Allison Mead A.
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ArticleHow tidal processes impact the transfer of sediment from source to sink : Mekong River collaborative studies(Oceanography Society, 2017-09) Ogston, Andrea S. ; Allison, Mead A. ; McLachlan, Robin L. ; Nowacki, Daniel J. ; Stephens, J. DrewSignificant sediment transformation and trapping occur along the tidal and estuarine reaches of large rivers, complicating sediment source signals transmitted to the coastal ocean. The collaborative Mekong Tropical Delta Study explored the tidally influenced portion of the Mekong River to investigate processes that impact mud- and sand-sized sediment transport and deposition associated with varying fluvial and marine influences. Researchers participating in this 2014–2015 project found that as sand and mud progress down the tidal portion of the river, sands in suspension can settle during reduced or slack flows as river discharge becomes progressively more affected by tides in the seaward direction. Consequently, deposits on the tidal river bed are connected to sand transport in the channel. In contrast, fine mud particles remain in suspension until they reach an interface zone where waters are still fresh, but the downstream saline estuary nonetheless impacts the flows. In this interface zone, as within the estuary, fine particles tend to settle, draping the sand beds with mud and limiting the connection between the bed and suspended sand. In the Mekong system, the interface and estuarine zones migrate along the distributary channels seasonally, resulting in variable trapping dynamics and channel bed texture. Therefore, the signature of fluvial-sediment discharge is altered on its path to the coastal ocean, and the disconnected mud and sand supply functions at the river mouth should result in distinct offshore depositional signatures.
ArticleA catastrophic meltwater flood event and the formation of the Hudson Shelf Valley(Elsevier B.V., 2007-01-04) Thieler, E. Robert ; Butman, Bradford ; Schwab, William C. ; Allison, Mead A. ; Driscoll, Neal W. ; Donnelly, Jeffrey P. ; Uchupi, ElazarThe Hudson Shelf Valley (HSV) is the largest physiographic feature on the U.S. mid-Atlantic continental shelf. The 150-km long valley is the submerged extension of the ancestral Hudson River Valley that connects to the Hudson Canyon. Unlike other incised valleys on the mid-Atlantic shelf, it has not been infilled with sediment during the Holocene. Analyses of multibeam bathymetry, acoustic backscatter intensity, and high-resolution seismic reflection profiles reveal morphologic and stratigraphic evidence for a catastrophic meltwater flood event that formed the modern HSV. The valley and its distal deposits record a discrete flood event that carved 15-m high banks, formed a 120-km2 field of 3- to 6-m high bedforms, and deposited a subaqueous delta on the outer shelf. The HSV is inferred to have been carved initially by precipitation and meltwater runoff during the advance of the Laurentide Ice Sheet, and later by the drainage of early proglacial lakes through stable spillways. A flood resulting from the failure of the terminal moraine dam at the Narrows between Staten Island and Long Island, New York, allowed glacial lakes in the Hudson and Ontario basins to drain across the continental shelf. Water level changes in the Hudson River basin associated with the catastrophic drainage of glacial lakes Iroquois, Vermont, and Albany around 11,450 14C year BP (~ 13,350 cal BP) may have precipitated dam failure at the Narrows. This 3200 km3 discharge of freshwater entered the North Atlantic proximal to the Gulf Stream and may have affected thermohaline circulation at the onset of the Intra-Allerød Cold Period. Based on bedform characteristics and fluvial morphology in the HSV, the maximum freshwater flux during the flood event is estimated to be ~ 0.46 Sv for a duration of ~ 80 days.