92 



to be the mouths of rivers which have cut their way through coastal 

 mountain ranges, either of their own accord or aided by glaciers as in 

 the case of Puget Sound. Shallow coastal embayments with little and 

 sporadic river flow are characteristic of the few estuarine systems of 

 the Southwest, except for San Francisco Bay, which receives fresh 

 water runoff from much of central California. 



Alaska presents the greatest variety in estuarine form and struc- 

 ture of any of the estuarine regions. Nearly all kinds of systems typi- 

 cal of other regions are found there. In addition, Alaska has the only 

 glaciated coast and most of the fjords found in the United States. 



The rivers entering the estuarine zone drain nearly 90 percent of 

 the U.S. land area. They carry to the sea sediments eroded from this 

 vast expanse and deposit much of it in the narrow band of 274 coun- 

 ties which comprise the basic political subdivisions of the estuarine 

 zone. These coastal counties form a strip of land averaging about 50 

 miles wide along the coast, except where the large embayments of the 

 Chesapeake Bay and Puget Sound make this strip reach more 100 

 miles from the ocean. 



The total area of the coastal counties is 552,000 square miles with 

 the bulk of this in the Alaskan estuarine region and the smallest part 

 in the Pacific Island estaurine region. In the Middle Atlantic, South 

 Atlantic, and Gulf regions, the coastal strip is low-lying plain com- 

 posed of easily erodable materials which tend to be deposited in the 

 estuarine zone and moved about by waves and currents. The ocean 

 coast is mostly sand throughout these regions, overlain near river 

 mouths by some mud and clay. The Mississippi delta is entirely mud, 

 clay, silt, and sand washed down from the heartland of the continent. 

 Sand, mud, and clay predominate in the embayments, with sand char- 

 acteristic of open waters and mud common in marshes. 



Rock, gravel, and sand are the common bottom materials along the 

 North Atlantic coast, with the rock overlain by fine mud and silt in 

 confined areas and sand common in the offshore areas. 



The Pacific coast counties form mountainous strips along the coast. 

 Sediments reaching the ocean in this region tend to be deposited in 

 broad tidal flats or bars where currents permit, or washed off into 

 the ocean where wind and waves motion is sufficiently vigorous. Bot- 

 tom sediments are rock and clay covered in some places with fine mud. 



The characteristic sediment of the Alaskan estuarine region is 

 glacial flour, that extremely fine material ground from the land and 

 carried along by glaciers. Many of the estuaries and much of the con- 

 tinental shelf off the western Alaskan coast are covered with this 

 material. 



Coral reefs, sand, and rocks are typical of estuarine bottoms in the 

 Pacific and Atlantic Islands. Except in extremely shdtered areas, 

 sediments are rare because of the continuous wind and wave action. 



A MORPHOLOGICAL CLASSIFICATION OF THE ESTUARINE ZONE 



The estuarine zone can 'be classified according to its local morphol- 

 ogy into major categories, several of which exist in each of the estua- 

 rine biophysical regions. Within each of these categories, the 

 similarities in structure reflect similarities in water movement, water 

 quality, and ecology which make it possible to apply lessons learned in 

 managing an estuarine system in one region to similar estuarine 

 systems in other regions. 



