II-7 



rents play their indirect roles in modifying the climates 

 of the estuarine zone. 



(7) Tide . The tide stands alone as a controlling force 

 and symbol of the estuarine environment. The combination 

 of tidal action and river flow gives rise to that unique 

 phenomenon called an "estuarine circulation pattern," which 

 means the fresh water flows in one direction in one layer 

 and the salt water flows in the opposite direction in 

 another layer with various degrees of mixing at the inter- 

 face between them. This type of circulation 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. 



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 

 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 estua- 

 rine zone appears to be composed of a bewildering variety of unique 

 systems. 



