COMPLEX FLOW DYNAMICS WITHIN A FRESHWATER TIDAL ECOSYSTEM: THE LOWER ANDROSCOGGIN PORTION OF MERRYMEETING BAY, MID-COAST MAINE

Merrymeeting Bay is a shallow freshwater tidal ecosystem formed at the confluence of the Kennebec and Androscoggin Rivers in mid-coast Maine. The bed topography of the bay is complex, reflecting the arrangement of glacially scoured bedrock and the tidal/riverine redistribution of glacial sand. During summer 2004, boat traverses were conducted during semidiurnal neap and spring tidal cycles (ranges of 1.2 and 1.7 m, respectively) utilizing an Acoustic Doppler Current Profiler to investigate current flow within the lower Androscoggin portion of the bay. Overall, tidal fluxes average an order of magnitude higher than river discharge, but flow is spatially non-uniform. During rising tides, high-velocity currents move up bay through a channel that wraps around the bay’s northwestern edge. In this channel, velocity magnitudes are a factor of two greater during flood than during ebb tide. Ebb flow, in contrast, is concentrated in shallow channels along the southwestern margin of the bay, at velocities some 30% higher than during the flood tide. Surface measurements reveal higher salinity, colder temperatures, and less turbidity in the flood-dominated channel over the course of the flood tide, versus lower salinity but higher temperature and turbidity in the ebb-dominated channels over the course of the ebb tide. Convergence fronts are common in the flood-dominated channel throughout the tidal cycle, reflecting density gradients and/or convergence of water moving via different paths through the complex topography. The distribution of sand bodies within the bay follows the major flow patterns. The flood-dominated channel is flanked to the north by a large flood-tidal delta, whereas ebb-oriented linguoid bars occupy the ebb-dominated channels to the southwest. These large bedforms, as well as channel floors, are mantled by dunes, some of which reverse direction with tidal flow. The complexity of flow presents challenges to sampling and modeling biogeochemical and sediment fluxes in this unique coastal environment.