San Francisco Bay

San Francisco Bay is a massive tidal estuary located in California, USA.

An estuary is a coastal area where freshwater from rivers meets the saltwater of the ocean. This bay serves as the namesake for the San Francisco Bay Area. It is a vital part of the regional environment and economy. The bay drains water from approximately 40 percent of the state of California. It is the second-largest estuary on the Pacific coast of the Americas. Only the Salish Sea in Washington and British Columbia is larger.

The movement of water through the bay follows a specific sequence. Water from the Sierra Nevada mountains and the Sacramento and San Joaquin rivers flows into Suisun Bay. From there, the water travels through the Carquinez Strait. It then meets the Napa River at the entrance to San Pablo Bay. San Pablo Bay connects at its southern end to the main San Francisco Bay. Finally, the water exits into the Pacific Ocean through the Golden Gate strait. This entire interconnected system is often referred to simply as San Francisco Bay.

The geography of the bay has changed significantly due to human activity. In the 1850s, hydraulic mining released huge amounts of sediment into the rivers. This sediment settled in areas with little current, changing the bay's shape. Later, people used landfill to fill in wetlands and inlets for building projects. This process reduced the bay's size by as much as one third between the mid-19th and late 20th centuries. This soil is often deep and damp. During the 1989 Loma Prieta earthquake, this soil caused liquefaction. Liquefaction occurs when saturated soil behaves like a liquid during shaking. Much of the damage to structures near the bay happened in these filled areas.

Several distinct islands sit within the waters of the bay. Alameda is the largest island and serves as a suburban community. It was created in 1901 when a shipping lane was cut for the Port of Oakland.

Angel Island served as an entry point for immigrants from East Asia. It is now a state park. Yerba Buena Island is a mountainous island. It contains a tunnel for the San Francisco–Oakland Bay Bridge. Treasure Island is an artificial, flat island located to the north. It was the site of the 1939 Golden Gate International Exposition. Alcatraz is an isolated island in the center of the bay. It is famous for its former federal penitentiary.

Geological forces shaped the bay over hundreds of thousands of years. The bay is thought to be a down-warping of the Earth's crust. This occurred between the San Andreas Fault to the west and the Hayward Fault to the east. About 560,000 years ago, a tectonic shift caused Lake Corcoran to spill through the Carquinez Strait. This carved out the canyons seen in the northern bay today. During the Pleistocene epoch, the bay filled and emptied many times. This happened because of sea level changes from melting glaciers. Between 15,000 and 10,000 years ago, the basin was a large river valley. As ice sheets melted 11,000 years ago, sea levels rose by about 1 inch per year.

The history of the bay includes many different cultures and explorers. The Ohlone people were the indigenous inhabitants of the area. The Chochenyo people called the bay "ommu." European mariners found the entrance difficult to see because of narrow gaps and fog. Spanish explorer Gaspar de Portolá first sighted the bay in 1769. He climbed the summit of Sweeney Ridge to see it. Juan de Ayala was the first European to enter the bay in 1775. He sailed the ship San Carlos through the Golden Gate. Many modern names, like Alcatraz and Point Reyes, come from his expedition's maps.

Today, the bay is a major center for industry and nature. The Port of Oakland is one of the busiest cargo ports on the west coast.

The San Francisco Bay Area is the second-largest urban area in the American West. It is home to approximately seven million residents. Despite this, the bay remains a critical ecological habitat. It serves as a nursery for Pacific salmon, California halibut, and Dungeness crab. The remaining salt marshes support many endangered species. In 1960, the Save the Bay movement began to protect these wetlands. This movement helped stop the trend of filling the bay for industrial use.