96 



Figure IV.1.23 illustrates each category. Table IV.1.7 shows the 

 numbers of different kinds of estuarine systems in each estuarine bio- 

 physical region. Unrestricted river entrances and embayments domi- 

 nate 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 em- 

 bayments 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. 



WATER MOVEMENT IN THE ESTUARINE ZONE 



The unique nature of water movement and circulation patterns in 

 the estuarine zone is 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. Where there is little or no fresh water inflow, water 

 moves toward and away from the shore, being reflected into currents 

 paralleling the shore in some cases. On ocean beaches, this parallel type 

 of water movement builds sandspits and barrier islands to begin the 

 transformation of drowned river valleys into embayments and coastal 

 marshes, as illustrated by figure IV. 1.24. 



Where fresh water runoff reaches the sea as a series of small streams 

 or as seepage across the surface, coastal marshes often form and cir- 

 culation patterns are weak and undefined. This situation may exist 

 where local coastal drainage runs off to the sea, where a drowned river 

 valley has filled in so much that the river channel is no longer defined, 

 or where sediment deposition at the mouth of a large river forms a 

 delta (fig. IV.1.6). 



Fiords are formed where a glacier, having gouged out a deep em- 

 bayment, melts as it reaches the sea and deposits the entrained dirt 

 and rock as a shallow sill across the entrance of the embayment (fig. 

 IV.1.25). This sill isolates the lower water of the fjord from the sea; 

 the only significant water movement is in the layers above the sill level. 



It is where moderately large rivers and streams meet the sea that 

 the unique estuarine circulation patterns occur mostly frequently. 

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

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

 such cases exhibits various degrees of stratification. 



Narrow channels and high fresh water flows result in a well-defined 

 sea water layer moving upstream along the bottom of the channel and 

 a nearly fresh layer moving toward the sea along the surface (fig. 

 IV.1.25). 



The Mississippi and Savannah Rivers are classic examples of this 

 "salt- wedge" circulation pattern. With this type of water movement, 

 salt and water from the bottom layer mix constantly into the top 

 layer, and more salt water flows in from the sea to replace it so that 

 the total amount of water in motion may be many times the river flow 



