Volume 129, Issue 11 e2024JF007778
Research Article

Holocene Lake Sediments Reveal Alluvial Fan History With Links to Climate, Wildfire, and Earthquakes

Daniel G. Gavin

Corresponding Author

Daniel G. Gavin

Department of Geography, University of Oregon, Eugene, OR, USA

Correspondence to:

D. G. Gavin,

[email protected]

Contribution: Conceptualization, Methodology, Software, Formal analysis, ​Investigation, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization

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William T. Struble

William T. Struble

Department of Geosciences, University of Arizona, Tucson, AZ, USA

Now at Department of Earth & Atmospheric Sciences, University of Houston, Houston, TX, USA

Contribution: Methodology, Formal analysis, ​Investigation, Writing - review & editing

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Mark A. Fonstad

Mark A. Fonstad

Department of Geography, University of Oregon, Eugene, OR, USA

Contribution: Writing - review & editing

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First published: 24 November 2024

Abstract

Flood hazards along alluvial rivers vary over time due to changes in both flow regime and channel morphology; however, their millennial-scale histories are difficult to study from incomplete and poorly dated alluvial stratigraphies. Thus, the role of external forcings (e.g., climate) in the magnitude of alluvial channel dynamics remains poorly understood. We developed a record of overbank flows of the Tolt River in Washington from a continuous 6,100-year sediment record obtained from a 33-m deep lake with an outlet dammed by alluvium. Overbank flows from the adjacent river result in fine laminations preserved in the lake sediment. Multi-century periods of overbank flows with fine (<2 mm; ca. annual) laminations account for 36% of the last 6,100 years. The only event recorded in the lake during the last 900 years is dated by 210Pb-verified varves to a historic atmospheric river event in December 1867. Tree-ring and radiocarbon-dated alluvial surfaces upriver from the lake are consistent with a significantly aggraded channel during silt periods in the lake. Although a sediment slump in the lake dates to a known earthquake, there is little other linkage between earthquake history and alluvial history. However, regional paleoclimate, local fire history, and landslide ages suggest that the continuous periods of overbank flows were sustained by aggradation from sediment input after fires and during dry climate periods. The lake record indicates an alternation of incision and aggradation and a much more dynamic channel history than observed over the last several decades following channel modification and dam development.

Key Points

  • An accurately dated lake sediment record reliably preserves 6,100 years of flooding from an adjacent alluvial river

  • Channel morphodynamics, rather than streamflow, explain the flood history at this site, including a major 1867 CE flood

  • Climate and fire history in the presence of frequent input from upstream landslides can explain the history of overbank flows

Plain Language Summary

Lakes positioned close to rivers can preserve sediments that indicate the times of past floods over thousands of years, but interpreting such records is complicated because river channels themselves can change over time. We analyzed a 6,100-year sediment record with clear evidence of flood deposits, the most recent in 1867 CE at a time of a very large rainfall event early in the settlement history of Washington State. However, the modern river channel adjacent to the lake is cut into the topography such that floods cannot reach the lake. Ages of trees near the river and carbon dates in the river sediment confirm that the river channel was aggraded at times when it flooded into the lake. We also compared the history of flooding with nearby studies of paleoclimate, landslides, forest fires, and earthquakes, all of which are known to impact river channels. This comparison suggests that past dry climates and forest fires sustained long periods of an aggraded channel. The magnitude of change in the channel required to explain these patterns is much greater than that observed over recent decades of monitoring.

Data Availability Statement

Magnetic susceptibility, pollen, loss-on-ignition, X-ray diffraction, and charcoal data are available on the Neotoma Paleoecology Database (Gavin, 2021a, 2021b, 2021c, 2021d). Core images, raw and interpolated sediment lightness, and the statistical analysis for computing the age-depth model are available in Gavin (2024).