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 (with the exception of 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.