HOW CHRONOSTRATIGRAPHIC DATA IMPROVES STREAM CLASSIFICATION IN WHITE CLAY CREEK, PA AND OTHER MID-ATLANTIC WATERSHEDS

Stream classification is a powerful tool for relating the geomorphic setting of a river to its hydraulic and sediment characteristics, allowing restoration practitioners to quickly assess channel function. While numerous classification schemes exist, one approach examines the origin of channel material (e.g., bedrock or alluvial rivers). Here we employ chronostratigraphic and geomorphic data to explore how the relation between history and geomorphic form may improve stream classification in the White Clay Creek (WCC) of southeast Pennsylvania and other mid-Atlantic streams. New data, including ten luminescence dates, from four sites bracket channel-adjacent landforms and basal deposits into three geomorphic regimes. First, during the late Pleistocene to early Holocene (8-12ka) we observe thick deposits of often coarse material representing a range of geomorphic processes. Features from this period include gently sloping debris fans of matrix-supported cobbles (~12ka), >2-m thick deposits of clast-supported cobbles (~12ka), cryoturbated laminated sand and mud (~11ka), and strath terraces (~8ka). Then in middle to late Holocene (2-5ka), deposits become generally thinner, finer-grained, and likely of alluvial origin. Deposits from both periods are buried by or adjacent to young sediments (typically <250yr.) that form overbank, lateral accretion, and milldam deposits. Bedrock exposures also appear frequently within the channel. Our observations suggest that (a) while the longitudinal profile of WCC is predominantly bedrock controlled, channel form is largely dictated by colluvium and alluvium, and thus classification as a bedrock stream is inappropriate, (b) the term “Anthropocene stream" only applies at smaller spatial scales (e.g., near milldams) due to the predominance of non-anthropogenically derived deposits, and (c) the interplay between the deposits from the three geomorphic regimes observed lead us to classify WCC as a partially confined, semi-alluvial stream due to the abundance of coarse-grained colluvium and bedrock, in addition to alluvium, that compose the channel. We emphasize the need to employ chronostratigraphic data into existing classification schemes, such as the Rosgen method, to better capture drivers of channel form that are independent of contemporary hydraulics.