Freeze-Dried, Rapidly Thawed Permafrost Landscapes, Breached Milldams, and Their Relation to Modern Wetland-Stream Restoration, Eastern US
With no equivalent in the Holocene or late Pleistocene sedimentary record, modern incised stream channel forms in the mid-Atlantic US south of Pleistocene ice margins represent a transient response to major changes in land use and anthropogenic base-level forcing that began centuries ago. Walter and Merritts (2008) documented late 17th-early 20th c. valley bottom sedimentation in the mid-Atlantic US from contemporaneous, widespread valley-bottom damming for water power. Our compilation of U.S. census data reveals >65,000 water-powered mills in 872 counties in the eastern United States by 1840, with 5 to 24 mills per km2 in hundreds of counties. From historic documents we map similar numbers of milldams at 1-5 km spacing along waterways.
Mill damming and reservoir sedimentation inundated, buried, and sequestered pre-settlement valley-bottom landscapes, the most common of which were carbon-rich Holocene wetlands. We map historic fill terraces regionally with lidar extraction techniques and quantify volumes of historic sediment. After historic dams are breached, streams incise through historic sediment, exposing the buried landscape and stratigraphic boundary between pre- and post-settlement valley bottom sediments. Deep incision exposes a Pleistocene periglacial colluvial landscape that can be connected to hillslope landforms and colluvial deposits via lidar topographic analysis and field work, respectively.
This unglaciated landscape has a strong signature of repeated frost cracking and downslope diffusive sediment transport that diminishes both southward (less to no Pleistocene permafrost) and northward (glaciated). Evidence of continuous permafrost and aridity during the late Pleistocene, notably from sand-filled thermal contraction polygons in bedrock, is widespread along an ~200 km wide (north to south) band. Near surface weathered material produced by frost cracking during cold glacial conditions was mixed with eolian silt and transported downslope during times of warming, permafrost thaw, and active layer stripping. Solifluction lobes, thermokarst landforms (e.g., retrogressive thaw slumps), and water tracks similar to those in Antarctica are widespread. We use radiocarbon and OSL dating with pollen and seed analyses to document how wetlands, small streams, and carbon sequestration became established on this periglacial rubble landscape, but find no geologic evidence of the classic model of single-channel meandering streams during late Pleistocene to Holocene time. Our north-south topographic analysis reveals that the signature of continuous permafrost ends near ~39° latitude. These findings have direct implications for stream and wetland restoration in formerly periglacial valley bottoms blanketed beneath historic sediment (see bsr-project.com).
Technical Talk:
Wicked Landscape Problems
Geomorphologists, policy makers, and engineering practitioners in the eastern US disagree on the cause of high suspended sediment loads in streams, with most blaming agriculture, stormwater runoff from urbanization, or a lack of trees along stream banks. Incomplete and contradictory information, different opinions, substantial economic costs, and the interconnectedness of high sediment loads with other phenomena makes this a wicked problem. Meanwhile, the eastern US leads the nation in dam removal and stream restoration, the former of which exacerbates high sediment loads and the latter of which is a multibillion dollar, controversial endeavor because of disagreements about how streams should be restored. We are working with local, state, and federal government agencies to use geomorphic understanding and multiple acquisitions of lidar (with DEM differencing) in order to determine what landscape processes are occurring, and at what rates, in order to resolve the wicked problem of high suspended sediment loads that degrade streams and bays. Ultimately, this work is leading to new ways to mitigate high sediment loads and restore streams.
Bio:
Dorothy Merritts is a professor in the Department of Earth and Environment at Franklin & Marshall College in Lancaster, Pennsylvania, and a geologist with expertise on the impact of geologic processes, climate change, and human activities on the form and history of Earth's surface. Her primary research in the eastern United States is in the Appalachian mid-Atlantic region, where she investigates the role of human activities in transforming upland woodlands and valley bottom wetland meadows of Eastern North America to a predominantly agricultural and mixed-industrial/urban landscape since European settlement. Associated with this work is developing new methods of wetland, floodplain, and stream restoration that rely upon geomorphic investigation. She also uses lidar to map the geomorphic evidence for continuous permafrost from the last glacial maximum throughout Pennsylvania and Maryland. In the western United States, she conducted research along the northern San Andreas Fault of coastal California for two decades; her international work focused on fault movements in South Korea, Indonesia, Australia, and Costa Rica. In 2004-2005 she was the Flora Stone Mather Visiting Distinguished Professor at Case Western Reserve University in Cleveland, Ohio. In 2011-2012 she was the Cox Visiting Professor at Stanford University. She is the author of an introductory textbook in environmental geology, numerous scientific papers and edited book chapters, and contributing author to several National Research Council reports. In 2017 she became President-Elect of the AGU Earth and Planetary Surface Processes Group.