rainy seasons. The sites include four burn areas that are the result of Oregon’s devastating 2020 Labor Day megafires: Riverside, Lionshead-Beachie Creek, Holiday Farm and Archie Creek; as well as the 2017 Eagle Creek burn area. Each of these fires burned west of the Cascades, on the rainy side of the mountain range. We conducted field surveys after several major rain events, most of which were the result of moderate to intense atmospheric river storms. Additionally, we used aerial photos acquired by the Oregon Department of Transportation for two of the burn areas to identify initiation areas and transport zones, which were otherwise difficult to see in the field. We identified 56 debris-flow events, 17 flood-scour events, 26 shallow landslides, 9 rockfall instances, and 226 drainages that did not display evidence of a flood or debris flow. These non-events are important to build a dataset of null events needed for rainfall threshold analyses. Following event identification, we further characterized sites containing debris flows using additional feature classes, including initiation points, transport lines, and deposition polygons. We identified 22 initiation points, 72 nonunique transport lines, and 39 deposition polygons. In this presentation, we will provide observations of the variety of mass movement events within the burn areas, as well as comparisons between the 2020-21 events and events the following year. We will also compare these post-fire events to non-fire debris-flow events within the same geographic extents in four out of five of the burn areas (excluding Riverside). Initial observations suggest differing initiation styles between non-fire debris flows and post-fire debris flows.