15. ESTUARIES 



W. M. Cameron and D. W. Pritchard 



1. Definition of Estuaries 



An estuary is a semi -enclosed coastal body of water having a free connection 

 with the open sea and within which the sea-water is measurably diluted with 

 fresh water deriving from land drainage. 



Traditionally the term "estuary" has been applied to the lower reaches of a 

 river into which sea-water intrudes and mixes with the fresh water draining 

 seaward from the land. The term has been extended to include bays, inlets, gulfs 

 and sounds into which several rivers empty and in which the mixing of fresh 

 and salt water occurs. 



Attempts have been made to enlarge the definition even further to include 

 those zones along an open coast in which the salinity is significantly lower than 

 in the open ocean. This extension allows the term "estuary" to include ex- 

 tensive regions off large rivers where the influence of the fresh-water drainage 

 from the land can be clearly recognized. However, such a broad definition does 

 not lead to a practical limitation of the term and we prefer our more restricted 

 definition as expressed in the opening paragraph. 



We shall also refrain from including as estuaries those embayments into 

 which there is a negligible river discharge or in which evaporation exceeds the 

 amount of fresh water draining into the system. Thus we exclude from con- 

 sideration salt-water lagoons where the salinity exceeds that of the adjacent 

 sea; we shall avoid the terms "inverse" or "negative" estuary as a suitable 

 designation for stratified bodies of water in which the haline circulation is 

 opposite in sense to that normally prevailing in estuaries. Thus, although we 

 recognize that the layered inflow and outflow through the Straits of Gibraltar 

 are controlled by factors analogous to those obtaining in typical estuaries, we 

 do not consider anything to be gained by labeling the Mediterranean Sea a 

 "negative estuary". 



On the other hand, we have included in this chapter a discussion of the 

 circulation patterns which occur in tributary embayments to estuaries, which 

 in regard to the increment of fresh water added within the embayment would 

 not be classified by our definition as estuaries, but in which the circulation is 

 definitely related to the estuarine character of the adjacent water body. 



It will be evident that many of the concepts developed here for estuaries will 

 apply, with certain appropriate modifications, to non-estuarine coastal em- 

 bayments and even to segments of the coast and of the open ocean. 



2. General Considerations 



The essential characteristics of an estuary result from the passage of river 

 water through the system to the open ocean. It is in the estuary that the fresh 

 water from the land meets the salt water from the sea, where mixing of salt 



[MS received July, 1960] 306 



