SOIL GEOMORPHOLOGY AND STRATIGRAPHY REVEAL MID- TO LATE HOLOCENE CHANNEL INSTABILITY ALONG THE DELAWARE RIVER VALLEY, USA

Several decades of research along the upper-central Delaware River valley concluded that the main channel was relatively stable and flanked by a 6,000-8,000 year old vertically accreting floodplain landform. Sixteen new radiocarbon ages, along with high-resolution (buried) soil stratigraphy and pedology, reveal distinct episodes of channel instability (i.e. widening, avulsion, and incision) following periods of widespread soil formation during the mid to late Holocene. Approximate lower-limit ages of channel widening and avulsion are based on age-dated basal channel fill and are (ka = Cal yr BP): 5.2ka; 4.2ka; 3.3ka; 2.0ka; 0.5ka; and 0.4ka. There is also evidence of incision around 0.3ka. With the exception of the 0.5ka and 0.4ka instability episodes, these events follow four major periods of soil formation: 10.7-9.4ka; 5.2-4.2ka; 3.0-1.8ka; and 1.1-0.3ka. δ¹³C values (VPDB) from soil organic carbon reveal important heavy isotope excursions corresponding with major soil units described above (e.g. -20 to -17‰). These data suggest the buried soils formed during warm and/or dry phases resulting in either moisture-stressed C3 grasses/forbs or establishment of C₄ grasses. The transition from dry to wet climate phases resulted in channel instability due to increasing stream power prior to re-establishment of forest-dominated ecosystems. Using archaeological, historical and paleoclimate records, the out-of-phase nature of the 0.5ka and 0.4ka events, coupled with the ~0.3ka incision event, is likely related to increased Native American land-use and subsequent European deforestation in concert with Little Ice Age climate change. We attribute the newly documented Holocene channel instability and drier/warmer climate phases along the Delaware River to the integrative approach used in this study: high-resolution alluvial stratigraphy, soil geomorphology, and radiocarbon chronology. Future research along major river valleys in the Mid-Atlantic region should use a similar approach when reconstructing river and floodplain response to climate change and land-use during the Holocene.