Sediment Transport Dynamics of a Shallow, Eutrophic Bay: Missisquoi Bay, Lake Champlain, VT

Missisquoi Bay, on the northeast portion of Lake Champlain (NY-Vermont), is a uniformly shallow bay with a mean depth of slightly less than 3 m. Changes in land-use practices since the 19th century - intensification of agriculture, increased animal husbandry, and rapid urbanization - have increased nitrogen and phosphorus loading in the bay, causing eutrophication and seasonal oxygen depletion. A 3-year monitoring program from 2012-14 utilized an array of ADCPs, water level gauges, vertical temperature strings, and meteorological sensors to monitor the bay’s hydrodynamics. From these data, four unique modes of circulation were found to exist, including periods of both vertically uniform and thermally stratified 2-layer flow. With respect to long-term sedimentation from such varied hydrodynamics, are there one or more dominant modes that control deposition or erosion? This question was addressed using two sediment studies, Sediment Trend Analysis® (STA) and acoustic sediment characterization software (SwathWay). Total grain size distributions of 369 grab samples were analyzed to perform a STA to identify the patterns of net sediment transport and their dynamic behavior (e.g., deposition, erosion, and dynamic equilibrium). SwathWay (Maritime Way Scientific, Ltd.) defined 12 distinct acoustic backscatterance signatures (classes or regions) from 3-years of Multibeam sonar data. Combining hydrodynamics, sediment grain size and density, STA results and acoustic zonations, a pattern has evolved which addresses the flow regime that is most representative of sediment transport and deposition, providing insight into the role sediment transport plays in eutrophication of the bay.