, partially enclosed coastal body of water, having an open connection with the ocean, where freshwater from inland is mixed with saltwater from the sea. One type of estuary, called a drowned river valley, can be caused by crustal subsidence or a rise in sea level. Chesapeake Bay is one of the largest estuaries of this type in the United States and was formed during the melting of the Pleistocene ice sheets (see Pleistocene epoch
, or drowned glacial troughs, form similar types of estuaries, particularly in Norway, Alaska, New Zealand, and other glaciated, mountainous coastal regions. Salt marshes and lagoons found behind barrier beaches, such as along the south shore of Long Island, and down faulted sections of the earth's crust, such as San Francisco Bay, are additional types of estuaries. The shape of an estuary affects the height of the tide; some estuaries (such as the Severn and the Bay of Fundy) are characterized by a wavelike tidal bore
. Estuaries represent one of the most sensitive and ecologically important habitats on earth. They provide sanctuary for many species of waterfowl, store nutrients for larval and juvenile marine life, and serve as breeding grounds for many desirable species of ocean fish. Since estuaries commonly provide excellent harbors, most of the large ports in the United States (New York, Philadelphia, Baltimore, Mobile, Galveston, Seattle, and San Francisco) are located in estuaries. However, the development of high-density population centers causes deleterious effects that can destroy the very properties of the estuary that made development of the region possible. Human impact on estuaries includes reclamation of tidal land by filling; pollution from sewage, solid waste, industrial effluent, and hot water; increased sedimentation filling the estuary; and alteration of the salinity of estuarine waters by withdrawal or increased influx of freshwater. Increasingly, federal and state governments are passing legislation to protect estuarine environments.