2022 QG&G CAREER AWARD: CHANGING VIEWS OF THE RIVER

A view of the temperate mid-Atlantic region (MAR) landscape of the eastern US as largely periglacial and anthropogenic in origin can inform current approaches to landscape restoration. Walter & Merritts (2008) determined that form-based stream restoration in the MAR Piedmont was disconnected from recognition of late Pleistocene to Holocene geologic and climate history that accounted for widespread accumulation of coarse hillslope-derived, frost-shattered colluvium in valley bottoms. These sediments had been misinterpreted as mostly fluvial gravels, despite their angularity, large size, and relative immobility. High Pleistocene periglacial sediment fluxes from solifluction during permafrost thaw covered many valley bottoms in the MAR with colluvial sediment. During Holocene warming, spring-fed wetlands spread across rubbly valley bottoms. Yet, many restored stream channels in the MAR have been designed to carry coarse sediment with the premise that it is fluvial in origin. Most MAR Holocene valley bottoms were buried with fine-grained sediment upstream of thousands of milldams during the 18^th-20^th C. After dam breaching and channel incision, km-long wedges of sediment formed high, eroding banks of mud. And yet, many restoration projects in the MAR have been designed to perpetuate single-thread meandering stream channels with high banks and inset gravel bars.

This understanding of MAR fluvial history leads to the need for reconsidering fundamental concepts of hydraulic geometry that relate stream channel shape and flow velocity to discharge. Since 2008, we have monitored an ~1 km-long valley bottom reach where ~22,000 m³ of slackwater sediment deposited upstream of an 18^th C. milldam was removed in 2011 to expose and revive the underlying buried Holocene wet meadows, groundwater springs, and small anastomosing stream channels. Hydraulic geometry relations were shifted from the pre-restoration channel condition of high depth to width ratios and velocity, to the post-restoration condition of high width to depth ratios and low velocity, similar to Holocene conditions. The wet meadow landscape restored in 2011 has remained resilient, resulting in carbon sequestration, groundwater recharge, and habitat improvement.