1500 years of lake sedimentation due to fire, earthquakes, floods and land clearance in the Oregon Coast Range: geomorphic sensitivity to floods during timber harvest period

Abstract

Sediment cores retrieved from landslide‐dammed Loon Lake recorded events back to the 5th century AD in a forested, mountainous catchment, thereby providing an opportunity to compare the impacts of known recent perturbations, including floods and timber harvesting with those of an early period in the cores, floods, fires, and earthquakes. High‐resolution multi‐parameter (grain size, %TC, %TN, and magnetic susceptibility) data allowed the core stratigraphy to be classified as background sedimentation and events. 137Cs and radiocarbon dating, as well as a varved record in the last 75 years provided age control. Mean mass accumulation rate from 1939 to 1978 AD, the time of peak timber harvest and a cool wet phase of the Pacific Decadal Oscillation, was 0.79 (0.74–0.92, 95% C.L.) g cm‐2 y‐1, significantly higher than mean rates of both the more recent contemporary period (coincident with the passing of the legislation that regulated harvesting practices in the region), 1979–2012 AD, at 0.58 (0.48‐0.70) and the entire early period, 0.44 (0.41–0.46). Several event deposits are coeval with independently estimated ages of eight Cascadia subduction zone earthquakes in the early period, including the 1700 AD Mw 9.0 event. These deposits are predominantly formed by hyperpycnal flows, as are the known event deposits in the contemporary period. The high mass accumulation rate and greater frequency of thick event deposits during the early contemporary period point to the extraordinary role of timber harvesting in priming the landscape for subsequent sedimentary delivery during floods. Copyright © 2017 John Wiley & Sons, Ltd.

Geomorphic and Sedimentary Effects of Modern Climate Change: Current and Anticipated Future Conditions in the Western United States

“…In sedimentary systems, antecedent events determine morphology, particle-size distribution and packing geometry, hillslope water content, and other parameters that affect geomorphic responses to the next rainstorm, windstorm, or flood event (Ashworth & Ferguson, 1986;Bakker et al, 2019;East, Logan, et al, 2018;Ellen & Wieczorek, 1988;Masteller & Finnegan, 2017;Tucker & Slingerland, 1997;Yellen et al, 2016). In some watersheds erosion during extreme rain exhausts the sediment supply, leading to less sediment export during subsequent, smaller storms, whereas in other basins one major storm sets up the landscape for even greater sediment export on the next storm (Page et al, 1994;Richardson et al, 2018). Disentangling relative effects of superimposed landscape disturbances remains extremely difficult, creating substantial uncertainty in our ability to detect or predict landscape response to climate change overall.…”

“…) andRichardson et al (2018) Lacustrine sediment volume accumulation rate (ΔV/Δt), from differencing of mapped surfaces Lacustrine record of river flood frequency (number of thick, coarse-grained deposits accumulating per unit time)Smith et al (2019) Q3: Changes to fluvial morphology? Riverbed elevation, h, over time (m above fixed datum, or Δh/Δt) Tunnicliffe et al (2018) Stage residual (either in m or dimensionless, as fraction of mean flow depth), interpreted to reflect change in bed elevation Anderson and Konrad (2019) and Pfeiffer et al (2019)Active channel width (m) channel braiding (dimensionless index based on andEast et al (2017) …”

High resolution lake sediment record reveals self‐organized criticality in erosion processes regulated by internal feedbacks

“…Soil sensitivity to erosion depends on many factors including slope exposure (Roering, ), vegetation cover, logging, stand replacing fires or triggered by large events such as earthquakes (Montgomery and Brandon, ; Dadson et al ., ; Pierce et al ., ; Valentin et al ., , Richardson et al ., ). When identified in the paleorecord, the different drivers of erosion variability may help explain the erosion time series (E 2 , E 10 and E 100 ), evidenced by our record (Figure ).…”

“…In addition, the availability of regional Late Holocene records for earthquake events (Morey et al ., ), Pacific Northwest summer temperature (Mann et al ., ), and regional winter precipitation and temperature (Ersek et al ., ), allow direct comparison between multiple external controls. Furthermore, the onset of logging in the catchment over recent decades, which increased sediment fluxes (Colombaroli and Gavin, ; Richardson et al ., ), enables a comparison of natural versus anthropogenic controls on sediment flux.…”

The Preservation of Climate‐Driven Landslide Dams in Western Oregon

“…In an effort to clarify the discrepancy that few bedrock landslides in the OCR have failed catastrophically in the past few 100 years but abound in the landscape, earthquake ground motion from the CSZ and shallow crustal faults has commonly been invoked to explain the distribution of deep-seated landslides in the Cascadia forearc (e.g., Leithold et al, 2018;Morey et al, 2013;Pierson et al, 2016;Richardson et al, 2018;Roering et al, 2005;Schulz et al, 2012;Schuster et al, 1992). However, no landslide has been definitively linked with ground motion from the last great earthquake, which occurred on 26 January 1700 AD (Atwater et al, 2005;Yamaguchi et al, 1997).…”