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. In the decades surrounding 1900, at the behest of local political officials and following Congressional orders, the U.S. 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. 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 islands 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. Large ships transiting the bay must follow deep underwater channels that are maintained by frequent dredging as the average depth of the bay is only as deep as a swimming pool—approximately . Between Hayward and San Mateo to San Jose it is . The deepest part of the bay is under and out of the Golden Gate Bridge, at . In the late 1990s, a 12-year harbor-deepening project for the Port of Oakland began; it was largely completed by September 2009. Previously, the bay waters and harbor facilities only allowed for ships with a draft of , but dredging activities undertaken by the United States Army Corps of Engineers in partnership with the Port of Oakland succeeded in providing access for vessels with a draft. Four dredging companies were employed in the US$432 million project, with $244 million paid for with federal funds and