319 



these latter embayments have large marsh areas, but the Middle 

 Atlantic, South Atlantic, and Gulf are the regions in which marshes 

 are the predominant 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 water and salty ocean water of slightly greater density under 

 the oscillating influence of the tide. There may be additional compli- 

 cating factors 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 patterns are predictable for each category of estuarine 

 shape. 



It is where moderately large rivers 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 rota- 

 tion 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. 



In the estuarine zone these two different solutions meet and mix. 

 Salt concentrations range from that of the oceans to the almost un- 

 measurable amounts present in some rivers. Where little stratification 

 exists, sea salt dominates mineral concentrations in estuarine waters; 

 in stratified systems, however, the small amounts of minerals entering 

 in the fresh water may be as important in some parts of the estuarine 

 zone as the much larger concentrations from the sea are in others. 



THE LIFE 



The governance of the dominating environmental factors, as mod- 

 ified by estuarine shape and water quality, result in an input of 

 energy into individual estuarine systems, and it is in the variety and 

 diversity of estuarine life that the input of energy to the estuarine 

 zone finds ultimate expression. Whether energy comes directly, as 

 in solar radiation stimulating photosynthesis, or whether it comes 

 indirectly, as with tidal flows or wind and rain pounding on the shore- 

 line, its absorption and conversion to other forms of energy (such as 

 food) are essential steps in the continuation of life in the water, in 

 the marshes, and on the land. 



Energy input from gravitational forces, as illustrated by tidal ac- 

 tion and river flow, depends primarily on local or regional conditions, 

 but direct energy input from solar radiation depends largely on 



