ANTHROPOGENIC ALTERATIONS TO SEDIMENT DYNAMICS AND CHANNEL MORPHOLOGY ALONG MILL CREEK, ERIE PA

In northwestern Pennsylvania, small streams drain the northern escarpment of the Allegheny Plateau and flow north into Lake Erie. Decreasing lake level during the Late Pleistocene and Holocene has resulted in base level lowering. As a result, many channels are cut in bedrock and numerous strath terraces with thin (<1 m) gravelly deposits are preserved. Mill Creek, characteristic of this stream morphology, has experienced basin-wide anthropogenic land use changes and modifications in the past 215 years. Prehistorically, the basin was largely forested and was clear cut for farming in the 19^thcentury concurrent with construction of mills and dams. Those changes initiated fine-grained sedimentation in channels and on floodplains. Pre-anthropogenic stream deposits are coarser, however, and serve as stratigraphic makers used to estimate historical rates of sedimentation.

In the early 19^th century, local residents constructed an earthen dam at the current location of Headwaters Park for ice harvesting and recreation. Upstream of the dam, channel aggradation of ~ 2 m had induced ~ 80,000 m³ of valley bottom aggradation. This dam failed catastrophically on August 3^rd 1915 after three days of intense rain; the dam was never repaired. Currently, upstream of the dam, Mill Creek has incised a narrow channel into the pond sediments and the sinuosity of the stream channel has decreased through time. Erosion of fine-grained deposits upstream of the dam has initiated downstream floodplain aggradation. Sediment thickness decreases downstream of the dam with the thickest sediments (1.5 m) within 10 m of the dam. Sediments become progressively thinner (0.6 m) farther downstream. The volume of sediments evacuated from the ponded reach is similar to the volume of sediments on the downstream floodplain suggesting the majority of the pond sediments are redeposited in close proximity to the dam. Deposition of fine-grained sediment has changed the channel morphology from a single threaded channel to a multithreaded channel. After initial post-dam aggradation, the channel has incised 1.5 m producing a terrace, which impedes erosion of much of the redeposited pond sediments. Thus, a century after the dam failure the majority of anthropogenic sediments have moved less than 250 meters.