The Impact of Dams on Sediment Transport from the Parker River Watershed to the Plum Island Estuary

Abstract

Though previous studies have shown saltmarsh adaptability to some degree of sea-level rise (SLR), sediment supply is critical to sustaining saltmarshes as SLR accelerates. Land-use activities, such as dams, often influence watershed sediment transport and delivery to the coast. Previous studies have suggested that, even in small watersheds, dams can significantly impact coastal sediment budgets. The Parker River watershed (PRW) in northeastern Massachusetts hosts 20 dams and several natural lakes, and drains into the Plum Island Sound Estuary (PIE). This research aims to evaluate the impact of dams and sediment transport in the PRW. Three approaches were used: theoretical modeling of sediment transport patterns using digital elevation models; spatial analysis of suspended sediment concentration (SSC) and remote sensing data; and empirical calculations of reservoir trap efficiency. Geomorphic modeling indicates that bankfull discharge can transport 20 μm grains (silt) as wash load throughout the PRW. Sediment deposition might happen at Crane Pond and in reservoirs, but removing dams would not change this pattern. Both SSC data and observations of satellite images during high-flow events indicate low supply and transport of sediment throughout the PRW. The estimates of sediment yield (Y) are low for the PRW. An empirical calculation indicates little-to-no trap efficiencies for all dams. Therefore, fluvial contribution to the sediment budget of the PIS estuary is limited and dam removals in the PRW are unlikely to change the rate of sediment delivery to the PIE. The proposed method of this study provides an additional scope to assess the ecological benefits of removing a dam and could be easily replicated for other locations for similar assessment. Future studies should assess sediment dynamics and management practices from a more thorough perspective incorporating the riverine, estuarine and shelf system.