Legacy of Gold Rush

No human impact on San Francisco Bay has been more extensive or had more dramatic effect than the sediment washed into the bay by nineteenth century gold miners. Miners wielding water cannons washed stupendous quantities of rock and soil streams as they mined the Sierra Nevada mountainsides. . . . William Brewer, a member of a survey team crisscrossing the state in 1862, struggled for words to describe the scale of the flows in his journal:

“The amount of soil removed in hydraulic mining must be seen to be at all appreciated. Single [mining] claims will estimate it by the millions of tons, the ‘tailings’ (refuse from the sluices) fill valleys, while the mud not only muddies the Sacramento River for more than four hundred miles of its course, but also is slowly and surely filling up the Bay of San Francisco.”

Brewer correctly predicted that mining debris would have its greatest effect at the bottom of the watershed. The impact was most obvious upstream, where boulders and gravel filled in the river beds. But immense quantities of fine sediments . . . flowed onward and into the bays. This fine material remained in suspension longest, settling out as it reached the calm and brackish waters of Suisun and San Pablo Bays. More than a century after the end of the hydraulic mining, large portions of San Francisco’s bay floor are paved with this former Sierra soil. . . .

The impact of this mud wave on bottom-dwelling species has never been calculated, but it must have been severe. Moving sediments smothered living animals and plants, as the bottoms of the bays were made permanently shallower. . . . The sand in effect “armored” the bottom with a sterile, highly abrasive surface, reducing habitat, preventing burrowing animals from penetrating the subsurface. . . .

This debris was partly responsible for one of the greatest climatic events in recorded California history. In 1862, rivers already filled with mining sediments had to absorb the greatest known rainstorms in California history. So much water fell from the sky . . . that the debris-choked rivers could not carry the flows. . . . The new state capitol in Sacramento was drowned. So much river water poured into San Francisco Bay that the estuary became nearly entirely fresh water for as much as two weeks. . . . Most estuarine species were probably wiped out at a stroke. . . .

^{Source: Booker, Matthew. “Real Estate and Refuge: An Environmental History of San Francisco Bay’s Tidal Wetlands, 1846-1972.” PhD diss., Stanford University, 2005.}