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terfering waves may develop which set up a complex pattern of standing, pro- 

 gressive and rotary waves on the free surface. Since tidal currents flow in 

 response to both the changing elevation of the free surface and the momentum 

 acquired during their earlier history, the current may not be in phase with the 

 wave form. To study this relationship, it is helpful to establish tide gauge sta- 

 tions around the perimeter of the estuary as well as to make current measure- 

 ments. 



In some instances the tidal currents are small and easily obscured by 

 currents due to other causes. In such cases an intelligible synthesis of the tid- 

 al system can be gained through use of Froude nnodels. 



In other instances the tidal currents dominate the current system to the 

 extent that the effects of river discharge, wind currents and so on, are obscure. 

 Since the river discharge pattern is important in a consideration of the flushing 

 rate, and the tidal motions may ebb strongly where the salinity is low and flood 

 strongly at another place to maintain the salt balance in the estuary, it is useful 

 to measure simultaneously the tidal volume inequality and the salinity. The ob- 

 servational requirements are stringent since the volumes of flow may be large 

 and the differences small. 



As though it were not enough that tides are reflected and progress in dif- 

 ferent phase than associated currents, the tidal signatures may be unsymmetri- 

 cal and change progressively from day to day with the changing configuration of 

 the astronomiical bodies causing them. Semi-diurnal tides occur along most 

 mid-latitude coasts but there are regions such as the Gulf of Mexico where di- 

 urnal tides predominate and then give way to semi-diurnal tides twice each 

 month. Since this tidal sequence may produce a more or less complete reor- 

 ganization of the tidal wave and current pattern every two weeks, rapid survey 

 techniques are required and data for statistical treatment are best taken over at 

 least a synodic period. Again the Froude model has served to show how the 

 tidal circulation changes with the passage of time. 



In shallow, narrow channels or estuaries through which the river dis- 

 charge is large, sea water may enter as a flat wedge moving under the lighter 

 mixed water containing the river discharge. A salt wedge intrusion influences 

 the vertical structure of currents. Over the course of a symmetrical tidal 

 cycle, the net flow in the salt wedge should be landward in order to carry the 

 salt required to increase the salinity of the predominantly seaward motions of 

 the river discharge. Since there may be a difference in temperature as well as 

 salinity at the interface between the salt wedge and the river discharge, the po- 

 sition of direct current measurement stations with respect to a salt wedge may 

 be chosen in the light of the distribution of temperature or salinity. 



The consequences of the existence of a salt wedge to both the circulation 

 and mixing of salt and fresh water have been studied intensively in recent years 

 by Keulegan (reprinted, 1950) and by Farmer (1951) in idealized models. The 

 staff of the Mississippi Waterways Experiment Station, Engineering Departmient 

 of the University of California at Berkeley, and other laboratories, have made 

 much longer series of observations both in the field and in models of field situa- 

 tions. The latter studies have been made primarily for engineering purposes, 

 but have provided basic data that have proved valuable for tests of the statistical 

 theories of flushing and investigations of the behavior of the salt wedge. 



The jet (von Arx, 1948) is another form of intrusion that may be impor- 

 tant in some well enclosed estuaries where wind mixing is significant, or the 

 river discharge small. Jet flow may occur through narrow passages when the 



