This dissertation advances knowledge of the mechanisms affecting sediment transport and deposition in microtidal rivers and marshes. Multiple field and dataset studies were conducted in the St. Jones River in central Delaware. Four component projects focused on forcing mechanisms influencing water properties and sediment fluxes on multiple timescales, and processes that affect sediment trapping and strata formation on the marsh surface.

The St. Jones River estuary is apparently well-mixed and tide-dominated, based on its depth, gauged discharge, and surface tide characteristics. Nevertheless, it is strongly impacted by offshore wind forcing and freshwater discharge. Oceanic forcing mechanisms exert a very strong control on sediment transport and related water properties in the study area, even 12 km upstream of the mouth of the river. Offshore wind stress in the along-shelf direction affects river sea level and salinity through Ekman set-up and set-down of Delaware Bay sea level. This remote wind effect is strongest over timescales of 2 to 14 days.

Freshwater discharge into the river is much larger than the amount indicated by a gauging station upstream of the head of tides, with a minimum of 63% of freshwater discharged into the estuarine portion of the river coming from ungauged sources. Sea level, salinity, and turbidity are strongly controlled by tides on less-than-daily timescales, but on monthly and longer timescales, water properties are affected by seasonal differences in freshwater discharge, atmospheric temperature, and storminess.

The direction of suspended-sediment transport changes with position in the river, and cannot be predicted from distortions of the vertical tide. Storms temporarily reverse the dominant direction of residual flow and suspended-sediment flux. Northeasters consistently generate water levels that exceed normal tides and turbidity levels. The position of the nearest high pressure system is critical in determining the magnitude of the effect on water level and turbidity of a weak or moderate northeaster.

The mechanisms controlling daily sediment deposition on the marsh surface can explain century-scale sediment accumulation in the study area. Rates and patterns of sediment deposition on the marsh surface were controlled by changes in grain/floc size and variations in overmarsh suspended-sediment concentration, rather than tidal hydroperiod. Daily sediment deposition can be scaled up to almost match longer-scale accumulation rates by assuming deposition only during spring tides. The results of these studies have implications for marsh modeling and restoration studies, and the response of the study river to climate change.