Transport dynamics of Missisquoi Bay, Lake Champlain, Vermont

Missisquoi Bay, located in both Canadian and US waters in 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 such as intensification of agriculture, increased animal husbandry, and rapid urbanization has increased the loadings of nitrogen and phosphorus causing eutrophication and seasonal oxygen depletion. Since monitoring began in 1992, Missisquoi Bay has displayed the greatest mean total phosphorus concentrations and chlorophyll-ɑ concentrations in Lake Champlain. A 3-year monitoring program (VT EPSCoR RACC 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 basic modes of circulation were found to exist. The first was defined as “wintertime sluggish” wherein water velocities were vertically uniform and on the margin of detectability by the ADCPs. The second “spring melt” mode was when all three river inputs were maximized with high-volume flows. The third and fourth modes were documented when stratified conditions could exist (May-November) and subsequently were divided up into those times when the water column was well mixed (mode 3 - “well-mixed summer”) and when stratified conditions led to highly dynamic 2-layer flow (mode 4 - “two-layer summer”). As a result, the circulation patterns within the bay are in constant flux due not only to the presence of any one of the four modes but, just as importantly, wind forcing. To understand sediment transport in the bay with regards to these modes of circulation, two sediment studies, Sediment Trend Analysis® (STA) and acoustic sediment characterization software (SwathWay) were done. Using grain size determination from grab samples in a 500 m grid, STA was performed to identify the patterns of net sediment transport and 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 collected over a majority of the bay. 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.