KIRK BRYAN AWARD LECTURE: THE RISE AND FALL OF MID-ATLANTIC STREAMS

Our study of small catchments and streams in the mid-Atlantic eastern US led to a new understanding of landscape evolution and geomorphic response to European settlement. Rather than resulting from long-term geological processes, many modern streams are artifacts of post-settlement anthropogenic activity: deforestation led to greatly increased soil erosion rates; construction of tens of thousands of low-head dams for waterpower led to regional base level rise of ~1.2 m and aggraded streams; and base level fall due to dam breaching led to incised streams with high banks of fine sediment. As massive volumes of eroded upland soil entered valley bottoms, sedimentation in millponds buried pre-settlement landscapes. Over a period of ~150 years, reservoirs filled with fine sediment, culminating in planar valley surfaces, or “valley flats”, with typical dimensions of roughly 100 m in width and several km in length. Sediment packages beneath valley flats thicken downstream to milldams and taper upstream, varying in thickness from 1-5 m, depending on dam height and distance from the dam.

Subsequent base level fall due to late 19^th-20^th c dam breaching (intentional or natural) led to channel incision into stored “legacy sediment”, exposing the pre- and post-settlement depositional history of valley bottoms. Incision stops at the regional groundwater table, which flows through a ca. 0.5 m thick Pleistocene periglacial lag gravel that overlies planar bedrock surfaces. In turn, these basal gravels are overlain by ca. 0.3 m of Holocene wetland soils that extend uninterrupted across the valley bottom. Wet meadows persisted at the groundwater level for millennia.

Microstratigraphy, radiocarbon dating, and seed macrofossil studies indicate long-term stability and resilience of the Holocene wetlands and organic-rich mucks that they produced. The classic model of a single-thread stream channel generating a broad floodplain over time is unsupported by this work. Instead, the transition from Holocene wet meadows to Anthropocene ponds and streams is delineated clearly in valley bottom stratigraphies. Understanding this stratigraphy and diagnosing the causes of bank erosion are crucial to developing strategies for stream and riparian ecological restoration. In this case, the past is the key to the future.