5. Physical properties and processes

### 5.1 Distribution of physical properties

Three distinct water types result from the physical processes in ES [10, 22]. The primary water type consists of offshore waters which enter the Slough with the flood tide and is characterized by cool temperatures ranging from 9 to 16°C, and salinities that range from slightly over 33 to almost 34 parts per thousand (ppt). The second water type consists of relatively fresh water mainly derived from agricultural runoff from the Old Salinas River channel through South Moss Landing Harbor. Because this water is of low salinity (<10 ppt), it is less dense than the waters from Monterey Bay and forms a thin surface layer. According to Smith [10], this water did not usually extend its influence beyond the South Harbor basin because pumping by the PG&E power plant was sufficient to maintain a net flow of offshore water into the harbor. As the pumping rates at the power plant have increased, the influence of fresh water entering the Slough from the South Harbor has correspondingly been reduced.

The third water type is formed in the upper Slough. During summer, this water is of higher salinity due to excess evaporation, and during winter, it is usually of lower salinity due to precipitation and runoff. Because this water type is formed in the upper Slough (5–10 km from the mouth), it may lie beyond the inland reach of the tidal prism and was found to have longer residence times than lower slough waters. Although the characteristics of this water type were well-documented from data collected in the 1970s, the properties and extent of this water type may have changed due to the increased tidal prism. For example, the reach of the tidal prism may extend further up the Slough today than it did 30 years ago. Because the volumes of water associated with the second and third water types are small in comparison to the offshore waters, their influence is primarily restricted to where they enter the Slough (South Moss Landing Harbor), or are formed (in the upper Slough). From measurements made over 25-h periods at the H1B and in the upper

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

Slough during 1971 and 1975, variations in temperature and salinity were highly correlated with tidal forcing at periods of 12 and 24 h [10, 22]. The ratio of the 12 and 24-h salinity amplitudes were similar to the corresponding tidal height amplitudes. The amplitudes for temperature and dissolved oxygen, however, showed a higher correlation at 24 h than at 12 h, suggesting that diurnal variations in heating/ cooling and biological photosynthesis/respiration contributed significantly to these variations. In the lower slough (0 to 5 km from the mouth), the influence of offshore waters decreased the effect of diurnal heating. As indicated earlier, tidal forcing causes the waters of ES to be well-mixed particularly along the main channel. Vertical profiles of temperature and salinity show little vertical stratification, except during periods of heavy rainfall in the winter [10].

The physical properties of ES vary on a seasonal basis. Seasonal changes in temperature and salinity in the upper slough are due to local influences, whereas seasonal changes in the lower Slough primarily reflect changes that occur in Monterey Bay. In Figure 15, temperature (upper panel) and salinity (lower panel) as a function of time and distance from the mouth are shown for the period July 1974 to May 1976. All sampling was done at high tide to remove the large tidal influences. The upper Slough is warmer than the lower Slough during the summer, and temperatures of 22°C have been observed. During winter, temperatures are cooler or about the same as offshore waters. In the upper Slough, temperatures during the winter as low as 12°C have been observed. A 14-month temperature time series (not shown) demonstrates that during the summer spring tide, pulses of relatively warm (>18°C) upper slough waters reach the lower Slough, and during winter, pulses of cool (<12°C) upper slough waters reach the lower Slough. ES, because of its direct connection to the bay, is also affected by El Nino conditions, and higher temperatures (2 to 4°C) may be observed during these episodes.

In the upper Slough, salinity (Figure 15, lower panel) is affected by precipitation, runoff, and evaporation. During late winter, when precipitation is greatest, salinities as low as 17 ppt have been observed. During late summer, when evaporation is maximum, salinities in the upper slough have reached 37 ppt. Thus, the characteristics of the Slough can vary from typically estuarine during periods of heavy precipitation in the winter, to an evaporative basin during the summer. We note that due to the occurrence of recent dry years along the central California coast, characterizing ES as typically estuarine during the winter may be less appropriate than characterizing the upper Slough as an evaporative basin during the summer.

Finally, Smith [10] concluded that the area above the tidal prism, i.e., the upper Slough, was essentially isolated from offshore influence in the lower Slough, and tended to develop a separate physical identity. Although increased tidal action might reduce the contrast between upper and lower slough water masses, recent work on phytoplankton community structure in ES shows that these waters have retained their separate identities (N. Welschmeyer, personal communication).

