SEASONAL PATTERNS OF COHESIVE PARTICLE CHARACTERISTICS IN THE CONNECTICUT RIVER ESTUARY

The fate of fine-grained, cohesive sediments in estuaries is largely determined by the particle settling velocity. Flocculation of fine sediments, or the formation of aggregates composed of silts and clays through inter-particle collision and adhesion, is a major control on the effective settling velocity of these muddy sediments, which are of particular interest in estuaries due to their impact on the fate of contaminants. In dynamic environments like estuaries, flocculation may vary widely in both time and space. The primary objective of this study is to quantify cohesive suspended-particle dynamics in the Connecticut River estuary and assess the impact on fine-sediment transport and trapping in a highly stratified, seasonally-varying estuarine system. Spatial and seasonal measurements of in situ floc size, density, salinity, suspended-sediment concentration (SSC) and bottom stress have been collected over three sampling periods with varying discharge regimes representative of seasonal variability. Three particle populations are apparent in the estuary each season, with major changes in floc density and size. In November 2013, a moderate-discharge event coupled with high sediment supply resulted in the largest observed flocs (median size 302 um). High SSC supports the building of fresh, large, loosely-bound flocs (median excess density 17.6 kg/m³). A typical spring freshet was captured in May 2014, where highest SSC and lowest-density flocs were observed throughout the estuary (13.9 kg/m³). Flocs form and grow (180 to 400 um) with increasing bottom stresses to 0.6 Pa, above which high stress causes floc breakup. This trend disappeared in September 2015, a period of low discharge and sediment supply. The smallest, most compact flocs (244 um; 60.1 kg/m³) were observed throughout the estuary with no correlation to bed stress, due to lack of sediment available for resuspension and new floc formation.