<b>Tree Throws, Spodosols, and Sand Flows: Analyzing Sand Flow Slides and Their Triggers</b> , <b>Haines, Alaska</b>

ABSTRACT The Lutak Spur (LS) is a prominent deltaic landform near Haines, Alaska, formed during the retreat of the Cordilleran Ice Sheet from the Last Glacial Maximum. This study evaluates its geologic evolution, soil development, and slope stability response to a December 2020 atmospheric river (AR) event that triggered multiple landslides. We posit that LS formed as an ice-contact kame delta at the margin of a retreating valley glacier. Post-glacial isostatic rebound exposed the LS surface, allowing Spodosols with iron-cemented (Fe-cemented) horizons to develop beneath forest vegetation. We used field observations, laboratory testing, surface drainage mapping, and modeling to evaluate tree throw as a possible landslide trigger. Direct shear testing indicated that Fe-cemented layers contribute cohesion to near-surface soils. On the basis of measured precipitation over a 2-day period and NOAA Atlas 14 precipitation frequency estimates, the December 2020 AR event approached the intensity of a 1,000-year storm. Hydrologic modeling indicated that peak discharges were approximately three times greater than those of a modeled 100-year storm. The SEEP/W modeling demonstrated elevated groundwater levels and pore water pressures (PWP) from infiltration. The SLOPE/W modeling indicated dry slopes were stable, but removal of the Fe-cemented layer reduced the factor of safety (FS); under saturated conditions, the FS dropped below 1.0. Field evidence, resident observations, and modeling support the hypothesis that tree throw along the slope crest disrupted the Fe-cemented layer, triggering failures. These results suggest that extreme precipitation, combined with vegetation disturbance, reduced slope stability through transient PWP increases and the loss of near-surface strength.