How the Bay was Born

the Bay area during the last ice age
The Bay Area during the last ice age

The Marin Headlands in 10,000 B.C.- no bridges, no San Francisco skyline, and no Bay. Instead, a large river meanders its way along a broad valley, then cuts west through a deep canyon later named the Golden Gate. From here, the river continues 30 miles west across a vast and lush plain, and pours into the Pacific Ocean.

It’s difficult to imagine the Bay Area without our beloved Bay, yet in terms of geologic time, the San Francisco Bay is an extremely young formation. 20,000 years ago, the Earth was in the depth of the last ice age. With so much water frozen in glaciers, the sea level was far lower than it is today and more land was exposed. Sea level was so low that the California coastline began 30 miles west of the Golden Gate.

Have you ever stood on Mt. Tam or another high view point and been able to spot the faint outline of the Farallon Islands? These Islands were once hills along the edge of the old coastline.

During the last ice age, the area now occupied by the Bay was a large river valley. Water draining from the Sierra Nevada and nearby mountain ranges formed a grand river that flowed through the Golden Gate, across the Farallon plain and into the ocean. Though the world was in an ice age, the climate in the Bay Area was not much different from today. The expansive Farallon plain and adjacent river valley supported an unimaginable diversity of large mammals; including mammoths, saber tooth tigers, American lions, bears, camels, buffalo, llamas and ground sloths. It was a true Eden, overflowing with life.

About 11,000 years ago the ice age came to an end. Glaciers around the world began to melt, and the sea level rose rapidly. The Farallon plain was flooded and seawater began to fill the San Francisco Bay at the rate of about 1 inch per year. As ice sheets in the Sierra Nevada melted, enormous amounts of water and sediment flowed down the Sacramento and San Joaquin rivers. Much of this sediment accumulated on the shores of the Bay, forming mudflats and marshes that supported plants and wildlife. By 5,000 B.C. the sea level had risen 300 feet and San Francisco Bay was born.

Since its birth, San Francisco Bay has been a defining feature of our region. But we must remember that our region is continually evolving—through natural processes as well as human influence. Over the last 150 years humans have greatly altered the landscape and continue to do so. Sea level, as a result of climate change, is expected to rise 16 inches by 2050, which will also raise the level of the Bay. In order to preserve Bay shoreline habitat for wildlife and protect shoreline communities from flooding, we need to re-establish as much tidal marsh as possible as quickly as we can. We’re working with partners around the region to restore 100,000 acres of wetlands, which scientists say is the minimum the Bay needs to be healthy. Join us to help keep the Bay thriving into the next ice age!

Volunteer with Save the Bay here.

Watch this clip from Saving the Bay on how the Bay was formed