PRE- AND POST-SETTLEMENT DEPOSITIONAL PROCESSES AND ENVIRONMENTS OF THE 3RD-5TH ORDER WHITE CLAY CREEK WATERSHED, PIEDMONT PROVINCE, PENNSYLVANIA AND DELAWARE, USA

We hypothesize that overbank deposition has dominated sedimentation since the mid-Holocene. Using diverse dating tools, we define 3 chronostratigraphic units, focusing on vertically accreted sediments: presettlement (before 1750), legacy (1750-1950), and modern (since 1950). We find the following lithofacies (oldest listed first): bouldery gravel (BG), pebble-sand gravel (PSG), massive fine sediment (fine-medium sand, silt, and clay) with some organic material (MFS – OM), organic sediment (OM) with little MFS, laminated fine sediment (LFS), and massive fine sediment (MFS). The current bed of the White Clay Cr. consists of fluvial and colluvial sand and gravel; the grain size distribution, roundness, and lithology of the basal (presettlement) BG facies is indistinguishable from these deposits. Presettlement wedge-shaped PSG gravel deposits, occasionally found overlying the BG facies, are similar to modern gravel bars. MSF deposits are currently forming through overbank deposition; modern MFS deposits have widely varying sand content and low organic content, and are indistinguishable from presettlement MFS. Laminated (LFS) deposits are exposed upstream of former mill dams, but these subaqueous millpond deposits pinch out rapidly upstream. Organic-rich OM facies are rare; only one laterally extensive, presettlement OM unit has been found, located behind a bouldery debris dam. Overall, MSF deposits dominate preserved sediments, supporting the hypothesis that overbank deposition has been the primary process of valley corridor sedimentation in our study area. The similarities between pre-and postsettlement gravels indicates that bed material transport processes before European settlement were broadly similar to those at present. Organic-rich deposits (presettlement wet meadows?) are unusual, and may have been restricted to headwater areas. Laminated subaqueous millpond deposits are localized and relatively minor. Stratigraphic data do not allow reconstructions of planform, but similarities in sediment transport processes between the modern and presettlement WCC suggest that the current sinuous, single-thread (but occasionally anastomosing) planform should provide a useful working model.