Depositional signatures of historical flood and human landscape disturbances in lakes of the northeastern U.S.

Sedimentary archives in lakes and ponds are widely used to reconstruct past climatic and environmental conditions, as well as to quantify the environmental impacts of human activity. In this study, we summarize the characteristics of sedimentary deposits associated with different types of disturbances including floods, landslides, timber harvest, and conversion of forested land to agricultural use. We evaluated sediment cores from a network of lakes across the northeastern U.S. The watersheds span a range of topographic and surficial geologic characteristics, and have land-use histories with differing types, timing, intensity, and duration of anthropogenic disturbance. Cores were analyzed to identify distinct event deposits and changes in clastic sediment input indicative of landscape disturbances. While most records span the past millennium, we focus specifically on the period of record that overlaps with historical and instrumental records of events that can be linked to specific sedimentary deposits. Neither hydroclimatic nor human land-use signals are ubiquitous across all watersheds. The identification of distinct flood deposits was limited to higher relief, mountainous watersheds with abundant glacial-age sediment. Distal flood deposits are typically thin (mm to cm scale) and characterized by sharp contacts between dominant gyttja and fine-grained clastic flood layers. Hydrologic disturbances associated with landslide activation (such as occurred during tropical storm Irene in 2011) result in similarly sharp basal contacts between gyttja and clastic sediment. However, these deposits are commonly thicker (10s of cm) and characterized by compositional grading from more clastic to more organic rich sediment, and have complex patterns of textural variability. These signatures reflect a multi-year duration of elevated sediment delivery as the landscape gradually stabilizes and vegetation returns. In contrast, human land cover alteration typically manifests in sediments as a gradual and often prolonged increase in clastic content. Thick (up to 10s of cm), often sandy, texturally graded clastic deposits are distinct from those formed by both hydrologic and human disturbances, and interpreted as a consequence of subaqueous mass movements.