FORMATION AND SEDIMENTATION OF LANDSLIDE-DAMMED LAKES WITHIN SMALL WATERSHEDS OF THE CENTRAL OREGON COAST RANGE

Over geomorphic timescales, the central Oregon Coast Range has been proposed to approximate steady erosion owing to the prevalence of steep, uniform topography and consistent erosion rates from cosmogenic nuclides. Surprisingly, few constraints on pre- and post-settlement erosion and sedimentation exist in the region. Landslide-dammed lakes in small, headwater catchments serve as pre-historic sediment repositories that track variations in sediment production and erosion in a region with minimal in-transit sediment storage ability.

To assess sedimentation flux in small drainages from the last several centuries, we collected sediment cores from three small (<4 hectares, <0.3 km²) lakes with standing drowned forests and landslide-dams visible from LiDAR in the headwaters of watersheds in the Tyee Formation, central Oregon Coast Range. Sediment cores recovered from Klickitat, Wasson, and Yellow Lakes were on the order of 1 to 2 meters in length and initial analysis of stratigraphy at Wasson and Klickitat Lakes indicate a rapid change from meandering sandy streams with abundant organic horizons to a gyttja (organic-rich mud) dominated lacustrine environment. An abrupt change in deposition style to distinct laminated bedding with increased dissimilar organic horizons occurs in the upper half of each core. This decrease in homogenous gyttja and increased deposition of coarse organics may capture a change in sedimentation following the onset of industrialized logging and major clear cutting after World War 2.

Preliminary dendrochronological dating of standing drowned trees has yielded preliminary lake formation dates of AD 1819 for Wasson Lake and AD 1751 for Klickitat Lake. Further analysis of sediment grain size, magnetic susceptibility, and charcoal distribution, aided by ²¹⁰Pb and ¹³⁷Cs dating will quantify sedimentation rates before and after fires and other natural perturbations that span the onset of industrialized logging over the last 100 years. This multi-disciplinary study provides greater insight into erosional and sedimentation patterns of small watersheds before and after logging and the role stochastic events play in landslide-prone mountainous catchments.