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Drifter Studies in Open Shallow Water Habitats of the San Francisco Bay and Delta
Jay I. Cuetara and Jon R. Burau, USGS, Sacramento, CA

Spatial variability in tidal forcing

The drifter studies shown in this series of web pages highlight the wide range of hydrodynamic forcing seen in the Bay/Delta system. The tidal forcing is greatest near the Golden Gate (tidal range of ~2 yards) where the tide from the Pacific Ocean propagates into the Bay. As one proceeds landward from the Golden Gate, the effect of the tide gradually diminishes until, on the eastern rim of the Delta, the flows become completely riverine. Figure 3 contrasts the tidal forcing between sea level records recorded at the Golden Gate and at the USGS-maintained flow-measuring station on the San Joaquin River at Jersey Point (see Study Area for location of Jersey Point). The drifters released in Suisun Bay (Figure 4) have relatively long and oscillatory (not shown) drifter paths (7.5 miles) typical of highly tidally forced systems. The drifters paths in Mildred Island (Figure 7) and Franks Tract (Figure 6), by contrast, are relatively short (2.8 miles and 1.9 miles, respectively) and typical of weak tidal forcing (and low river flows). The Jersey Point discharge and RMS discharge are given as a reference for each drifter study so that the magnitude of tidal flows between drifter studies may be compared.

Spring-neap cycle

Another important feature of the tides is the 14-day period "spring/neap" tidal cycle. Although the tides rise and fall daily, the amount of the rise and fall changes daily with the spring/neap tidal cycle and with the seasons. The spring/neap cycle modulation of the tides is the single most important characteristic in the tidal cycle, and can represent a two-fold difference in the magnitude of the tidal variations (Figure 4). The amplitude of the spring/neap modulation changes throughout the year: therefore, not all spring or neap tides are equal in magnitude. When comparing the drifter paths shown on these web pages one must take into account not only the daily tidal differences (tidal discharge subplots are presented for each drifter study), but also the magnitude of a given spring or neap tide represented by the Root Mean Square (RMS) tidal discharge subplots. The RMS tidal discharge is a measure of the tidal energy and allows one to quantitatively compare the relative amount of energy in a given part of the spring/neap tidal cycle. Low points in the RMS tidal discharge plot represent neap tides and peaks represent spring tides.

The drifter studies in Franks Tract and Mildred Island highlight the difference between the spring and neap exchanges and residence times that occur on flooded islands. Even though the studies shown on these web pages were performed during a moderately strong spring and moderately weak neap (see RMS tidal discharge subplots), the tidal excursion on the spring tide is much greater than on neap tides in both of the flooded islands studied.

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