CIRCULATION DYNAMICS OF MISSISQUOI BAY; A NEW LOOK AT THE QUESTION OF WATER QUALITY AND CAUSEWAYS

Lake Champlain's Missisquoi Bay is a uniformly shallow bay with a mean depth slightly less than 4 m. Three rivers that discharge into Missisquoi Bay (the Missisquoi, Pike and Rock) have drainage areas totaling 2900 km² and are primarily located in agricultural settings. In the mid-1800s, a large rock-filled causeway was built across the 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 basin, so too does the amount of phosphorus accumulation and unsightly/unhealthy 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. Over a decade later, a 3-year hydrodynamic monitoring network of ADCPs, water level gauges, vertical temperature strings and meteorological sensors were placed throughout the bay. 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 generally <1 cm/s. The second mode, “spring melt”, is where all three river inputs were maximized with high-volume flows while the third and fourth modes were confined from May-Nov when stratified conditions could exist. During this time, modes 3 and 4 were characterized by well-mixed and stratified conditions, respectively. While mode 3 was expected due to the shallow depth of the bay, mode 4 was not and led to highly dynamic 2-layer flow. 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 management policy. Presently, we are in the process of calibrating an atmospherically coupled 3D hydrodynamic model for the Restricted Arm that will hopefully shed further light on the issues of causeway removal and water quality from Missisquoi Bay to Malletts Bay.