## 5.2 Diffusion and mixing

The tides contribute to horizontal as well as vertical mixing in ES. The effects of horizontal mixing can be quantified by estimating the coefficient of eddy diffusivity, KX. The magnitude of the along-channel diffusivity has been variously estimated using both physical and chemical parameters. When salinity is used to estimate KX, both fresh water discharge and evaporative fluxes will affect estimates of KX, if they are significant. Smith [10] estimated eddy diffusivities for ES using salinity data acquired during June and October of 1971, periods with no precipitation. Smith used the one-dimensional advection-diffusion equation balancing the seaward eddy diffusive salt flux with the landward advective salt flux to produce

Figure 15. Seasonal variation in temperature (upper panel) and salinity (lower panel) in ES between 1974 and 1976 (redrawn from Broenkow [22]).

the local time rate of change in salinity. He calculated the non-tidal landward velocity from observed evaporation rates from a nearby reservoir. From his results, the mean diffusivities for the summer can represented by a second-order polynomial as

$$
\ln\left(K\_X \times 10^{-3}\right) = 0.095 - 1.65X + 9.00X^2,\tag{3}
$$

where KX is the eddy diffusivity (m<sup>2</sup> /s) and X is distance from the mouth (km). Smith's table correctly sets the eddy diffusivity at 1 km from the mouth as infinite which is the case when lower slough waters exit the Slough and do not return. Monthly mean KX values, obtained over the length of the Slough, decreased by almost two orders of magnitude from the lower Slough (�<sup>500</sup> � <sup>10</sup><sup>3</sup> cm<sup>2</sup> /s) to the head of the Slough (�<sup>6</sup> � <sup>10</sup><sup>3</sup> cm<sup>2</sup> /s). These values from Smith are also in good

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

agreement with similar estimates of KX obtained in other well-mixed estuaries [23]. The relatively high values of eddy diffusivity in the lower Slough demonstrate the importance of the tides as the dominant forcing mechanism. Because similar results were obtained in successive months, Smith concluded that a balance between evaporation and tidal diffusion provided a satisfactory explanation for the increase in salinity that was observed.

Reilly [24] calculated Reynolds fluxes to estimate eddy diffusivity in the lower Slough. In September 1975, he measured currents and salinity in the main channel at two depths, 1 m above the bottom and 1 m below the surface, based on a 50-hour time series acquired 3 km inland from the H1B. Reilly decomposed observations of salinity and the along-channel component of velocity into mean, periodic, and turbulent components, following Hansen [25]. He then obtained estimates of the salt transport by taking the product of the various components of salinity and velocity, with the cross-sectional area of the channel at the location where the measurements were acquired. Cumulative fluxes of salinity for the periodic (i.e., tidal) and fluctuating (i.e., turbulent) components are shown in Figure 16.

The periodic Reynolds fluxes (upper panel) promote a seaward salt flux, whereas the turbulent Reynolds fluxes (lower panel) promote a landward flux. We note that Reilly's estimates of eddy diffusivity compare favorably with those obtained by Smith. However, because Smith's analysis was based on an integral taken over the summer season, his method is to be preferred.

## 5.3 Residence time

The flushing or residence time of slough waters is of considerable importance regarding the fate of pollutants and other dissolved materials. As indicated earlier, most of the water that leaves the Slough on the outgoing tide does not re-enter on the incoming tide. Thus, the residence time for waters in the lower Slough is short, on the order of a tidal cycle or a few cycles at most. However, the degree to which waters from the upper slough mix with waters from the lower slough is not

#### Figure 16.

Cumulative salinity fluxes in ES from Reilly's Reynolds flux calculations are shown [24]. Observations were made over a 50-hour period that were acquired near the "Dairies". Upper panel shows the harmonic salt flux UpSpA, where Up and Sp amplitudes are computed from harmonic analysis of the M2 and K1 tidal periods. Lower panel shows turbulent salt flux U<sup>0</sup> S0 A, where U<sup>0</sup> and S<sup>0</sup> are turbulent residuals following harmonic analysis. The trend lines indicate a seaward tidal flux and a landward turbulent flux. Note the different scales used to represent the fluxes.

well-established and is almost certainly seasonally dependent. During summer, the waters in the upper Slough become somewhat isolated from the waters in the lower Slough. In winter, during periods of precipitation, inflows from connecting tributaries and runoff enter the Slough and circulate into the lower Slough where they become part of the tidal prism. Smith [10] estimated that this sequence of events probably took upwards of a month in 1970, following a period of major precipitation. He also used his previously-derived eddy diffusivities (Section 5) to obtain estimates of residence times of about 30 days in the upper Slough during summer.

Because the tidal prism has increased since Smith's work, residence times in the upper slough are likely to have decreased. Near-surface temperature measurements taken across the channel between Parsons Slough and Kirby Park in July 2002 indicate that in areas outside the main channel, temperatures are slightly higher along the banks. This suggests that circulation in wider portions of the Slough may be weaker than flow along the main channel.
