Effect of Tributary Creeks on Estuarine Dispersion

Tidal dispersion has been found to be the dominant mechanism of upstream salt transport in the North River, a salt-marsh estuary located in Marshfield, MA, USA. A field study was conducted to determine whether the presence of side creeks could explain the vigorous tidal dispersion, based on the Okubo (1973) tidal trapping mechanism. Time-series measurements of salinity, velocity and water level were obtained in the main channel and the adjoining creeks to investigate differences in tidal phase of currents and salinity through the spring-neap cycle. Additionally, surveys were conducted with an acoustic Doppler current profiler (ADCP) and a conductivity-temperature-depth (CTD) sensor over flood and ebb through the tidal cycle to measure velocity and salinity across transects of the main channel and throughout the side channels.

The key finding is that the creeks lower the salinity of the main channel during the ebb tide, thereby shifting the phase of the salinity to favor up-estuary salt transport in the North River. Consistent with the Okubo (1973) mechanism, the tidal phase of the currents in the side creeks leads that of the main channel. This phase shift results in extraction of lower salinity from the main channel during early flood, and re-injection of that low salinity water into the high-salinity water of the main channel during early ebb. The fresher outflow ( 4 PSU difference between the side and main channels) forms fronts in the main channel at the junctions, which can result in recirculation zones that affect mixing and depositional processes. While the creeks provide a measurable phase shift of salinity of the North River, there must be other significant trapping volumes to account for the high dispersion rates. Mosquito ditches as well as tidal over-topping of the marsh during spring tides may also contribute significantly to the total dispersion.