Ever wonder what’s beneath the Golden Gate Bridge and what it might have looked like thousands of years ago? Michael Ellis says it may have been very spectacular.
Did you know that there might have been a huge waterfall under the Golden Gate Bridge? During the last ice age, 20,000 years ago, the San Francisco Bay was not a bay, but a broad valley with a river, which we now call the Sacramento, draining the Central Valley through the Carquinez Strait and heading out to the Pacific through the Golden Gate. Except it didn’t meet the ocean outside the Gate at Pt. Bonita, as it does now. Instead, it ran for another 27 miles across a broad plain past a series of hills (now called the Farallon Islands) before finally draining into the Pacific.
The deepest part of the Bay, at 350 feet or more, is just west of the Golden Gate Bridge. To the east, bedrock at Racoon Strait between Angel Island and Tiburon is only 140 feet deep. So in just over two miles, this ancient river dropped more than 200 vertical feet. This most likely would have created a series of cascades, if not a waterfall. And right under the bridge is an extraordinarily deep hole. The existence of the hole is a bit of a conundrum to geologists, some of who suspect that a good-size waterfall may have created this large cavity in the bedrock. The sheer volume of water passing through this narrow cut in the Coast Range Mountains must have been immense. Today, runoff from nearly 40 percent of California’s land area drains out through the San Francisco Bay. During ice ages, there would have been even more water in the river to carve out the canyon. Waterfall or no, these rushing waters would have been a remarkable sight, and sound.
As sea level rose at the end of the last ice age about 8,000 years ago, the ocean invaded this river valley, creating the modern San Francisco Bay. Evidence of the old riverbed can be seen during winter storms when large waves break over the Potato Patch, a 24-foot deep shoal made up of sand carried by the ancient Sacramento River on its way out to the Pacific.
And next time you so easily traverse the Golden Gate Bridge imagine a waterfall perhaps the size of Niagara thundering far, far below. It must have been incredibly impressive and perhaps our ancestors were witnesses.