QUANTIFYING SEDIMENTATION PATTERNS OF SMALL LANDSLIDE-DAMMED LAKES IN THE CENTRAL OREGON COAST RANGE

Changes in sedimentation patterns spanning the last 250 years are preserved in two landslide-dammed lakes in small watersheds (<10 km²) of the central Oregon Coast Range. These two watersheds are accurate representations of typical coastal streams that host anadromous fish which have been affected by logging and road development since the mid-20^th century. Both landslide-dammed lakes host drowned Douglas-fir trees that have been precisely dated using dendrochronology with Klickitat Lake forming in AD 1751 and Wasson Lake forming in AD 1819. Perturbations from wildfires, logging and road development, and climate directly affected linear and mass sediment accumulation rates, and are identified in the sediment record using ¹³⁷Cs, high-resolution charcoal stratigraphy, local fire records, and aerial photography. A constrained cubic-spine using identified charcoal and ¹³⁷Cs horizons demonstrates that each lake has high linear sedimentation accumulation rates (0.2 – 4.5 cm y^-1) and variable mass accumulation rates (0.12 – 1.47 g cm^-2 y^-1). Early sediment accumulation rates remained low until the mid-19^th century when stand-replacing wildfires increased short- and long-term linear sedimentation and mass accumulation rates. Mass accumulation rates peaked in the mid-20^th century coinciding with several years of above-average peak discharge events associated with wetter and cooler conditions across the Pacific Northwest. Early logging and road development in the mid-1960s increased sediment accumulation rates at both sites, especially when coinciding with years of above average precipitation. A comparison of the temporal and spatial distribution of previously published sedimentation records of larger landslide-dammed lakes in the Tyee Formation indicates higher sedimentation rates in the steeper, debris-flow dominated sandstone of the southern Tyee Formation, and lower sediment mobilization and deposition in the thinly bedded ­­siltstone and sandstone units of the northern Tyee Formation. These results are critical in understanding the temporal and spatial distribution of sedimentation patterns in small coastal streams that are the centers of major ecological rehabilitation for harboring anadromous fish and other threatened native species.