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among- living organisms, from the microscopic plants and animals 

 which convert solar energy directly and are eaten by other organisms, 

 to the fish and wildlife which are the ultimate life forms in the manless 

 estuarine environment. 



Solar radiation and gravitational forces control the natural environ- 

 ment through a complex series of mechanisms. In the estuarine zone 

 this control exhibits itself through seven major environmental factors 

 that exist throughout the estuarine zone. 



(1) Continental Shelf. — The submerged land next to the continent 

 slopes gently to a depth of about 600 feet, then it drops more rapidly 

 to form the deep ocean basins. This fringe of slightly sloping sub- 

 merged land, which along much of the Atlantic and gulf coasts would 

 appear quite flat to the naked eye, is called the "Continental Shelf" ; its 

 width and general configuration along the U.S. coastline affects the 

 force with which ocean waves strike the shore and consequently the 

 manner and degree of shoreline erosion and accretion. 



(2) Ocean currents. — The major ocean currents passing near or 

 impinging on the continent exert strong, if subtle, effects on the estu- 

 arine zone through their temperatures, which affect continental land 

 temperatures, and through their nutrients, which govern the nature 

 and productivity of offshore and estuarine fisheries. The cold Labrador 

 Current water from Maine to Virginia, warm Gulf Stream water along 

 the South Atlantic and Gulf coasts, and the California Current along 

 the Pacific coast all have noticeable effects on coastal land and water. 



(3) GoastliTie slofe. — The configuration of the coastline itself, even 

 though subject to additional molding by the flow of rivers to the sea, 

 is closely related to the shape and structure of the Continental Shelf. A 

 wide Continental Shelf is generally associated with lowland next to 

 the coast, while a narrow shelf is associated with mountainous terrain. 

 These associations throughout the estuarine zone of the United States 

 have produced estuarine systems characteristic of particular regions. 

 Glaciation in New England, Washington, and Alaska; old mountain 

 ranges and a sedimentary coastal plain from New Jersey to Texas, 

 and the young, steep ranges of the Pacific coast are all continental fea- 

 tures having different impacts on the estuarine zone. 



(4) River -flow. — The estuarine zone is also shaped through erosion 

 and sediment transport by fresh water making its way to the sea. All 

 along the coastlines are streams and rivers carrying water from land 

 runoff' to the sea. These waterways ran^e from the Mississippi River, 

 which drains 41 percent of the contermmous land mass of the United 

 States, down to tiny trickles across a beach. The volumes of water and 

 sediment moved reflect not only the total amount of precipitation and 

 its annual cycle, but also the sizes and slopes of drainage basins and 

 the types of soil over which the rivers flow. 



(5) Sedimentation. — The general outlines of many estuaries, la- 

 goons, and embayments in the estuarine zone were formed by erosion 

 from land runoff during the last ice age when sea levels were much 

 lower than they are now. As the sea level rose, the drowned river 

 mouths became zones of mixing, sediment deposition, and erosion where 

 the rivers and tidal currents met. These erosion and sedimentation 

 processes molded the estuarine zone into its present shape and con- 

 tinue to change it. 



