Missisquoi Bay circulation dynamics and 3D hydrodynamic modeling of the restricted arm of Lake Champlain; a question of water quality and causeways

Missisquoi Bay is a uniformly shallow bay with a mean depth of slightly less than 4 m. With a surface area of 78 km², it is the 3rd-largest physical feature of the lake with the Main Lake (683 km²) and Northeast Arm (269 km²) being the first two major sectors. Three rivers that discharge into Missisquoi Bay (the Missisquoi, Pike and Rock) have drainage areas totaling 2900 km² that are primarily located in agricultural settings. In the mid-1800s, a large rock-filled causeway was built across 1200 m wide southwest channel which provided the only access to Lake Champlain. Only ~200 m of the original channel was left open for water movement. As farming practices continue to develop within the drainage basins, so did the amount of phosphorus accumulation and unsightly/unhealthy algal blooms of blue-green algae. Even though numerical modeling of the bay showed that there would be no significant improvement in water quality even if the entire causeway was removed, the narrow channel was opened up by an additional 100 m by the state of Vermont in an effort to balance the public outcries of highly eutrophic conditions as well as the rock-filled causeway becoming a breeding site for an endangered species. This model was verified against a single ADCP record of ~6 weeks and a few surface drifter tracks during that time period. During a 3-year monitoring program, arrays of ADCPs, water level gauges, vertical temperature strings and meteorological sensors were used to monitor the bay’s hydrodynamics. From this program, four basic modes of circulation were found to exist. The first is defined as “wintertime sluggish” wherein water velocities are vertically uniform and on the margin of detectability by the ADCP. The second mode is “spring melt” where all three river inputs were maximized with high-volume flows. The third and fourth modes were confined when stratified conditions could exist (May-November) but 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”). All of the four modes exhibited unique circulation dynamics that if modeled correctly, would provide greater insight to the chemical, biological and sedimentological transports within the bay as well as creating a more informed public and management with regards to phosphorus dynamics related to the causeway. Presently, we are in the process of verifying a fully 3D hydrodynamic model for the Restricted Arm that will shed further light on the issues of causeway removals and water quality from Missisquoi Bay to Malletts Bay.