About an hour south of the Bay Area, Elkhorn Slough winds 7 miles inland from Moss Landing on Monterey Bay. Wikipedia describes it as California's second-largest estuary and the first estuarine sanctuary in the United States. Outside San Francisco Bay, it has the largest stretch of tidal salt marsh in the state.
The wildlife
The California Department of Fish and Wildlife says more than 340 bird species, 100 fish species and over 500 documented invertebrate species use the slough. It also supports more than 100 Southern sea otters, which feed, rest and raise pups in its calm water. Wikipedia notes that the slough has California's greatest concentration of sea otters. It is also home to the endangered Santa Cruz long-toed salamander and the threatened California red-legged frog.
How it formed
The slough sits in an old river valley. Its present shape dates to 1946, when jetties built at Moss Landing Harbor opened it directly to the ocean. It is named for the tule elk that once lived in the area, and it was designated a wetland of international importance under the Ramsar Convention in 2018.
Visiting the reserve
The Elkhorn Slough Reserve covers more than 1,700 acres on the east side of the slough at 1700 Elkhorn Road, near Watsonville. CDFW lists hours as Wednesday through Sunday, 9 a.m. to 5 p.m. It is closed Monday and Tuesday, and admission is free. Three loop trails cover about 5 miles, with marsh boardwalks and overlooks, and all of them start at the visitor center.
Docent-led walks run at 10 and 11 a.m. every Saturday and Sunday, with binoculars and bird books to borrow. An early bird walk is offered at 8:30 a.m. on the first Saturday of each month. The reserve is managed by CDFW with NOAA and the nonprofit Elkhorn Slough Foundation. It is one of 30 National Estuarine Research Reserves in the country. Horses are not allowed, and there is no fishing or hunting on the reserve.
Wikipedia lists migratory shorebirds, sharks and rays among the slough's seasonal visitors.