In the decades surrounding 1900, 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 to 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 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.