Document Type

Thesis

Date of Degree Completion

Spring 2018

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Lisa Ely

Second Committee Member

Megan Walsh

Third Committee Member

Joshua Roering

Abstract

Up to 250 years of sedimentation patterns in headwater streams are preserved with detail in landslide-dammed lakes of the central Oregon Coast Range. I hypothesize that both anthropogenic and natural perturbations should increase linear and mass sediment accumulation rates and be discernible spatially and temporally in the sediment record with use of 137Cs, high resolution charcoal stratigraphy, and aerial photography. Klickitat Lake and Wasson Lake are landslide-dammed lakes in small watersheds (<10 >km2) that contain drowned Douglas-fir stumps that are used for accurate dendrochronology and precise timing of the lake formation. An age-depth relationship was developed using 137Cs and identifiable fire events, which demonstrates that each lake has high linear sedimentation accumulation rates (0.2 – 4.5 cm y-1) and variable mass accumulation rates. Both lakes exhibit similar changes in mass and sediment accumulation, with rates remaining low after formation of each landslide-dammed lake and stabilizing after the initial lake filling. Stand-replacing wildfires of the mid-19th century increased short- and long-term sedimentation and mass accumulation rates, but they are not a primary driver in sediment mobilization and deposition. Sedimentation to the lakes systematically and gradually increased after the 1920s. Rates peaked in the mid-20th century associated with above-average peak discharge events with wetter and cooler conditions across the Pacific Northwest. Sediment deposition to the repositories has since decreased, but remains elevated compared to pre-industrial logging and road development. Comparisons to other landslide-dammed sediment repositories in the region suggest higher sedimentation rates in the steeper southern Tyee Formation, and lower sediment mobilization potential in the deep-seated dominated northern Tyee Formation. Results of this study indicate that small landslide-dammed lakes are viable tools for assessing sediment mobility patterns in headwater channels and can be used to further understand sedimentation and erosional patterns in the central Oregon Coast Range.

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