LIDAR-BASED MAPPING OF LANDSLIDES ALONG THE CISPUS RIVER, WASHINGTON: REVEALING REGIONAL STRUCTURE AND VOLCANIC HAZARDS

Located within the Gifford Pinchot National Forest, the Cispus River drainage fuels a flourishing forestry industry and a small population centered about the Cispus Learning Center about 15 km south of Randle, Washington. Flowing southward from its source in the Plio-Pleistocene Goat Rocks Volcanic Field, the Cispus River meanders west beyond Mount Adams where it incises through Oligocene basaltic andesite volcanics and enters the Cowlitz River. Though at risk of lahars from Mount Adams (Scott et al., 1995), the Cispus River drainage remained poorly studied until Pope (2020) interpreted poorly sorted terrace deposits as resulting from a late-glacial/postglacial lahar from Mount Adams. After entering the Cispus River drainage via Adams Creek, the Cispus Lahar grew to a minimum depth of 30 m at the Cispus Learning Center and over two times more voluminous than Mount Rainier’s Electron Mudflow (circa 1500 CE), depositing a series of poorly sorted reverse graded terrace deposits along much of the lower Cispus River. Because these depositional terraces were likely produced during a single lahar, the terraces can be used as a time stratigraphic indicator for defining volcanic and landslide hazards along the Cispus River. Using LiDAR data, landslides were categorized as predating, postdating, or of unknown association to the Cispus Lahar. Landslides postdating the lahar range widely in volume but can be much larger than landslides predating the lahar. Having been heavily weathered and truncated, only the largest of these pre-lahar landslides remain in reduced size. Though pre- and post-lahar landslides are not found together, post-lahar landslides are generally larger but fewer than pre-lahar landslides, suggesting that the erosive nature of the Cispus Lahar triggered hillslope failure which may conversely reactivate pre-lahar landslides in the future. Towards the south, larger landslides generally occur along slopes striking northwest-southeast, while to the north they are along slopes striking northeast-southwest. This delineates folds along the Cispus River that were likely produced during Miocene uplift of the Cascade Range (Sisson et al., 2014), facilitating prediction of high-risk slopes. Such mapping enables identification of previous volcanic and landslide hazards and future risks.