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Wednesday, January 23, 2008

A Fascinating Historical Perspective of the Changing Geology of the San Francisco Bay-Delta Region

To bay or not to bay?

Can you imagine what San Francisco Bay looked like 15,000 years ago?

Actually at that time– during the last ice age– San Francisco Bay wasn’t a bay at all. Instead, it was a valley dotted with grazing antelope. Hills jutted up here and there (destined to become the Bay’s islands). The Sacramento and San Joaquin Rivers of the Central Valley joined forces in the vast marshy Delta, flowed west through a 300-foot deep gorge in the Coast Range (now the Golden Gate), and across a broad coastal plain to the ocean. California’s coastline was out past the Farallon Islands.

With the end of the ice age and the melting of the great ice sheets, the oceans began rising. By about 10,000 years ago, the rising Pacific had intruded through the Golden Gate and begun to form the Bay. (This wasn’t the first Bay, by the way. During the Pleistocene, as the ice sheets alternately retreated and advanced, San Francisco Bay was alternately flooded and drained as sea levels rose and fell.) By 6,000 years ago, tidal influence had extended to the Delta.

Around the shallow margins of the newly formed Bay, tidal marshlands began to form. Between Vallejo and Novato about 55,000 acres of cordgrass and pickleweed took hold. In the south Bay was another wetland tract totaling about 50,000 acres and due south of where the city of Fairfield now resides were another 60,000 acres. As enormous as these expanses of marsh were, they were dwarfed by the Sacramento and San Joaquin River Delta. Forming a jagged triangle with apexes at Sacramento, Antioch, and Tracy, the Delta spanned a massive 345,000 acres. For 4,000 years, the wildlife of these wetlands thrived in the rich waters of the Estuary.

More recently, the Bay landscape has undergone further radical change– though on a much shorter time scale and due to man-made rather than natural forces. What had existed for four millennia was destroyed in the geologic blink of an eye. In about 100 years beginning in the mid-1800s, 92% of the Estuary’s wetlands were drained. Of San Francisco Bay’s original 196,000 acres, only 35,000 acres remained by 1960. The devastation of the Delta was even more complete, with only 10,000 acres left by mid-century.

Some of those drastic man-made changes are now beginning to be changed back, with a goal to restore 100,000 acres of the Bay’s lost wetlands in the coming decades– the largest coastal wetland restoration project in the United States.

The Bay Institute’s Bay Restoration Program Manager Marc Holmes recently spoke with “Your Wetlands” about the Bay’s changing landscape and current restoration efforts. You can learn more by listening to the podcasts:

Changing landscapes

Restoring the landscape

Ann Dickinson is Communications Manager for The Bay Institute (, a nonprofit research, education, and advocacy organization dedicated to protecting and restoring San Francisco Bay and its watershed, “from the Sierra to the sea.”

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