Alviso Slough Trail
Alviso Marina Park
1195 Hope St. [Directions]
Alviso, CA 95002
The nation’s first urban National Wildlife Refuge is a jewel on the San Francisco Bay. Its 30,000 acres of open bay, salt pond, salt marsh, mudflat, upland, and vernal pool habitats are constantly changing. It’s a great place to hike or watch birds. It’s also a great place to see up close how a salt pond is turned back into wetlands.
Look around you. What do you see? hear? smell? Perhaps you’d be surprised to learn that there are hidden realms in this landscape, evidence of past human activity, even a mystery or two. A salt pond looks quite different from above. There are myriad species of organisms living in the rust-red, purple, or emerald green water. Here you’ll discover clues to some of the stories this area has to tell about its ecology and its past.
See if you can spot any of the birds, insects, or other animals that live in the dense stands of bulrushes. Later, during your hike, you can compare this restored marsh habitat to some salt pond habitats. There’s a lot that scientists don’t know, too. Maybe you can help find some of the answers!
Visit the marsh during the spring or fall migratory seasons and you might see brandt, pintails, mallards, or canvasbacks. Other water birds that feed and live here include herons, seagulls, avocets, and stilts.
Special Thanks to The Exploratorium for connecting QUEST with Dr. Wayne Lanier and Cris Benton, of the Hidden Ecologies blog. Hidden Ecologies is part of the Exploratorium’s Invisible Dynamics project.
Who Needs Wetlands?
Everyone needs wetlands. Even though humans considered them wasteland in the past, today we know wetlands are among the most complex, unique, and necessary communities in nature. Much of the life on Earth depends on their existence.
Coastal wetlands serve many important functions. They provide food and shelter to enormous numbers of fish, migratory birds, and other wildlife, including many endangered species. They put nutrients into the oceans. They offer protection to juvenile fish and other species. They buffer storm surges and prevent flooding and shoreline erosion. They also filter pollutants and particles out of the water and help to keep it clean.
Making Salt from Bay Water
Salt makes up about 2.5 percent of the bay’s water. By collecting the water in ponds and allowing the water to evaporate, you end up with salt and some other mineral byproducts.
During high tide, bay water floods over the levees and into the first of a series of salt concentrator ponds. The sun slowly evaporates the water, making it saltier and saltier. Salt crystals begin to form when the salt concentration reaches 25.8 percent. The water is moved from one pond to the next as its salinity increases.
The salt concentration is carefully controlled in each pond. When all the water is gone, each pond holds a bed of salt five to eight inches deep. The salt is then harvested, purified, and packaged for farm, industry, and home use. It takes about five years to go from bay water to harvested salt.
Why Are the Ponds Green, Pink, and Orange?
Salt is usually white, right? Out here, the salt ponds can be red, pink, orange, or deep green! The giant blocks of bright colors aren’t salt but the colorful life forms that live in very salty water. Different organisms thrive in different salt concentrations. The color of each pond changes as the water evaporates and the pond becomes more saline.
Dunaliella algae can live in water with low salinity. They turn the salt green.Archaebacteria turn the salt pink or red. These can even live in completely dry salt crystals. At moderate to high salt levels, tiny brine shrimp turn the salt orange. Their orange color comes from substances, called carotenoids, that give tomatoes, carrots, and pink flamingos their bright colors.
Turning Salt Ponds Back into Wetlands
To create wetlands where there now are salt ponds, the levees enclosing the ponds must be opened, or breached, to allow the tidal water to once again flow in and out. Tidal circulation transports and mixes decomposing plant matter, other nutrients, and oxygen, and helps to turn the marshland into one big smorgasbord for wildlife.
The South Bay Salt Pond Restoration Project is the third-largest wetlands restoration project in the country and the largest on the West Coast.
In July 2004, ten salt ponds were opened, with nine more in March 2005. In October 2006, a levee was broken open near the ghost town of Drawbridge. The area you see (where) is slowly turning back into wetlands. Already the numbers of waterfowl and shorebirds, such as sandpipers and wigeons, and have increased dramatically. Experts estimate that it will take decades for mature wetland habitats to develop.
Concerns and Challenges
It takes a lot of work to restore and protect salt marshes. These vitally important habitats face dangers from encroaching development to aggressive non-native species to pollution. Many of the species that are native to salt marshes are threatened or endangered because their territory and food supplies are disappearing. People are working hard to reverse the damage caused here by past human activity.
The wetlands restoration projects in the Don Edwards San Francisco Bay National Wildlife Refuge is a large one by any measure. But one of its most important features is its urban context. Surrounded by communities and industry, this project gives millions of people an up close and personal look at maturing, developing wetland habitats.
Not a lot is known about how a salt pond turns back into a mature wetland, but many people are interested in documenting the process. Much was learned with the restoration of the 200-acre LaRiviere marsh to the east, near the Visitor Center.
The Wetland’s Smallest Inhabitants
Some of the most important inhabitants of wetlands– and the planet– are so tiny you can’t see them individually, but, collectively, they make up a huge portion of the biomass on Earth.
Bacteria Are Your Friends
Out of the 100 million or so species of bacteria on Earth only about 100 are known to be harmful to humans. While bacteria are often just associated with disease and dirt, quite the reverse is true. You couldn’t live without them! Right now there are hundreds of millions of bacterial cells living in your digestive system.
Because they’re found in such large numbers, and in habitats ranging from ice caps to hot springs, it’s not too surprising that bacteria are plentiful in wetlands. Some of the bacteria found along the Alviso Slough trail include cyanobacteria, aquatic organisms that manufacture their own food through photosynthesis.
Over 3.5 billion years ago, there wasn’t much oxygen in Earth’s atmosphere. When cyanobacteria developed during the Archaean and Proterozoic Eras, they were the big guys on the block–the “dinosaurs” of their day. The dramatic increase in oxygen in the atmosphere generated by these organisms shaped the course of evolution, allowing much larger creatures, such as trilobites–and mammals–to develop.
A patch of cyanobacteria is sometimes referred to as “blue-green algae,” but they aren’t related to algae.
Consequences of Habitat Change
Changing any habitat affects wildlife. In fact, one of the main arguments against restoring all of the ponds back to tidal marsh is the need to preserve some of the familiar salty habitat–the shorebirds are used to it!
Some salt ponds will remain, not just to produce salt but also to provide essential habitat and shelter for dozens of species of birds, particularly during stormy weather. California’s coast has been dredged and developed for years, decreasing the acreage of protected flats like the salt ponds. These ponds are critical resting places for migrating species like the Western sandpiper.
South Bay Wetlands
Centuries ago, the salt marshes and other wetlands surrounding the bay were much larger. Many were filled in to make more room for development. Levees closed off other wetlands to make salt evaporation ponds. The first salt ponds were created in the bay in 1854. By the late twentieth century only 10 percent of the bay’s original wetlands remained, and many of those were in danger.
In 2003, the state of California, the federal government, and private donors purchased from Cargill Salt 16,500 acres of salt ponds, an area about half the size of San Francisco. Over the next several decades, the South Bay Salt Pond Restoration Project will return most of the ponds to native wetland habitat.
There are lots of volunteer opportunities at the Refuge Headquarters and Visitor Center in Fremont, the Environmental Education Center in Alviso, the Native Plant Nursery, and many of the satellite refuges.
For more information, visit the volunteer page or call the Volunteer Program Coordinator at (510) 792-0222.
Latest Visitor Photos
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