Longitudinal Variation in Legacy and Pre-settlement Floodplain Deposits of White Clay Creek, PA and DE, USA.

Authors: Melinda Daniels*, Stroud Water Research Center, Diana Oviedo-Vargas, Stroud Water Research Center, Marc Peipoch, Stroud Water Research Center, Bernard Sweeney, Stroud Water Research Center, Charlie Dow, Stroud Water Research Center
Topics: Geomorphology, Earth Science, Water Resources and Hydrology
Keywords: floodplains, legacy sediments
Session Type: Paper
Day: 4/6/2019
Start / End Time: 9:55 AM / 11:35 AM
Room: Hoover, Marriott, Mezzanine Level
Presentation File: No File Uploaded

Floodplains in the mid-Atlantic Piedmont of the United States are characterized by post-settlement alluvium deposits originating from colonial-era and ongoing hillslope deforestation, agriculture and other land disturbances. These deposits, often referred to as legacy sediments, overlay a comparatively organic-rich, pre-colonial buried floodplain soil. Debate has emerged regarding the spatial ubiquity and thickness of the legacy sediment mantle in modern floodplains, and concerns have been raised regarding their potential to contribute to nutrient and sediment pollution within modern streams. Considering the management concerns regarding the potential for legacy sediment to serve as a source or nutrient-rich fine sediment, it is imperative that we improve our understanding of the nature and extent of these floodplain deposits. In this study, we use spatially extensive longitudinal field sampling within the White Clay Creek stream network in Southeastern PA and Northwestern DE, to characterize the nature and spatial variation of legacy sediment and buried soil thicknesses along the network continuum. At different points along the 2nd-4th order stream network, floodplain transects were cored and analyzed for thickness, organic content (AFDM), C, N and P and other elemental content. Our results indicate that both legacy and pre-settlement deposit thicknesses vary greatly along the stream network continuum, but elemental organic content was comparatively uniform. Legacy sediments were deepest immediately adjacent to the modern active channel in the narrow natural levee zone and then decreased in thickness by as much as half behind the levee zone. Legacy sediment thickness was most correlated with the degree of valley confinement.

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