83 



Tide ranges, i.e., the difference between high water and low water, 

 are not so uniform. These are largely a matter of shape, size, and 

 bottom material in individual estuarine areas. Ranges vary from the 

 barely noticeable rises and falls of some lagoons along the Gulf of 

 Mexico to the tremendous 28-foot range in Alaska's Cook Inlet.^ 



Even with small tidal ranges and small estuaries, the volumes of 

 water being moved by tidal flow are fantastic. At Charleston, for 

 example, in 6.5 hours 25 billion cubic feet of water move into or out 

 of the harbor in one tidal cycle (IV-1-3). This is more than enough 

 volume of water to supply the entire population of the United States 

 with water for 1 day. The volume of water flowing into or out of Great 

 South Bay on Long Island in one tidal cycle is adequate in volume to 

 supply the city of New York for 1 week. 



The combination of tidal action and river flow gives rise to that 

 unique phenomenon called an "estuarine circulation pattern," which 

 usually means that fresh water flows in one direction m one layer and 

 salt water flows in the opposite direction in another layer with various 

 degrees of mixing at the interface between them. This type of circula- 

 tion pattern is of great importance in some of the estuaries along the 

 Atlantic and gulf coasts, and to a large extent governs the capacity 

 of such estuaries to rid themselves of waste materials. 



Section 3. The Biophysical Estuarine Regions 



Each estuarine system along the coastline is affected to some extent 

 by all of these dominating environmental factors. In some cases, as in 

 the example already given, the dominance of one particular factor is 

 readily apparent. It is much more often the case that the competing 

 environmental factors are so evenly balanced that none can be said to 

 dominate and the estuarine zone appears to be composed of a bewil- 

 dering variety of unique systems. 



Yet, as an individual person can be identified as a member of the 

 human species by general common characteristics and as a member of 

 particular race by more specific characteristics, so can individual 

 estuarine systems be recognized as belonging to regional and national 

 groupings. 



Table IV.1.5 summarizes the dominating environmental factors in 

 the estuarine zone of the United States. Combinations of environmen- 

 tal conditions characteristic of various parts of the coastline permit 

 the grouping of the national estuarine system into ten biophysical 



^ A tidlal bore, a single breaJiing wave bringing in the flood tide, is cliaracteristic of 

 Turnagain Arm of Cook Inlet at certain times. This is the only tidal bore in the United 

 States. 



