Dispersion by tidal trapping is enhanced by stratification

The North River estuary, Massachusetts, is a small marsh creek network in which tidal dispersion processes dominate the salt balance. We investigated the tidal trapping mechanism due to tributary creeks as a possible explanation for the vigorous tidal dispersion. We deployed moorings to measure time-series of salinity, velocity, and water level in the main channel and creeks, and we also conducted shipboard surveys with an acoustic Doppler current profiler (ADCP) and a conductivity-temperature-depth (CTD) sensor to measure velocity and salinity profiles. Results from this field study were compared with model output from an idealized estuary simulated with the Regional Ocean Modeling System (ROMS). During flood tide, saltwater propagates up the main channel and gets ‘trapped’ in the creeks. Whereas the main channel has weak stratification due to strong tidal mixing, the creeks show persistent stratification and a robust vertically sheared exchange flow, due both to weaker tidal currents and stronger along-axis salinity gradient. The strong salinity gradient arises from the time-dependent boundary forcing at the mouths of the creeks, with negligible freshwater input from the heads of the creeks. Since the main channel is longer and deeper than the tributary creeks, the tide propagates more like a progressive wave in the main channel and like a standing wave in the creeks. Thus, the creek reverses flow direction before the main channel, releasing fresher water back into the main channel during ebb tide. This reduction of salinity in the main channel during ebb results in a net landward salt flux over a tidal cycle. The tidal trapping mechanism is enhanced by the exchange flow within the creeks that enhances the salinity contrast between the outflowing creek water and the main channel. The effective dispersion rate is approximately twice as high as theoretical parameterizations that do not consider the exchange flow. The trapping mechanism due to creeks may account for up to half the total tidal dispersion.