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

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.