68 



are clearly visible and dramatic. They are a matter of slight differences 

 in degrees of temperature, of concentrations of certain chemical com- 

 pounds, or of speed of motion. Yet they help to explain why lobsters 

 grow in Maine and not along the coast of South Carolina, and they 

 form one basis for regarding the national estuarine system as a unified 

 whole, not as a group of unique coastal systems. 



STRUCTURE OF THE COASTLINE 



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. 



The northern part of the North American Continent was once 

 covered by an ice sheet of continental dimensions, Avhich left its impress 

 on the estuarine zone as far south as New York City on the Atlantic 

 coast and Puget Sound on the Pacific coast. These massive glacial 

 rivers, sometimes over 1 mile thick, cut their way to the ocean, ter- 

 minating somewhere in the vicinity of the edne of the Continental 

 Shelf on both coasts (fig. IV.1.8).' 



The result of their passage is the sharply sculptured and generally 

 steep shoreline associated with the New England, Puget Sound, and 

 southeast Alaska regions. The submarine topography of these regions 

 is similar to that above the water, except where earth and rock have 

 eroded from the land above the water and been deposited on the land 

 mider the water. The estuarine zone along formerly glaciated coasts 

 is a region of deep, heavily indented embayments, many islands, steep 

 rocky shores, predominantly evergreen forests reaching nearly to the 

 water, irregular bottom topography, and vistas of great scenic beauty 

 (fig. IV.1.9). 



The unglaciated parts of the Atlantic coast and of the gulf coast 

 consist of relatively flat terrain in which coastal embayments and 

 marshes are the predominant estuarine features. These are coasts 

 formed primarily of sediments eroded from ancient mountains, and 

 along which the embayments and marshes form traps for sediments 

 the rivers bring down to the sea. 



The estuarine zones along these coasts may be of many forms, but 

 the general impression is one of great expanses of shallow water and 

 aquatic vegetation, extensive sand dunes and sandy ocean beachfront, 

 and narrow and carefully maintained navigation channels with port 

 facilities well inland (fig. IV.1.10). 



The Pacific coast of the conterminous United States is actively 

 being eroded by wave action against the exposed shoreline. The major 

 coastal feature is narrow beach or rocks at the base of steep bluffs. Deep 

 embayments behind headlands or shallow indentations in the coast are 

 typical of the estuarine zone. 



The southern coast of Alaska is the only part of the United States 

 with glaciers existing in the estuarine zone. Glacier-fed estuaries have 

 much floating ice, usually in the form of small icebergs, and very steep 

 sides. The water is icy cold and often milky with sediment from earth 

 and rock ground to a fine flour by the movement of the ice across the 

 land (fig. IV.1.11). 



