Longitudinal sections of potential density (s₀) in the East River from CTD surveys on June 17, 2003 and June 25, 2003. The surveys are taken at opposite states of the tide: the survey on June 17 began 2 h after SBF and the survey on June 25 began 2 h after SBE. Both surveys were made from south (the Upper Bay) to north (Long Island Sound) and lasted approximately 3 h. Small lines of dots indicate sample points. A consistent structure is evident in the two surveys, displaced according to the tidal phase. In general, the northern half of the East River manifests the vertical stratification typical of an estuarine circulation, while the southern half is vertically homogenous with the exception of a lens of fresher water from the Hudson River that intrudes up the East River 2 to 6 km depending upon the tide.

Tidally synchronized longitudinal profiles of SF₆ concentrations in the East River for six days following the injection at SBF on June 17, 2003. The static locations of secondary features (peaks and valleys) in the tracer concentration suggest little residual circulation during this period. The uneven spacing between the surveys from each day indicates a highly variable decay rate (note logarithmic scale).

Daily comparisons of tidally synchronized longitudinal profiles of SF₆ concentrations in the East River following the flood injection (open circles) and the ebb injection (closed circles). Differential flushing behavior of the tracer as a function of tidal phase during injection is clearly evident, particularly in the southern (negative kmp) half of the river. The profile from day 3E is particularly clear in this regard; north of the injection point, SF₆ concentrations are similar for both injections, but south of the injection point, the ebb injection is sharply reduced.

Vertical profiles of SF₆ concentration in the East River from three regions: (a) northern half of the river; (b) injection point; (c) southern half of the river. Each day is represented by a different symbol, and open symbols represent profiles following the flood injection, while closed symbols represent profiles following the ebb injection. Where more than one profile was taken on the same day in the same region, the same symbol is used. The vertical structure in the southern half of the river and at the injection point is weak, but in the northern half of the river (the portion closer to Long Island Sound), a strong gradient is evident that depends neither upon the tidal phase of the injection nor the tidal phase of the data collection.

Vertical SF₆ profiles in the Hudson River following the flood (a) and ebb (b) injections. Different symbols indicate different locations, and lighter symbols indicate later observations. Distance up the Hudson River (from the Battery) is indicated as “HR kmp”. In (c), normalized SF₆ concentrations are plotted against salinity for the full set of profiles (excepting one incomplete profile from HR kmp 33). The peak SF₆ concentrations are found in an intermediate water layer (or along the bottom, further up the Hudson River) at salinities matching those in the East River, suggesting that the tracer-tagged water was carried into the Upper Bay, sandwiched between high-salinity oceanic bottom water and low-salinity surface water from the Hudson River, and carried up the estuary. Two days after the injections, ebb injection concentrations were much larger than those from the flood injection, but in later days these values evened out.