315 



( 6 ) Climate. — Solar energy striking the earth sets up complex cycles 

 oi' water and energy flow from the oceans to the sky and the land and 

 back again. That part of the energy cycle occurring in the atmosphere 

 gives rise to the various combinations of weather phenomena which 

 make up local climates. Land, sea, and sky are mutually dependent in 

 producing specific climates, and the great ocean currents 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 direc- 

 tion in one layer and the salt Avater flows in the opposite direction in 

 another layer with various degrees of mixing at the interface between 

 them. This type of circulation pattern is of great importance in some 

 of the estuaries along the Atlantic and Gulf coasts, aiitl 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 estuarine 

 zone appears to be composed of a bewildering variety of unique system. 



Yet, the dominating environmental factors listed above form a set 

 of natural guiding principles which govern the general characteristics 

 of the estuarine zone of the United States, and the occurrence of 

 various combinations of these environmental factors permits the 

 grouping of the national estuarine system into 10 geographical zones, 

 each governed by a different combination of environmental conditions. 



Characteristics of the hiophysical regions 



North Atlantic estuarine region. — Canadian border to Cape Cod. 



Cool, fertile waters with a large tidal range strike a steep, indented 

 coast with deep water close inshore, but protected from the full force 

 of the ocean waves by a wide continental shelf. Moderate precipita- 

 tion with heavy snow-fall leads to heavy spring river runoff which 

 dominates local circulation. Natural erosion and sedimentation are 

 not severe problems, and the evolution of drowned river valley estu- 

 aries is in an early stage in this region. 



Middle Atlantic estuarine region. — Cape Cod to Cape Hatteras, 

 exclusive of Chesapeake Bay. 



A wide, gently sloping continental shelf with a smooth shoreline 

 is cut by the entrances of several major river systems carrying mod- 

 erate amounts of sediments. The same cool, fertile waters as in the 

 North Atlantic estuarine region wash this coastline but with a smaller 

 tidal range. The evolution of drowned river valleys into coastal 

 marshes is in a secondary stage in the larger estuarine systems, with 

 sand spits and barrier islands forming. 



Chesapeake Bay estuarine region. — All the Chesapeake Bay system 

 from Cape Charles and Cape Henry Island. 



