RECORDS OF LATE HOLOCENE LANDSCAPE EVOLUTION IN THE FRENCH CREEK WATERSHED, NORTHWESTERN PENNSYLVANIA

Floodplains, alluvial fans, and lakes within the French Creek watershed in northwestern Pennsylvania provide evidence of changes in late Holocene landscape stability related to prevailing environmental conditions and land use/land cover history. In contrast to the prevalence of middle Holocene lateral accretion facies, soil-sediment sequences indicate increased vertical accretion of floodplains, punctuated by several phases of landscape stability and pedogenesis during the past ~ 3.5 ka. A similar story is documented by alluvial fan sediments, which indicate several discrete depositional events separated by weakly developed soils over the past ~ 2 ka. The timing of fan deposition is modestly correlative with phases of floodplain aggradation. Lake sediment cores also document several phases of increased clastic sedimentation and high magnetic susceptibility, proxies for landscape instability and/or hillslope soil erosion, over the past ~ 3 ka within the watershed.

Independent paleoenvironmental proxies for the area suggest the late Holocene prior to Euro-American settlement was a time of cool and wet climate under which a pine/hickory/beech forest predominated. This paleoenvironmental setting would have been conducive to frequent overbank flood events. Deforestation and the transition to an agriculturally-dominated landscape beginning in the late 1700’s significantly altered geomorphic and hydrologic processes within the watershed. All three sediment records definitively document the profound impact of Euro-American settlement and subsequent anthropogenic perturbations in the area.