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Photo by William Bowen
The current San Francisco Estuary is the third incarnation of the water body over geologic history. Each iteration has formed in response to global warming periods between ice ages. The most recent Estuary is geologically very young. During the last ice age, the coastline of what now is California was about 25 miles west of the current shore. Sea level was several hundred meters lower than it is today, as much of the ocean’s water was bound up in frozen glaciers. As global temperatures began to warm about 18,000 years ago, sea levels slowly began to rise. About 8,000 years ago the expanding Pacific Ocean waters reached the Golden Gate. Sometime between 2,000 and 3,000 years ago the Estuary that we recognize today was formed. This was long after humans had established communities around the Bay. In fact, there is archeological evidence that the villages of aboriginal Californians living along the Bay shoreline were gradually moved to higher elevations over the millennia as sea level slowly rose.
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