88 



The Mississippi forms one of the major deltas of the world. This 

 delta 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. 



Paciftc 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, show what erosion and sedi- 

 mentation might have done along the southwest coast if rainfall were 

 greater in that area (fig. IV.1.20) of easily erodable mountains. 



Paciftc Northioest estuarine region. — Cape Mendocino to the 

 Canadian border. 



The continental shelf and coastal configurations are similar to those 

 of the Pacific Southwest, but ocean water temperatures are lower here; 

 the movement of the California Current away from the coast is not as 

 pronounced, and heavier rainfall has resulted in some major rivers 

 cutting through the coastal mountains to form deeply embayed estua- 

 rine systems. See figure IV.1.21. Extensive erosion and sedimentation 

 have caused wide tidal flats, bars, and shoals to be typical of these 

 systems. 



The straits of Juan de Fuca and Puget Sound, which were glacier- 

 formed, do not have as severe sedimentation as exists along the ocean 

 coast, and have retained much of their original configuration. 



Alaska estuarine region. — All of Alaska including the Aleutian 

 and Bering Sea Islands. 



The dominant factors in this region are temperature and precipita- 

 tion. Water temperatures are near freezing, and much of the precipi- 

 tation falls as snow. The continental shelf is wide all through the 

 region, and tide ranges are verj large. The southeast and south coasts 

 have active glaciation and consist primarily of glacier-cut embayments 

 and fjords ; the west and north coasts are much flatter and have been 

 modified to some extent by sediments eroded from the interior, includ- 

 ing glacial silt, and by the grinding action of pack ice during winter. 



Pacific Islands estuarine region. — The Hawaiian Islands, American 

 Samoa, Guam, 



This region consists of tropical ocean islands of volcanic origin. 

 Dominating factors are lack di a continental shelf, full exposure to 

 oceanic conditions, and pleasantly warm temperatures. Coral reefs 

 and beach and bluff configurations are typical (fig. IV.1.22). 



MANAGEMENT AND THE BIOPHYSICAL REGIONS 



The environmental factors upon which this subdivision of the 

 national estuarine system is made all represent transport of solar or 

 gravitational energy to the estuarine zone. Inherent in this subdivision 

 is acceptance of the fact that the input of energy — upon which all life 

 is based — differs in quantity and type in the several regions of the 

 estuarine zone. 



In managing estuaries for human benefit, these regional differences 

 in energy form and quantity represent the environmental realities 



