Alviso Slough in the San Francisco Bay

Caitlin Dempsey

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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, are blue carbon sinks, and serve as critical nurseries for many marine species.


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A view of a wetland with marshes on either side of a body of water.
Alviso Slough is where the freshwaters of the Guadalupe River meet the saline waters of the San Francisco Bay. Photo: Caitlin Dempsey.

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.

Salt ponds of the San Francisco Bay

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.

Satellite imagery showing multiple colored salt ponds in the SF Bay.
The San Francisco Bay is lined with multi-colored salt ponds that are currently being restored back to wetlands. Satellite image: NASA.

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.

The restoration project, one of the largest of its kind in the United States, involves multiple government and private entities and aims to convert a majority of the altered shoreline back to its natural state of tidal wetlands and marshlands. 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.

The marshland restoration project, planned over multiple decades as long as funding continues, involves removing levees and dredging channels to allow the inundation of salt ponds with tidal water. 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 a salt pond with the sky reflected into the pond.
A view across salt pond A12, Alviso Marina County Park. Photo: Caitlin Dempsey.

The water in the salt ponds around the Alviso Slough is now managed not for salt production but to provide optimal habitat for not only for the terrestrial marsh birds but also for the migrating and wintering birds species that pass through the marshland.

Like many wetland ecosystems around the world, Alviso Slough faces challenges related to environmental degradation, including pollution and sewage contamination, habitat destruction, and the impacts of climate change, such as sea-level rise.

Conservation efforts are ongoing to restore and protect the slough’s natural habitats. These efforts include projects to improve water quality, reducing contaminants such as mercury, restore tidal wetlands, and enhance the resilience of the native species to predation by corvids and gulls.

A winter visit to Alviso Marina

Located just past the northern boundary of Santa Clara, the road that leads visitors to the pier that fronts Alviso Marina County Park passes through the historic town of Alviso. Remnant buildings from the Bayside Cannery business along with Victorian homes line the streets.

The cannery was established in 1906 and was notable for being the first cannery in the world to can green instead of white asparagus in 1921. Bayside Cannery became the third largest cannery in the United States by 1931 and was sold in 1936.

A view from a marshland of office buildings on the other side of the water.
Silicon Valley buildings not far from Alviso Slough. Photo: Caitlin Dempsey.

The tech office buildings of the Silicon Valley that can be seen from the slough are reminders that this marshland is surrounded by urban development.

A short boardwalk leads towards the dirt path that is Alviso Slough Trail. This 8.8 loop around the salt ponds gets quite muddy during the rainy season so wearing hiking boots with deep threads is recommended. Sections of the Alviso Sloughs Trail as well as the entirety of the Mallard Slough Trail are closed due to restoration work until the summer of 2025.

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.

A white pelican flying in the cloudy blue sky.
An American white pelican flying over Alviso Marina. Photo: Caitlin Dempsey.

These are some of the birds I was able to observe during my short visit to the slough.

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).

A pair of white crowns sparrows perch on reeds.
White-crowned sparrows perched among the reeds. Photo: Caitlin Dempsey.

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 vegetation where the Guadalupe River empties into the slough. Northern Harriers fly close to the ground, moving back and forth as they fly across fields and marshes, listening for the sound of small prey.

I was struck by how silently most of the birds move through the area. Even the gulls, typically known for their loud piercing keow call, swooped silently down, one after the other, onto an exposed mud flat in the middle of a salt pond.

On the banks of the salt pond, a California ground squirrel (Spermophilus beecheyi) sat motionless in the grass, one paw raised up, ready to dart at a moment’s notice.

Tucked away, but not hidden among the browns and greens of the marsh, a brilliantly white snowy egret (Egretta thula) sat motionless among the reeds.

Several species of ducks such as the Ruddy Duck (Oxyura jamaicensis) and Common Goldeneye (Bucephala clangula) as well as grebes and American coots could be seen flying and swimming around Alviso Slough and the channels that extend inland from the slough.

Alviso Marina Park is an easy and short day trip for birding in the San Francisco area. While the entrance of the park with it’s picturesque ochre colored entrance to the short boardwalk is a popular local for photo shoots, if you walk beyond the boardwalk onto the trail system, you can leave the noise of content creators behind and enjoy the sounds and sights of a busy marshland.

References

Carlowicz, M. (2016, February 11). From salt production to salt marsh. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/87379/from-salt-production-to-salt-marsh

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.