Elkhorn Slough Reserve, California

Geology

Elkhorn Slough was apparently eroded by a large river draining into the Santa Clara Valley and perhaps the Central Valley of California during low stands of sea level less than a million years ago. The slough is cut by the San Andreas Fault near the Monterey and San Benito County lines. During the last glacial maximum and the resulting low stand of sea level, local drainage in the Elkhorn Valley cut a stream about 30 m below the present sea level. This period, (about 17,000 years ago) had greater rainfall and local runoff than we experience today. As sea level rose, tidal water invaded the channel of the Elkhorn River.

By the year 6000 B.C., the channel was a high-energy tidal inlet. The inlet gradually filled with fine sediment while vegetated marshes developed along its landward margins and advanced toward the center of the slough during the last 5000 years. A quiet water estuary, much larger than the present slough, covered the region less than 3000 years ago.

For thousands of years the Elkhorn Slough was part of a much larger wetland system covering the mouth of the Pajaro Valley, the Salinas Valley and the area in between, including the present Elkhorn Slough. The slough was a large shallow embayment and a quiet water estuary with little tidal influence. Freshwater input was much greater than saltwater. 90 percent of the coastal wetlands around the slough were drained during the Reclamation Period.

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