A MODEL OF FLOODPLAIN EVOLUTION IN THE FORMERLY GLACIATED OHIO RIVER HEADWATERS OF NORTHWESTERN PENNSYLVANIA

A model of floodplain evolution in formerly glaciated landscapes is presented here for the French Creek watershed in the Ohio River headwaters of northwestern Pennsylvania. Thirteen ¹⁴C assays help constrain floodplain evolution and indicate the prehistoric alluvial fills are multi-aged, a phenomenon typical along laterally mobile, meandering streams. A typical stratigraphic sequence consists of late Holocene vertical accretion deposits overlying either late or middle Holocene lateral accretion deposits. An apparent lack of alluvium older than ~ 5-6 ka suggests that lateral migration has removed much of the earlier Holocene fill and transferred it to the Allegheny River system where some of the sediment is stored as low-lying terrace and floodplain fill.

Numerous archaeological sites and pedological data suggest a relatively stable floodplain environment for the past several thousand years. A pervasive, dark, and sometimes over-thickened, buried A horizon developed in fine-grained vertical accretion deposits commonly marks the contact between prehistoric floodplain deposits and post-settlement alluvium (PSA). The unusually dark prehistoric soil often contains prehistoric artifacts, sometimes thickens near archaeological sites, and may be the product of Native American utilization of the floodplain environment. The PSA stratigraphic unit is related to land conversion from forest to a largely agricultural landscape beginning in the late 1700s and peaking in the mid to late 1800s. When taken together, it appears that humans have been impacting the French Creek valley in northwestern Pennsylvania for an extended period of time, which is consistent with other studies in the eastern United States.