2. The setting

ES is located in central California at the eastern side of Monterey Bay mid-way between Monterey and Santa Cruz. Several tributaries (i.e., lesser sloughs) including Bennett, Moro Cojo, and Tembladero Sloughs feed directly into ES or Moss Landing Harbor. Originally, ES was somewhat removed from direct tidal flow and the Salinas River discharged directly into Monterey Bay just north of the entrance to Moss Landing Harbor (Figure 1). In the late 1800s, the lands surrounding ES were altered through the construction of ditches and dikes to make the low-lying marshes suitable for agriculture. Around 1900, a 150-ha area near the mouth of the Slough was diked to form evaporative salt ponds. In 1908, the mouth of the Salinas River was diverted about 7 km to the south, removing any residual tidal influence from ES. When the U.S. Army Corps of Engineers created the channel between Monterey Bay and ES in 1946, the old Salinas River channel was dredged to form what is now Moss Landing Harbor (Figure 2) and the Slough was opened to full tidal circulation.

ES is about 10 km long, with depths ranging from 8 m at the harbor entrance, to 0.5 m near the head of the Slough (Figure 1). The average depth is approximately A 30-Year History of the Tides and Currents in Elkhorn Slough, California DOI: http://dx.doi.org/10.5772/intechopen.88671

Figure 1.

Elkhorn Slough. This map was originally drawn by John Hansen from aerial photographs in 1971 and has been updated subsequently. Circles show Smith's [10] sampling stations. Crosses show tide stations.

3 m. The main channel has a width of about 200 m landward of the Highway 1 Bridge (H1B), to 90 m near the entrance of Parsons Slough. The ES drainage area is small: 585 km<sup>3</sup> [1], and significant fresh water enters the Slough only during winter months. The sinuous path of ES, with four major bends, has a major impact on its circulation. Because it is a complex, shallow waterway comprised of a main channel, extensive mud flats, Salicornia marsh, and small tidal channels, it is difficult to obtain reliable estimates of its volume, tidal prism, surface and crosssectional areas, and bathymetry. Figure 1 is a map of ES based on an aerial photograph taken in 1971 (Hansen, personal communication) with recent updates from 1993 aerial photographs. At low tide, the Slough consists essentially of a narrow channel whereas at high tide, waters entering the Slough inundate the surrounding mud flats, greatly increasing the surface area covered by water.

A number of prominent features in the aerial photograph of ES are shown in Figure 3: the lower Slough with the entrance cutting across the beach near the

Figure 2. Moss Landing Harbor, adapted from Duke Energy Moss Landing [10].

center; the Duke Energy Power plant with several reserve fuel storage tanks paralleling the lower slough; the now abandoned National Refractories and Minerals plant with a large tailing waste pond composed of mainly calcium carbonate precipitate, and Moss Landing Harbor with a small fishing fleet and pleasure boat moorings. The Highway 1 Bridge near the Duke power plant has been the site of a number of current meter studies. Near the main northward bend in the channel (Figures 1 and 3), a linear strip of land supports the Southern Pacific railroad and separates the main Slough from Parsons Slough and the South Marsh. This relatively large body of water is connected to the Slough through a 50-m wide channel under the railroad trestle. Because Parsons Slough and the South Marsh serve as one body with respect to ES, we will usually refer to them as Parsons Slough from this point on unless there is a reason to refer to them separately.

#### Figure 3.

A more recent aerial photograph of Elkhorn Slough taken in September 2002, looking eastward from an altitude of 1500 m (courtesy of Scott Benson).

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

During the past 25 years, major changes have occurred in the Slough's morphology, and these have further altered its tidal response. During the winter months when the Salinas River flows into the bay, a channel is bulldozed across the beach 7 km south of Moss Landing to prevent local flooding in the surrounding towns of Castroville and Moss Landing. The sources of freshwater entering the Slough during this period include rainfall and agricultural runoff from its small drainage basin. Moss Landing Harbor receives a continual supply of low salinity water from agricultural runoff, and before 1982, when the Monterey Regional Outfall began operation, treated sewage discharge from Castroville entered the system via Tembladero Slough south of Moss Landing Harbor. These waters now enter the south harbor at low tide through a tide gate. Recently, another tide gate limiting the flow of waters into Moro Cojo Slough has been removed, and that area is again open to tidal action. Recent changes in bottom depth at one location are readily apparent in the four profiles shown in Figure 4. These profiles, taken at the Highway 1 Bridge, show that bottom depth and channel width have both increased significantly since 1972.

The Duke Energy of North America Power Company operates the largest power generation plant (2.5 gW) in California at Moss Landing (Figures 2 and 3). Before Duke Energy acquired the plant in 1998, it was operated by the Pacific Gas and Electric Company (PG&E power plant or simply PG&E). At that time, seawater coolant was pumped from the east side of Moss Landing Harbor at a maximum rate of 2.7 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> /day, with a temperature of 7–10°C above ambient [11]. The original five power generating units (1–5) discharged coolant waters into Elkhorn Slough, while coolant waters from units 6 and 7 were discharged into the head of Monterey Submarine Canyon within 200 m of the harbor entrance. In 1998, discharges into Elkhorn Slough were eliminated. In July 2002, two new 1.1 gW gas turbine generation units became operational and their coolant waters were discharged through the offshore outfall (Figure 2). Until recently, the National Refractories and Minerals (NRM) operated a magnesium extraction plant, just south of the Duke Energy

Figure 4. Bottom profiles at Highway 1 Bridge (H1B). Depths are referenced to MLLW.

plant. The National Refractories plant ceased operation in the year 2000, but the tailing pond of CaCO3 remains.

The last large-scale change to the hydrography of Elkhorn Slough was implemented by the California State Department of Fish and Game in 1983. Dairy pastures of 160 ha (1.6 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>2</sup> ) were returned to tidal flooding by digging a channel across former dikes and excavating about 10% of the marsh to a depth of 2 m below MLLW. This area, called the South Marsh, is now part of the Elkhorn Slough National Estuarine Research Reserve (ESNERR). More recently, ES may have been affected by the 1989 Loma Prieta earthquake. Cracks in the surrounding marshlands have been identified that appear to be related to this event (R. Kvitek, personal communication) and there is evidence that the Salicornia marsh has settled (G. Greene, personal communication).
