Alviso Slough in the San Francisco Bay

By neub9
7 Min Read

The term “slough” (pronounced slew) has different meanings depending on the geographic region of the United States. While all definitions of the word are generally used to refer to a wetland, often a swamp or shallow lake system, the exact definition of what a slough means varies. In other areas of the United States, the word “slough” might be used more broadly to describe a swampy or muddy condition of an area. In these regions, slough might refer to any wetland or marshy area, without the specific connotation of being a coastal or tidal waterway. Along the West Coast, especially in states like California, Oregon, and Washington, “slough” commonly refers to a coastal or tidal creek. These sloughs are found at the intersection of fresh and saltwater where salt marshes and tidal wetlands form. These sloughs are often important ecological zones, serving as habitats for a diverse range of flora and fauna, including migratory birds. They play significant roles in water purification, flood control, and serve as critical nurseries for many marine species. Alviso Slough is where the freshwaters of the Guadalupe River meet the saline waters of the San Francisco Bay.

One such slough can be found at the southern base of the San Francisco Bay where freshwater from the Guadalupe River mixes with saltwater flowing into the bay from the Pacific Ocean. Part of the Don Edwards San Francisco Bay National Wildlife Refuge, Alviso Marina County Park is home to sloughs and salt ponds in the area.

The San Francisco Bay is an urban estuary with remnant salt ponds created about 50 to 150 years ago. Originally formed for salt harvesting starting around 1854, the geometrically shaped ponds, visible on satellite imagery, range in levels of salinity and colors. The San Francisco Bay is lined with multi-colored salt ponds that are currently being restored back to wetlands.

The color variation in salt ponds is due to different organisms thriving at various salinity levels. Dunaliella algae, which flourish in low-salinity water, gives the ponds a green hue, while archaebacteria, capable of surviving in dry salt crystals, color the waters pink or red, and tiny brine shrimp at moderate to high salinity levels turn the salt orange. 80% of the original salt marshes in the San Francisco (SF) Bay Area were lost due to dyking, farmland, urban development and the creation of salt ponds. More recent efforts are focused on restoring the natural wetland conditions in parts of the south end SF Bay Area.

Efforts to revert the SF Bay from salt ponds to tidal wetlands is the largest restoration ever undertaken in an urban environment. The South Bay Restoration Project, initiated in 2008 and expected to span 50 years, is restoring the natural buffer zone in an area surrounded by urbanization.

In 2002, a total of 15,100 acres of industrial salt ponds were acquired from Cargill, Inc. for restoration and management by the California Department of Fish and Game, the U.S. Fish and Wildlife Service, the U.S. Geological Survey, and the California Coastal Conservancy. Over time, the resumption of the natural ebb and flow of the tides brings sediment and a return of the marsh vegetation. Some of the ponds remain as salt flats, as certain birds species have successfully adapted to these conditions during the 160 years of salt production in the region. As is common for sloughs, Alviso serves as a vital habitat for numerous species of birds, fish, and other aquatic organisms. It plays a significant role in the estuary’s ecological health, providing nursery grounds for fish and other marine life, filtering pollutants, and helping to buffer coastal communities from floods. A view across salt pond A12, Alviso Marina County Park.

The slough is a popular place for birding in the Silicon Valley. Alviso Slough’s geography as a migratory bird stop along the Pacific Flyway makes it important habitat for a variety of migratory and wintering bird species as well as resident endangered species like the western snowy plover (Charadrius nivosus nivosus) and the California Clapper Rail (Rallus longirostris obsoletus). Over 280 species of birds at found can be seen at some point during the year within the Don Edwards San Francisco Bay National Wildlife Refuge. I paid a visit to the slough on a sunny but cold winter day in February. The day before, the end tail of a series of atmospheric rivers had battered the area, dropping record breaking amounts of rain over the course of a few days. Other than a very soggy and muddy trail and a few remnant clouds in the sky, no telltale signs of the storm were visible. An American white pelican flying over Alviso Marina.

Walking through the boardwalk to the trail, I was greeted with the rhythmic trilling and singing of the song sparrows (Melospiza melodia) and the raspy calls of marsh wrens (Cistothorus palustris). Other common song birds that can be seen at the slough include golden (Zonotrichia atricapilla) and white-crowned Sparrows (Zonotrichia leucophrys), yellow warblers (Geothlypis trichas), and savannah sparrows (Passerculus sandwichensis). White-crowned sparrows perched among the reeds. An American white pelican (Pelecanus erythrorhynchos) swooped by over the slough. A northern harrier (Circus hudsonius) undulated through the air as it hunted over the marsh.

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