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Isolation from direct oceanic effects in much of the greatly branched 

 system, the many subsystems with major river flows, and the reduced 

 concentration of the ocean salt throughout the Bay and its tributaries 

 make this a unique estuarine system. This is a drowned river valley 

 with numerous similar tributary systems in various stages of evolution. 



South Atlantic estuarine region. — Cape Hatteras to Fort Lauder- 

 dale, Fla. (about 26° North Latitude) . 



The generally wide continental shelf is brushed by the warm waters 

 of the well-defined Gulf Stream. The low-lying coastal plain termi- 

 nates in barrier islands and marshes in which large amounts of sedi- 

 ments are being continually deposited by moderate-sized rivers fed by 

 heavy summer rainfall. Many of the drowned river valley estuaries 

 have evolved all the way to coastal marshes. Tidal ranges are small 

 to moderate, depending on local conditions. 



Carribean estuarine region. — Fort Lauderdale to Cape Romano (the 

 Florida peninsula south of 26° North Latitude) , plus Puerto Rico and 

 the Virgin Islands. 



High temperatures, heavy rainfall, and warm ocean currents along 

 practically nonexistent continental shelves result in tropical estuarine 

 environments throughout this region. Coral reefs and mangrove 

 swamps are the typical coastal features of south Florida, while the 

 islands are mountainous and are fringed with coral reefs and beaches. 

 Tidal ranges are small. 



Gulf Coast estuarine region. — Cape Romano to the Mexican border. 



A wide continental shelf extends all the way around this large 

 embayment, in which warm tropical waters are moved gently by weak 

 currents and small tidal ranges. Heavy rainfall over most of the area 

 brings sediments from the broad coastal plain to be deposited in the 

 estuarine zone. Most of the drowned river valleys have evolved to a 

 point intermediate between those of the Middle and South Atlantic 

 regions — barrier islands are extensive and have large shallow bays 

 behind them. 



The Mississippi, carrying drainage from 41 percent of the con- 

 terminous land mass of the United States, forms one of the major 

 deltas of the world and is unique among the estuarine systems of the 

 United States, both in its size and in the extent to which it has built 

 out over the continental shelf. 



Paci-fic Southwest estuarine region. — Mexican border to Cape 

 Mendocino. 



Because of the narrow continental shelf, periodic upwelling of deep 

 water close inshore as winds force the California current offshore 

 brings cool, fertile water near the coast for several months of the year. 

 The coastline has a typical beach and bluff configuration with only a 

 few shallow embayments and the unique earthquake-born valley of 

 San Francisco Bay, which, in the delta formed by the confluence of 

 the San Joaquin and Sacramento Rivers, shows what erosion and 

 sedimentation might have done along the southwest coast if rainfall 

 were greater in that area of easily erodable moimtains. 



Pacific Northioest estuarine region. — Cape Mendocino to the Cana- 

 dian border. 



The Continental Shelf and coastal configurations are similar to 

 those of the Pacific Southwest, but ocean water temperatures are 



