RESPONSE OF RIVER COASTAL MARGINS TO HUMAN INDUCED VARIATIONS IN SEDIMENT SUPPLY AND ACCELERATED SEA-LEVEL RISE

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  • March 20, 2019
Creator
  • Jalowska, Anna
    • Affiliation: College of Arts and Sciences, Department of Marine Sciences
Abstract
  • My dissertation emphasizes that human-induced modifications to riverine sediment budget and changes in the rate of sea-level rise strongly influence bayhead-delta evolution. The response of bayhead deltas to these alterations is difficult to predict, but important to understand because they can lead to submergence and erosion of deltaic environments and loss of important habitat. Floodplains and the bayhead delta of the Lower Roanoke River, NC have the largest, most pristine bottomland-hardwood forest ecosystem remaining in the mid-Atlantic region. The majority of this ecosystem is currently at 0 m above sea level (MASL) and is very vulnerable to sea-level rise and changes in sediment supply. The Lower Roanoke River has been impacted by human activities (land clearing followed by river damming), and climate change. Using multidisciplinary tools, I was able to reconstruct the geological history of the Roanoke Delta (core descriptions and radiocarbon and 210Pb geochronologies), and define sources, fates and pathways of suspended sediments in the floodplains and delta environments on seasonal (radionuclide based sediment fingerprinting) and decadal to monthly (210Pb geochronology) time scales. My research reconstructed two episodes of retreat in the Roanoke delta during the past 6000 years. The first event occurred around 3700 BC when marine transgression flooded the Roanoke Paleovalley. The flooding formed an interdistributary bay that slowly filled with sediments until the 17th century. During the 1600s, when the first European settlers began to clear forest and farm the drainage basin, the delta rapidly accreted and the interdistributary bay filled with legacy sediment from increased agricultural runoff. The regression was also facilitated by the low rates of sea-level rise until ca 1860AD. The second episode of bayhead-delta retreat started during the 19th century and continues today. Improved agricultural practices and dam construction decreased the amount of sediment delivered to the bayhead delta. Additionally, the rate of sea-level rise increased to 0.21 cm/yr at that time. Under these conditions, the delta entered an erosional phase and during 1954–2012, the rate of delta loss was 2469 m2/yr. Decreases in sediment supply and more frequent inundations associated with the sea-level rise, led to a dramatic change in the function of delta plains. My research shows that at a 0 MASL elevation, the frequency and extent of flooding control erosion and deposition in the delta plain, and lead to loss of their ability to retain sediments. Hence, delta plains, regarded to be sediment sinks and sites of long-term sediment storage, become a source of sediment to the upper delta, as sea level rises and the delta retreats. This mechanism exposes a unique distribution of sediments in eroding deltas. In contrast to the previous paradigm emphasizing a unidirectional seaward dispersal of eroded deltaic sediments during transgression, my research illustrates that a landward-directed sediment migration pathway could occur, allowing nourishment and fortification of the upper bayhead delta to accelerated sea-level rise.
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  • In Copyright
Advisor
  • White, Brian
  • Rodriguez, Antonio
  • Cable, Jaye
  • McKee, Brent
  • Miller, Richard
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2016
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Date uploaded
  • July 5, 2017

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