http://en.wikipedia.org/wiki/Bay_mud



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Note added at 1 hr (2014-01-10 14:09:47 GMT)
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http://en.wikipedia.org/wiki/San_Francisco_Bay

Bay fill and depth profile[edit]

San Francisco Bay's profile changed dramatically in the late 19th century and again with the initiation of dredging by the US Army Corps of Engineers in the 20th century. Before about 1860, most bay shores (exception: rocky shores such as those in Carquinez Strait, along Marin shoreline, Point Richmond, Golden Gate area) contained extensive wetlands that graded nearly invisibly from freshwater wetlands to salt marsh and then tidal mudflat. A deep channel ran through the center of the bay, following the ancient drowned river valley.
In the 1860s and continuing into the early 20th century, miners dumped staggering quantities of mud and gravel from hydraulic mining operations into the upper Sacramento and San Joaquin Rivers. GK Gilbert's estimates of debris total more than eight times the amount of rock and dirt moved during construction of the Panama Canal. This material flowed down the rivers, progressively eroding into finer and finer sediment, until it reached the bay system. Here some of it settled, eventually filling in Suisun Bay, San Pablo Bay, and San Francisco Bay, in decreasing order of severity.
By the end of the 19th century, these "slickens" had filled in much of the shallow bay flats, raising the entire bay profile. New marshes were created in some areas.


(1) Richmond-San Rafael Bridge, (2) Golden Gate Bridge, (3) San Francisco-Oakland Bay Bridge, (4) San Mateo-Hayward Bridge, (5) Dumbarton Bridge
In the last years of the nineteenth- and first decades of the twentieth-centuries, at the behest of local political officials and following Congressional orders, the US Army Corps began dredging the Sacramento and San Joaquin Rivers and the deep channels of San Francisco Bay. This work has continued without interruption ever since, an enormous federal subsidy of San Francisco Bay shipping[citation needed]. Some of the dredge spoils were initially dumped in the bay shallows (including helping to create Treasure Island on the former shoals to the north of Yerba Buena Island) and used to raise an island in the Sacramento-San Joaquin Delta. The net effect of dredging has been to maintain a narrow deep channel—deeper perhaps than the original bay channel—through a much shallower bay. At the same time, most of the marsh areas have been filled or blocked off from the bay by dikes.