CONTROLS ON ESTUARINE MORPHODYNAMICS: A CASE STUDY ON MERRYMEETING BAY, KENNEBEC RIVER ESTUARY, MAINE

Three boat teams collected concurrent measurements of current velocity, salinity, and water temperature 25 km above the mouth of the Kennebec River estuary, Maine at Merrymeeting Bay during a 13 hr semi-diurnal tidal cycle of a near-perigean spring tide superimposed on low river flow. These data, in concert with side-scan sonograms and sediment samples, were used to examine assumptions of widely-accepted estuarine models and to test the hypothesis that Merrymeeting Bay serves as a sediment depocenter during summer months of low river flow, but is excavated during spring freshets to supply the lower estuary with bedload sediment. The data from the summer hydrography and side-scan sonar cruise (July 2001) revealed that Merrymeeting Bay is a shallow bay choked with coarse-grained sediments molded into a suite of bedforms with nearly ubiquitous flood-orientations. Finer-grained deposits exist along the periphery and upstream of the Bay in low-energy tidal flats. Embayment geometery, salinity, water temperature, fresh-water discharge, current velocity, and bedform data all suggest that flood-velocity asymmetry, set up by strong flood-directed tides superimposed on low freshwater flows, is the most important control on bedload sediment transport within this high-latitude bay that connects two of Maine’s largest rivers with the lower Kennebec River estuary. Consequently, during summer months, little sediment is transferred from the Bay to the lower estuary. In this manner, the bedrock constriction that links Merrymeeting Bay to the lower estuary acts as a release valve through which sediment moves to the lower estuary during large scouring events such as spring freshets.