Elkhorn Slough is an estuary about 7 miles (11 km) long that connects with Monterey Bay at the community of Moss Landing, about 17 miles (27 km) southeast of Santa Cruz and 16 miles (26 km) north-northeast of Monterey, California. Elkhorn Slough is the third-largest estuary in California after San Francisco Bay and Humboldt Bay. It was named after the Tule elk that were abundant here before the arrival of Spanish explorers and missions. The estuary has a maximal tide range of about 5 feet (1.5 m) and the primary source of freshwater is Carneros Creek, with secondary contributions from McClusky Slough and Moro Cojo Slough. The watershed drains 45,000 acres (18,000 ha) and more than 8,000 acres (3,200 ha) are protected by private and public ownership. The nonprofit Elkhorn Slough Foundation is the single largest landowner in the watershed, with nearly 3,600 acres (1,500 ha). Other conservation partners include the Elkhorn Slough State Marine Reserve, Elkhorn Slough State Marine Conservation Area, and the Elkhorn Slough National Estuarine Research Reserve. The community of Moss Landing and the Moss Landing Power Plant are located at the mouth of the estuary. Moss Landing was originally called Moss, after Charles Moss a Texas ship captain who with a partner built a wharf. The Moss post office opened in 1895 and changed its name to Moss Landing in 1917. A dredged channel and additional piers allowed the community to become a busy whaling and fishing port and a location for fish canneries. The Moss Landing Power Plant opened in 1950. The California State University system founded Moss Landing Marine Laboratories in 1966, and in the mid-1990s the Monterey Bay Aquarium Research Institute was moved to Moss Landing from Pacific Grove.

Elkhorn Slough occupies the western reaches of Elkhorn Valley, a relic river valley eroded during the early Pleistocene by water draining out of the Santa Clara or Great Valley of California into Monterey Bay. During the mid to late Pleistocene, Elkhorn Valley was isolated from its headwaters by movement along the San Andreas Fault. During the Last Glacial Period, local drainage in Elkhorn Valley incised a stream channel at least 95 feet (29 m) below present-day sea level. The subsequent sea level rise during the Holocene flooded the incised channel, creating a high-energy tidal inlet at the mouth of Elkhorn Slough approximately 8,000 years ago. As sea level continued to rise, the main channel of Elkhorn Slough began to fill in with fine-grained estuarine sediments. A quiet backwater estuary, considerably larger than the present-day Elkhorn Slough, existed approximately 3,000 years ago. As the slough was slowly filling, salt marshes developed along its landward margins and have progressively advanced toward the center of the slough during the past 5,000 years. Humans began using Elkhorn Slough and its watershed nearly 8,000 years ago. The Ohlone people historically gathered a wide assortment of plant and animal food from the slough and uplands, including clams and oysters, acorns and blackberries, and sea lions and elk. Their most significant ecological impact was on the region’s grasslands that were burned to reduce woody vegetation. During the Spanish and Mexican periods of occupation, the grasslands surrounding the slough were used for cattle grazing. Over time, activities such as whaling, sea otter fur trade, duck hunting, salt production, the shellfish industry, and fishing invariably resulted in overharvesting of these resources. In the mid-1850s, Elkhorn Slough was a minor tributary to the much larger Pajaro–Salinas River system which shared a common entrance to Monterey Bay north of Moss Landing. Beginning in the late 1880s, farmers and ranchers turned their attention to Elkhorn Slough’s wetlands as potential farming areas. Low-lying lands and portions of the tidal slough were ‘reclaimed’ by farmers through an elaborate system of dikes and levees. The diversion of the Salinas River between 1908 and 1910 decreased seasonal flooding in the slough area. By 1940, some 50 percent of the wetlands associated with Elkhorn Slough had been converted to productive farmland. In 1946, the construction of jetties at Moss Landing Harbor provided a direct link between the Monterey Bay and Elkhorn Slough. As a result of the changed flow regime, the perimeter salt marshes began to retreat as the estuary evolved into its present hydrological regime.

Many of the most extreme changes in Elkhorn Slough since the arrival of Europeans in the 1700s are a direct result of human activities, specifically the increasing demands on natural resources that have introduced new stresses, triggering impacts such as pollution, sedimentation, and introduction of nonnative species. Other indirect effects of increasing population growth and industrial agriculture have brought nutrient and pesticide runoff, increased soil erosion, and growing demand for freshwater. Farming practices have also led to increased soil erosion in the watershed, resulting in high levels of pesticides, including DDT, flushed into the estuary. One of the highest management priorities is the restoration of wetlands. Since 1979, much of the pastureland that had been created by diking and draining was restored to salt marsh. When Moss Landing Harbor was built in 1946, Elkhorn Slough changed from a depositional system in which sediments were gradually accumulating to an erosional system. In 2010, an erosion control project included building a steel weir at the mouth of the Parson’s Slough, a fork of the Elkhorn, and is intended to reduce erosion of the marsh caused by harbor dredging and the redirection of the Salinas River. A variety of management strategies and policies have been used over the past thirty years to reduce the ecological impact of human activities on Elkhorn Slough. These strategies include educational and research programs, technical and financial assistance programs for landowners and farmers, legislated environmental standards and regulations, land use planning processes, and purchases of conservation easements or entire properties. Elkhorn Slough relies on collaborative partnerships to implement the management strategies and these include government agencies, non-governmental organizations, citizen groups, and individual landowners concerned with Elkhorn Slough and the watershed. Read more here and here. Explore more of Elkhorn Slough and Moss Landing here: