San Pablo Bay is a shallow tidal estuary that forms the northern extension of San Francisco Bay in northern California in the United States. It receives the waters of the Sacramento and San Joaquin rivers, via Suisun Bay and the Carquinez Strait on its east end, and it connects to San Francisco Bay on its south end. The bay is heavily silted from the contributions of the two rivers, which themselves drain most of the Central Valley of California. At the Napa Sonoma Marsh San Pablo bay also receives the waters of Sonoma Creek , Petaluma River, and the Napa River, the latter of which flows into the Carquinez Strait via the Mare Island Strait near its entrance into the bay.
The bay is approximately 10 miles (16 km) across and has an area of approximately 90 square miles (240 km²).
The bay is shared between Contra Costa, Solano, Sonoma and Marin counties, the boundaries of which meet near the center of the bay. Communities on San Pablo Bay include San Pablo, Pinole, Hercules, Rodeo, Vallejo, Novato, and San Rafael. The city of Richmond is on the peninsula separating San Pablo Bay from San Francisco Bay.
The undeveloped portions of the periphery of the bay are plentiful with salt marshes and mudflats. The bay is a primary wintering stop for the canvasback duck population on the Pacific Flyway, as well as a migratory staging ground for numerous species of waterfowl. Much of the northern shore of the bay is protected as part of the San Pablo Bay National Wildlife Refuge.
Endangered species that are found in the bay include the California brown pelican, California clapper rail, and salt marsh harvest mouse. Saltwater fishes found in the bay include striped bass, surfperch, sturgeon, starry flounder, leopard shark, topsmelt, and anchovy. The bay is a popular destination for recreation fishing.
On the southwestern end, near the town of San Rafael, was the site of a Chinese shrimp-fishing village, where some 500 people lived in the 1880s, sending some 90% of their catch of bay shrimp back to China. The location is now part of China Camp State Park.
The airspace above the bay is frequently used for flight training due to its proximity to Oakland International Airport and other local airports, but being outside of San Francisco's Class B airspace.
Because of its great size but shallow waters, San Pablo Bay frequently has difficult boating conditions. The prevailing western wind meets strong currents both at Carquinez Straits and, at the opposite end of the bay, near the Richmond Bridge, to produce large waves, with few areas of retreats for most boats.