Located at the southern end of the San Francisco Bay lies the Alviso Marina County Park. Alviso Marina is located where the freshwater of the Guadalupe River flows into the tidal marsh area of the San Francisco Bay.
In addition to a boat launch area, Alviso Marina features a boardwalk area that leads onto the Alviso Slough Trail. This dirt trail loops around several of the salt ponds that form the southern section of the Don Edwards San Francisco Bay National Wildlife Refuge.
The trails, like all trails near and around the baylands are very flat with almost no elevation change. This makes walking the trails fairly easy for people of all ages and most physical conditions. There are no trees in this area so there is no shade for relief from the sun on a hot day.
The entire loop around the salt ponds is a little under 9 miles but about half of the Alviso Slough Trail is closed off due to construction until June of 2025.
Birds at Alviso Marina
Alviso Slough and the salt ponds provide refuge for a range of shorebirds, corvids, and raptors. While they don’t reach the large flocking numbers common further up the bay around Charleston Slough and the Palo Alto Baylands, a variety of gulls, double-crested cormorants, egrets, great blue herons, American white pelicans, western sandpipers, and other shorebirds are commonly seen here. In the marsh vegetation, song sparrows, marsh wrens, and savannah sparrows are the most vocal songbirds.
Walk out to the levees
The trails near the entrance to Alviso Marina aren’t the best places for birding. The yellow wooden entrance structures and boardwalk that lead to viewing platforms and out to the trail are popular sites for visitors to congregate around and take selfies from. The boardwalk takes you over the marsh where song sparrows and marsh wrens are the most vocal birds that can be heard.
Salt ponds at Alviso
Just beyond the picturesque yellow portals visitors to Alviso Marina are greeted by the contrasting bright white salt flats and magenta hued waters of salt pond A12. Microscopic algae, Dunaliella salina and halobacterium, give the salt ponds their pinkish tint.
Map of the salt ponds at Alviso Marina County Park
With the exception of a few isolated ravens, the only birds that use these salt flats to flock are gulls. This is a resting place for gulls which glide in silently from the nearby Alviso Marina Slough to land on the salt. Hundreds of gulls can be seen huddled on the salt flats.
To the left as you walk along that section of the Alviso Slough Trail is the Alviso Slough. Northern harriers can be seen hunting over the marshes between the trail and the slough. Higher up, turkey vultures kettle in search of carrion. In the reeds sparrows and wrens can be heard chirping and calling.
American white and brown pelicans fly silently over the trail as they move between the levees further up the trail and the open bay off beyond the slough.
Most visitors to the trails tend not to venture far. Many turn back after about 1.5 miles, at the first junction where the Alviso Slough Trail has a fork to the right that leads towards the levee trails surrounding the salt ponds A12 and A13.
This means that if you are willing to keep walking beyond this point to access points for the A9-A10-A14 ponds, you will be able to do some birding in relative solitude and quiet.
So, to get to this quieter spot for birding, even on a busy weekend day, keep walking along the Alviso Slough Trail until you reach the levee trail on the right that separates ponds A10 and A9 (the levee trail between A10 and A11 shown on the map above has eroded).
The trek from the entrance at Alviso Marina County Park to the A9/A10 levee trail is about 3.5 miles. At a decent pace, this will take most people about 1 1/2 hours to walk to.
Single-track levee trail
The levees in this area are narrow strips of built up land with single-track compressed dirt paths. The distance from the start of the levee trail to the center of the A9-A10-A11-A14 ponds is just under one mile (roughly 4,600 feet).
Two of the other three levees that jut out from the center have eroded away in part. The fourth levee that connects back to the Alviso Slough Trail to the north is intact. On my visit, this trail was occupied by a large pod of American white pelicans so I opted not to walk along it and disturb them.
A visit in late September to bird means that you might catch sight of winter shorebird arrivals. During my walk, a long-billed curlew was hanging around the willets on this levee.
In the shallows immediately adjacent to the levee in pond A9, I saw least sandpipers in large flocks foraging in the sticky algae covered mud.
A lone black necked stilt stands out among these small sparrow-sized birds.
Snowy egrets, great white egrets, and great blue herons can be seen hunting for fish on the banks of the levee trail and in the shallow waters of the upper left quadrant of pond A10.
