3-14 



denser saline water upstream along the river bottom. Salinity generally 

 increases downstream and from top to bottom at any point in the brackish 

 fjart of t}i(.' fjKtuary. Tho distribution of salinity chancj(?s with the 

 tidal stage and the amount of fresh water inflow. 



Though various attempts have been made to classify estuaries 

 into types (Officer, 1977) , such typif ications must remain somewhat im- 

 precise, as there is a continuous range in nature of such character- 

 istics as geometry, bathymetric configuration, and physical character- 

 istics of circulation and mixing. Furthermore, estuaries are by defi- 

 nition the interface of river and sea, making them extremely dynamic and 

 changeable. Nevertheless, despite the inherent problems, a distinction 

 can be made in terms of the vertical salinity distribution. Estuaries 

 can range from a "well-mixed" condition in which there is essentially no 

 variation in the salinity in a vertical column, to a "stratified" con- 

 dition with a halocline between the upper and lower portions of a water 

 column. In New Haven Harbor, conditions range from unstratified to 

 weakly or partially stratified (salinity change of a few parts per 

 thousand (ppt) from surface to bottom) to a "strongly or highly stra- 

 tified" situation (salinity change of at least 5 to 10 ppt from surface 

 to bottom) . 



The driving forces for estuarine circulation are longitudinal 

 surface slope (acting in a "down estuary" direction) and the longitudi- 

 nal density gradient force which is a function of ambient salinity and 

 temperature (acting in an "up estuary" direction) . These two driving 

 forces are balanced by the internal and bottom frictional forces. 

 For the condition in which the river runoff is small, as in New Haven, 

 the net effect is that the surface slope force will be dominant in the 

 upper portion of the water column, producing a seaward flow, and that 

 the density gradient force will be dominant in the lower portion of the 

 water column, producing a landward flow. In some cases there can be an 

 important contribution from a third driving force, wind stress at the 

 surface, and in fact wind effects are often important in New Haven 

 Harbor circulation. 



