7. Conclusions

Elkhorn Slough is unique: unlike estuaries which have evolved over hundreds or thousands of years, this estuary was transformed from a sluggish backwater in 1946 when Moss Landing Harbor was formed, to a vigorous, rapidly growing estuary that has become a habitat for many fish, marine, mammals and sea birds. This transformation has taken place in less than 50 years and continues today.

ES is an ebb-dominated embayment which produces asymmetric tides. The ebb tidal currents are stronger than the flood currents and the duration of the transition from high to low tides is shorter than the reverse. The presence of extensive mudflats and Salicornia marsh distorts the incoming tide through water storage on the mudflats and through increased friction. Thus, the mud flats slope downward may be an important factor in contributing to the tidal asymmetry in ES. The distortion experienced by the incoming tidal wave produces a number of shallow water constituents including the M3, M4, and M6 overtides, and the 2MK3 and MK3 compound tides. The degree to which ebb domination is due to the form of the incoming tide, i.e., mixed, mainly semidiurnal, or to the overtides and compound tides that are generated is an open question. Tidal currents are maximum along the main channel and their vertical structure indicates that slough waters are well mixed between the bottom boundary layer and the surface. Tidal currents near the H1B at maximum ebb have increased from approximately 75 to 150 cm/s over the past 30 years. This increase in current speed can be attributed primarily to the increase in tidal prism which has increased from approximately 2.5 to 7.6 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> between 1956 and 2003. The increase in tidal prism is the result of both man-made changes to the Slough, and the continuing process of tidal erosion. We note that these changes are not independent as far as the circulation of ES is concerned. When the man-made effects of increasing the surface area of the Slough occurred in 1983, the corresponding increase in the tidal prism required that the tidal currents increase in accordance with the tidal prism. Thus, that the mud flats slope downward may be an important factor in contributing to the tidal asymmetry in ES.

As in most estuaries, the tidal response in ES shows both standing and progressive wave character. Since the incoming tide is subject to frictional dissipation as it progresses up the Slough, the combined response of both waves consists of a mixture of a standing wave (without amplification) and a progressive wave. From harmonic analysis of the currents in the lower slough, we find that currents lag tidal elevation by 84° and 88° for the M2 and K1 constituents, respectively. Thus standing wave behavior appears to dominate, in agreement with Dyer [8] who indicates that most estuaries display characteristics that are consistent with standing wave behavior.

The physical properties of ES vary seasonally and with location. Temperature and salinity in the lower slough reflect primarily the influence of Monterey Bay waters, whereas the temperature and salinity of the waters in the upper slough (>5 km from the mouth) tend to reflect the influence of heating and precipitation (or their converses) from the atmosphere. During the summer, both temperature and salinity are higher in the upper slough due to local heating and excess evaporation, respectively. During the winter, salinities can reach values of less than 20 ppt during periods of heavy precipitation.

Our knowledge of the circulation and distribution of physical properties in ES are, to a large extent, based on data collected during the 1970s and 1980s. However, the results presented here show that the Slough is changing rapidly. In the past,

## A 30-Year History of the Tides and Currents in Elkhorn Slough, California DOI: http://dx.doi.org/10.5772/intechopen.88671

waters in upper ES were distinct from lower ES with the high tide interface located near the entrance to the Parsons Slough and South Marsh additions. As the tidal prism increases, this boundary between upper ES and Monterey Bay waters will move inland, and waters in the lower slough will be more ocean-like. With future observations, we will be able to confirm or reject these ideas.

Few measurements have been made in Parsons Slough and the adjoining South Marsh. This overlooked area contributes over 30% to the tidal prism for ES. From recent observations, we have observed vigorous tidal flows entering (60 cm/s) and leaving (>100 cm/s) Parsons Slough through its entrance located under the narrow railroad trestle (Figures 9 and 10). Recent current measurements near the entrance indicate that relatively large volumes of water are exchanged between Parsons Slough and ES itself. It is recommended that new observations of water elevation, temperature, and salinity in the South Marsh/Parsons Slough area, and current measurements through the entrance to Parsons Slough be acquired to better understand this relatively new portion of ES and its contribution to the overall water budget of the Slough per se.

The volume of water taken in by the Duke Energy Power Plant on a daily basis is relatively large compared to the tidal prism of ES. For an intake rate of 2.0 <sup>10</sup><sup>3</sup> m3 /min, the total volume of water taken in by the power plant is almost 50% of the tidal prism over a tidal day. However, the intake has essentially no effect on the Slough itself, but profoundly affects the circulation of Moss Landing Harbor and increases the current speeds for the incoming tide through the harbor entrance by up to 10 cm/s.

Residence time for waters in the lower slough is relatively short, on the order of a tidal cycle or perhaps several cycles at most. Summertime diffusive residence times for the upper Slough based on data collected in 1973–1974 were of the order of 30 days. Because of the increase in tidal prism in ES since that time, residence times in the upper Slough have almost certainly decreased. New observations in the upper Slough will be required to address this important question.

As of the early 2000s, few if any observations have been made of the ES discharge plume in Monterey Bay. Cursory observations show that the plume is discharged to the southwest and becomes entrained in the circulation of Monterey Bay. At its maximum extent, the plume may extend as far as 3 km offshore. These sediment laden plumes provide further evidence of the erosional processes at work inside the Slough. On daily time scales, the plume provides a clear indication of the sediment erosion that takes place within the Slough. On longer time scales, this erosion eventually contributes to sediment transport through Monterey Submarine Canyon. For sediment, nutrient and water budgets, we must know more about the fate of the plume as it becomes entrained in Monterey Bay waters, and learn more about the pathway taken by waters which enter the Slough and Harbor on the incoming tide.

It is interesting to note that the four estimates of the tidal prism for ES that have been made over the past 47 years show a somewhat linear increase over time (Figure 18). However, changes in the tidal prism have often occurred rather abruptly due to human intervention, such as the restoration of Parsons Slough and South Marsh in 1983; as a result, the slope of the trend in Figure 18 is not constant, as might otherwise be inferred. We believe the accuracy of the tidal prism estimates is about 20%, although more conservative estimates have been reported for other estuaries, e.g., O'Brien, 15% [37]. In lieu of a better predictor, the slope of the linear trend in Figure 18 is approximately 0.1 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> /year over the 47 years between 1956 and 2003. We also note that although the uncertainty of the individual estimates of the tidal prism may be relatively large, the uncertainty of the slope
