Elkhorn Slough National Estuarine Research Reserve, California

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Elkhorn Slough National Estuarine Research Reserve (www.elkhornslough.org)


caption This map shows the protected lands within the watershed as of 2/2010 (http://www.elkhornslough.org/esf/properties/)

The Elkhorn Slough National Estuarine Research Reserve is one of 27 National Estuarine Research Reserves established nationwide as field laboratories for scientific research and estuarine education. Elkhorn Slough is one of the relatively few coastal wetlands remaining in California. The main channel of the slough, which winds inland nearly seven miles, is flanked by a broad salt marsh second in size in California only to San Francisco Bay.

The reserve lands also include oak woodlands, grasslands and freshwater ponds that provide essential coastal habitats that support a great diversity of native organisms and migratory animals. More than 400 species of invertebrates, 80 species of fish and 200 species of birds have been identified in Elkhorn Slough. The channels and tidal creeks of the slough are nurseries for many species of fish. At least six threatened or endangered species utilize the slough or its surrounding uplands, including peregrine falcons, Santa Cruz long-toed salamanders, California red-legged frogs, brown pelicans, least terns and sea otters.

Additionally, the slough is on the Pacific Flyway, providing an important feeding and resting ground for many types of migrating waterfowl and shorebirds. The slough and surrounding habitat are renowned for its outstanding birding opportunities.


Many habitat types are located within a short distance from the slough. Upland hills with oak, pine, eucalyptus, grassland and maritime chaparral surround the slough. Several thousand acres of salt marsh, tidal flats and open water comprise the main channel of the slough. Beach and sand dunes separate the estuary from Monterey Bay. Riparian habitat is also found on the reserve. Agricultural lands and residential areas border the reserve. The close proximity of these varied habitats supports a remarkable diversity of plant and animal species in a relatively small area.


caption Pickleweed takes salt water up through the roots and stores excess salt in the top "pickles." In the fall, this part turns red and falls off, ridding the plant of the extra salt. (http://www.fws.gov/refuge/San_Pablo_Bay/Pickleweed_Profile.html) The largest wetland habitats in Elkhorn Slough are covered with salt marsh plants. Pickleweed accounts for over 90 percent of the plant cover. Other salt marsh plants common to the slough are salt grass, alkali heath, the succulent janmea and fat hen. Macroalgae is also prevalent in the salt marsh. Dense mats of green algae often cover the upper mudflats at the end of the pickleweed marsh, forming a green band from spring to early fall.

There are only small patches of eel grass near the mouth of the [slough]]. Before the harbor opened, eel grass covered large areas of the slough around the mouth and inland along the channel, where the water was clear and shallow. Longtime residents recall that dense beds of eel grass or related species were present even at the head of the slough. Much of the upper slough is covered with pickleweed, salt grass, mixed winter grasses and willow trees. Willow is the only common tree in the reserve's riparian habitats. Near the upper edge of the salt marsh, there are small patches of cattails, rush and sedges. Coast live oak is the most common tree in the upland habitats around the slough. Hemlock, thistle, wild mustard, harding grass and many other non-native species dominate the upland areas, living testimony to cultural imparts on the habitat. On thin soiled ridge tops oak forest is replaced by chaparral assemblages that are dominated by manzanita.


caption Western sandpiper Calidris mauri (By Alan D. Wilson (naturespicsonline.com ([1])) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons) Marine invertebrates are extremely abundant in Elkhorn Slough. A species-rich, abundant marine invertebrate fauna inhabits the mouth of the slough, the channels, mudflats and salt marshes. The upper or back-bay invertebrate fauna is dominated by infaunal species that brood their young. Several species of large infaunal clams are abundant along the main channel. The gaper clam and the Washington clam are most abundant at the slough's mouth. Dense beds of rough paddock clam dominate the areas around Seal Bend and Moonglow Dairy. Many polychaete worms live in both the intertidal zone and the subtidal channels. The intertidal mudflats also harbor dense assemblages of small tube-dwelling and mobile invertebrates.

There is a rich fish fauna in Elkhorn Slough, dominated by marine and estuarine species. The fish population peaks in both the spring and summer. The most common species are the staghorn sculpin, English sole, starry flounder and several species of perches. Leopard sharks, gray and brown smoothhound sharks and several species of rays also live in the slough.

caption This rare species – Santa Cruz long-toed salamander, Ambystoma macrodactylum croceum – is a federally listed endangered species found only in Santa Cruz and Monterey Counties. (http://sightings.elkhornslough.org/2009/01/24/santa-cruz-long-toed-salamander/) Shorebirds of the reserve are similar to those observed in other coastal wetland and salt marsh habitats along the California coast. Among the most abundant shorebirds in the slough are the western sandpiper, least sandpiper, marbled godwit, dowitchers, willet, American avocet, black-bellied plover, sanderling and long-billed curlew. The small western sandpiper is the dominant species, accounting for at least 75 percent of the slough's bird population.

Several marine mammal species inhabit Elkhorn Slough, including seals, sea lions and sea otters. A number of terrestrial mammals reside in the reserve as well: raccoons, opossums, striped skunk, longtail weasel, red and gray fox, brush rabbit, California ground squirrel and various other rodents.

