HOLOCENE FLOODPLAIN EVOLUTION IN NORTHWESTERN PENNSYLVANIA

Multiple cutbank and back-hoe trench exposures throughout the French Creek watershed in northwestern Pennsylvania were used to reconstruct floodplain evolution during the Holocene. Two distinct stratigraphic units are recognized within the French Creek watershed; a prehistoric unit and a historic unit of post-settlement alluvium (PSA). Land conversion from forest to largely agricultural cover beginning in the late 1700s and peaking in the mid to late 1800s is largely responsible for the production and deposition of PSA as modern point bars and vertically accreted alluvium. Usually a distinctive dark, and sometimes over-thickened, buried A horizon developed in fine-grained vertical accretion deposits marks the contact between the prehistoric and historic units within vertical stratigraphic exposures. Thirteen ¹⁴C assays derived from the French Creek floodplain and two tributaries suggest the prehistoric alluvial fills are multi-aged, a phenomenon typical along laterally mobile, meandering streams.

Numerous archaeological sites and pedological data suggest a relatively stable floodplain environment for the past several thousand years. The dark prehistoric soil that usually separates the historic and prehistoric stratigraphic units, or is at other times the surface soil, may be the product of Native American utilization of the floodplain environment. Although archaeological and pedological evidence indicates some patches of pre-late Holocene alluvium exist, a lack of widespread alluvium older than ~ 4-5 ka suggests that lateral migration has removed much of the earlier Holocene fill. The removal of alluvium from the alluvial valley has thus hindered a thorough understanding of geomorphic, paleoenvironmental and archaeological records.