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Tuesday, October 13, 2009

Environmentalists Worry S.F. Bay Is Becoming Too Clean

From an Oct. 5 article by Kelly Zito in the San Francisco Chronicle:

Millions of tons of sand and clay that Gold Rush miners scoured from the Sierra Nevada have finally flushed out to sea after more than a century in San Francisco Bay, according to the San Francisco Estuary Institute.

Less sediment means that more sunlight can permeate the bay's waters. That, in turn, spurs growth of phytoplankton, the microscopic plant matter that makes up the base of the food chain. An overabundance of phytoplankton can result in a massive die-off with plant life sinking to the bottom of the bay to decompose. That process eats up valuable dissolved oxygen necessary for fish.

Free-flowing sediment serves another purpose:

as a camouflage for certain fish, according to Peter Moyle, a UC Davis zoologist who has studied California estuaries for 35 years. For instance, the beleaguered delta smelt, a thumb-size fish that resides mainly in Suisun Bay and farther east, depends on all those minuscule particles to hide it from prey.

Miners employed high-pressure water cannons to break down rocks and hillsides in search of gold. Much of that runoff bled into Central Valley rivers, through the delta and out to San Francisco Bay, where it built up along the bay floor and drifted in the water for generations.

By the late 1990s, however, that sediment had washed out of the system and water clarity dramatically increased. A particularly wet 1998, in fact, created a tipping point by pushing much of the remaining, erodible material into the Pacific, Schoellhamer said.  The 20th century proliferation of levees throughout the delta and dams on California's rivers means that little sediment naturally flows into the bay; instead, it collects near dam bases.

The flushing of Gold Rush-era silt from the bay has had at least one bright spot: Levels of mercury contamination have declined as well. Mercury, a toxic metal that can build up in fish tissue, was once used to separate gold from rock.

To read the full report by the San Francisco Estuary Institute on sediment in the bay, go to www.sfei.org.

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