Endangered Species

Five threatened or endangered species occur in the slough: the California brown pelican, California least tern, Santa Cruz long-toed salamander, southern sea otter and American peregrine falcon.

Tidal Range and River Flow

Today, Elkhorn Slough is a tidally flushed seasonal estuary with little freshwater input. Strong tidal currents scour every major wetland habitat. Today, tidal currents suspend and transport large quantities of sediment from the slough into Monterey Bay during each low tide. At low tides, a muddy plume reaches a mile or more into Monterey Bay. Elkhorn Slough is a highly modified system.

The majority of freshwater enters into the north end of the slough through Carneros and Watsonville Creeks, two intermittent creeks with substantial flows during the winter and virtually none during the summer. Direct runoff from surrounding hills in the winter and agricultural return flows from irrigated fields contribute additional freshwater. A potentially important amount of freshwater may enter the slough from the Salinas River via the Old Salinas River Channel and the Moss Landing Harbor.

Carneros Creek flows along a drainage ditch in a highly modified riparian corridor. After the rainy season, some adjacent lands are plowed and cultivated right to the channel edge. There are major inflows of eroded soil into the channel and the adjacent flats flanking the channel. In the past the channel has filled with sediment and vegetation and was dredged in 1957.

Soil Types

Predominant soils in the watershed include Arnold loamy sand, Elkhorn fine sandy loam, Santa Ynez fine sandy loam and Alviso silty clay loam.

Arnold soils consist of somewhat excessively drained soils that formed on hills and uplands in old marine sand dunes or in materials weathered from soft sandstone. Capability classes are IV and VII. They are generally found in the hills in the northeast part of the watershed. While they are primarily in native vegetation, they are also used for strawberry production in some locations.

Elkhorn soils are well drained soils on marine terraces and dune-like hills. Capability classes are II and III. Santa Ynez soils consist of moderately well drained soils that formed on terraces in alluvium derived from sandstone, and are in capability class IV. The Elkhorn and Santa Ynez soils are found in the western and southern parts of the watershed. Most of the cropland is on these soils. The Alviso soils are found in the wetlands adjacent to the sloughs.


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 meters 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.

Cultural History

caption Aerial view of Moss Landing, Monterey County, California, USA. The Elkhorn slough runs the area and about 6 miles (8 km) inland. The huge Moss Landing Power Plant is visible at the center. (By U.S. Army Corps of Engineers, photographer not specified or unknown [Public domain], via Wikimedia Commons)

The first Native Americans migrated into the Monterey Bay area between 10,000 B.C. and 6,000 B.C. There is some evidence of native presence around the slough as early as 6000 years ago. Thousands of years before Spanish and Mexican settlers arrived, Ohlone Indian villages thrived in the slough's mild environment. Indian middens around the slough are full of the shells of marine invertebrates and the bones of birds and mammals.

The Spanish brought cattle whose grazing habits, along with seeds in fodder brought from Europe, had a tremendous impact on the habitat. Many of the nonnative plants present today became established at this time, displacing native species and completely changing the landscape.

American settlers moving into California farmed the slough's fertile, coastal valley during the 1800s, beginning what is known as the Reclamation Period. They built a system of dikes to drain additional land for crops and dairy pastures. Chinese immigrants were also among the first to dike, ditch and drain the local wetlands for agricultural use. Wheat, sugar beets, potatoes and other products were shipped by schooner to gold-boom San Francisco from the small port of Moss Landing established at the mouth of the slough. By the 1870's the hefty coastal schooners were replaced by a railroad built along the edge of the estuary. More dikes and evaporation ponds were created at the slough's mouth to concentrate and extract precious salt. While the largest area of wetland habitat was reclaimed before 1900, additional significant wetland areas were reclaimed in the 1930s and 1940s.

The construction of Moss Landing Harbor was completed in 1947. The opening of the harbor was the most significant known impact on the hydrology and wildlife of Elkhorn Slough. Prior to the construction of the harbor, tidal action in the slough was very restricted, the slough existing as a shallow, quiet-water embayment. The new jetty at the harbor mouth now causes the slough to drain quickly, exposing shallow flats of eel grass at low tide, which had formerly been underwater at all stages of the tidal cycle. The increase in tidal flow has contributed to significant erosion of the main channel and bordering mud flats.

Recognizing the pressing need to protect the slough from further development, the California Department of Fish and Game purchased 1,000 acres in 1980, calling it Elkhorn Slough Ecological Reserve. Because of the slough's critical location and resources, the Reserve became the 6th estuarine area to become part of the National Estuarine Research Reserve System in 1979. In 1983, dikes used to drain the land were broken and, once again, tidal waters flowed deep into the marsh. By 1985, reserve lands increased to 1,330 acres. The 400 cows that once resided there have been replaced by hundreds of resident and migratory wildlife species.


The reserve is owned and managed by the California Department of Fish and Game in partnership with the National Oceanic and Atmospheric Administration. Watershed-wide conservation and reserve programs are also supported by the Elkhorn Slough Foundation, a non-profit, membership-supported organization.

Further Reading


Disclaimer: This article is taken wholly from, or contains information that was originally published by, the National Oceanic and Atmospheric Administration. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the National Oceanic and Atmospheric Administration should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.






Center, N. (2014). Elkhorn Slough National Estuarine Research Reserve, California. Retrieved from http://www.eoearth.org/view/article/152398


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