SEDIMENTATION PATTERNS IN THE DAMARISCOTTA RIVER ESTUARY

The tidally dominated Damariscotta River estuary is located on the south-central Maine coast. The elongate, north-south orientation of the estuary is characteristic of the indented shoreline in this region and a consequence of the bedrock structural framework, comprising Paleozoic high-grade metasedimentary rocks. The river system relief is ~95 m overall and a series of pegmatite sills separate the estuary into seven distinct basins. The basins were sequentially isolated during fall from the high-stand of sea-level associated with retreat of the Laurentide ice sheet, and then gradually reincorporated into the marine environment with the post-glacial sea level transgression. Holocene sediment within the estuary is found in three distinct zones: 1) a stable inner zone with accumulation of sediments on tidal flats and in marshes, 2) a transitional middle zone with tidal ravinement reworking of estuarine sediments including oyster bioherms, and 3) a high-energy, deeply scoured outer zone in which sediment is preserved only below wave base. Previous studies employing seismic reflection profiles, side-scan sonar, and sediment cores have led to a well-understood model of estuarine evolution and stratigraphy. The present study uses multibeam bathymetry, sediment cores and grab samples, resulting in high-resolution bathymetric and surficial sediment maps of the estuarine system and an improved understanding of sedimentary processes within the estuary. Bathymetry and backscatter maps reveal plumose tidal scour features bordering the thalweg of the middle zone of the estuary, as well as fan-like drainage networks in the intertidal to subtidal transitional zone in multiple coves. Analyses of sediment cores from each estuarine zone, including using ²¹⁰Pb and ¹³⁷Cs dating, reveal modern sediment accumulation rates, which are particularly relevant in the context of disturbances to the system, such as deforestation associated with colonization, and the introduction of aquaculture to the system.