Impacts of geographic variability and geologic history on the distribution of post-settlement alluvium (PSA) across the upper Midwest, USA

Abstract

Accelerated floodplain sedimentation related to agricultural development of uplands has produced post-settlement alluvium (PSA) along rivers throughout the upper Midwest, U.S.A. Landscape characteristics, surficial sediments, and soils in the region vary geographically in relation to differences in geologic history, yet the extent to which this geographic variability influences PSA accumulation remains unexplored. This study uses existing data to assess how non-dimensional PSA thickness varies with landscape characteristics, surficial sediments, soils and climate. Geographic variability is associated with three subregions: 1) areas glaciated during the Late Wisconsin Episode (LWE), 2) areas glaciated during Pre-Illinois and Illinois Episodes (PI&IE), and 3) the Paleozoic Plateau (PP), an area where evidence of Quaternary glaciation is highly localized and does not influence geomorphic characteristics of the landscape. These subregions differ significantly in average geomorphic characteristics, including mean watershed slope (WS), mean local relief (LR), fraction of non-contributing area (NCA), pre-settlement drainage density (DD), and mean normalized river steepness (KSN). Native vegetation type also differs systematically between the subregions, creating significant differences in the frequency of alfisols (Alfi) and molisols (Mol). Thickness of last glacial loess (Loess) also varies across the region, although not systematically between the subregions identified. Non-dimensional PSA thickness differs significantly among the subregions, increasing systematically with landscape age, reflecting faster upland erosion rates and stronger connectivity of uplands to river corridors in older landscapes relative to more recently glaciated landscapes. Non-dimensional PSA thickness is significantly positively correlated with LR, KSN, WS, Loess, Alfi, and Mol and significantly negatively correlated with NCA. Non-visibly distinct PSA is present in some LWE watersheds characterized by significantly lower KSN and WS relative to other LWE watersheds in which PSA is visibly distinct. PSA thickness and visibility reflect catchment-wide landscape characteristics and watershed-scale river steepness, which emphasize the importance of geographic setting, geological history, and landscape geomorphic characteristics for understanding historical river sediment dynamics. Spatial variability in PSA thickness also serves as an indicator of river system sensitivity to land-use change, providing insight into the relative impact of humans on rivers within different geographic settings.