15 



The estuarine zone can be classified according to its local mor- 

 phology into 10 major categories, several of which exist in each of the 

 estuarine 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 m other regions. The morphological categories are: 



1.1 Smooth shoreline without inlets 



1.2 Smooth shoreline with inlets 



1.3 Smooth shoreline with small embay ments 



2.1 Indented shoreline without islands 



2.2 Indented shoreline with islands 



3 Marshy shoreline 



4 Unrestricted river entrance 



5.1 Embayment with only coastal drainage 



5.2 Embayment with continuous upland rivBr inflow 

 6 Fjord 



Unrestricted river entrances and embayments dominate the estua- 

 rine zone and are rather evenly distributed throughout all the regions, 

 with the common type of estuarine system being a coastal embayment 

 with drainage from only the local coastal area. Many of these latter 

 embayments have large marsh areas, but the Middle Atlantic, South 

 Atlantic, and Gulf are the regions in which marshes are the predomi- 

 nant feature in some parts of the estuarine zone. 



The water 



The unique nature of water movement and circulation patterns in 

 the estuarine zone are the result of the meeting and mixing of fresh 

 river and salty ocean water of slightly greater density under the oscil- 

 lating influence of the tide. There may be additional complicating fac- 

 tors such as temperature and wind action, but the resulting circulation 

 nearly always reflects the interaction of river flow and estuary shape 

 with the tidal flow of the ocean water. General water movement pat- 

 terns are predictable for each category of estuarine shape. 



It is where moderately larger r-vers and streams meet the sea that 

 the unique estuarine circulation patterns occur most frequently. Large 

 fresh water flows in well-defined channels tend to slide over the top 

 of the denser sea water without rapid mixing. Water movement in 

 such cases exhibits various degrees of stratification. 



With wider channels, smaller river flows, and greater tidal ranges 

 more mixing occurs and other forces come into play. Embayment 

 shape, bottom configuration and material, and the effects of the earth's 

 rotation all may play a role. In some estuarine systems of this type, the 

 degree of stratification may change with changes in river flow, tem- 

 perature, wind, or other transient conditions. 



Estuarine water quality is the product of both land and water. From 

 the land, erosion and solution in river water bring suspended and dis- 

 solved minerals, while decaying vegetation adds dissolved salts, but 

 negligible quantities of organic matter. 



