Investigating the Influence of Coastal Islands on River Water Distribution and Mixing in Western Long Island Sound

The Saugatuck, Norwalk, and Five Mile Rivers enter western Long Island Sound (LIS) adjacent to a chain of islands. These islands can influence the exchange of river water with the broader estuary. An observational and modeling approach is taken to determine the influence of these islands on the flow pathways and mixing of these coastal river waters. Prior research used a LIS- wide application of the Regional Ocean Modeling System (ROMS) and passive tracers to track the distribution of waters from individual major rivers (e.g. Connecticut and Housatonic) and groups of smaller coastal rivers. The western group of small coastal rivers (including the Saugatuck, Norwalk and Five Mile Rivers) contributes less than 10% of the freshwater in western LIS. The farther removed but larger Connecticut and Housatonic collectively account for more than 75% of the freshwater. These small coastal rivers, however, were found to enhance coastal stratification in early summer more so than the larger rivers. The LIS-wide model, however, lacked adequate resolution to include the island chain and its effects on circulation and freshwater distribution. Recent observations have suggested that these islands can enhance mixing in specific areas around the islands. Observed surface salinities tend to be fresher inshore of the island chain and show a clear freshening after major rain events. A new high resolution nested grid is applied to this area in ROMS to resolve the islands and to isolate their effects. Model output and data collected during the summer of 2015 low discharge season are used for comparative purposes and to determine the overall influence of these islands on the flow pathways and mixing of coastal river waters.