The influence of the Kennebec River discharge on estuarine and reverse estuarine flow in eastern Casco Bay, Gulf of Maine

Harpswell Sound and the New Meadows estuary are narrow coastal embayments on the eastern side of Casco Bay, in the Gulf of Maine. Hourly oceanographic data was collected at both locations, including ADCP profiles of current velocity vs. depth. It was found that net circulation in both inlets displays both estuarine and inverse estuarine characteristics at various times. During periods of low discharge from the neighboring Kennebec River there is little connection between the two inlets, with New Meadows developing a three- layered circulation regime and Harpswell Sound having periods of negligible net flow punctuated by brief wind-driven events. However, when the discharge of the Kennebec River is high both inlets experience considerable freshening of surface water with corresponding increases in vertical stratification. In addition both inlets act in concert, experiencing near-simultaneous alternation between estuarine and inverse estuarine current regimes of relatively high strength. It is hypothesized that this coordinated reverse estuarine flow is caused by the passage of the leading edge of a bolus of fresher Kennebec discharge past the entrances of the inlets. Water flowing down the sloping surface of this bolus is deflected by the Coriolis force, running into the inlets at the surface and triggering a compensating outflow at depth. Conversely, coordinated estuarine flow is caused by the passage of the trailing edge of the bolus past the entrances, causing the sea surface slope to switch direction.