24 



to place and time to time within the same system. Therefore, estuarine cir- 

 culations must either be studied in great detail or studied statistically. 



Within the past few years progress has been made in a statistical ap- 

 proach to the problem of the flushing rate. Flushing, the progress of river wa- 

 ter through an estuary, has been assumed to be indicated by the average distri- 

 bution of salinity between the river mouth and the open sea. The recent theories 

 of estuarine flushing by Ketchum (1951), Arons and Stommel (1951), Stommel and 

 Farmer (195Z), and others have been based on observations in widely different 

 field situations such as the fjord type estuary (Tully, 1949), the coastal plane 

 estuary (Pritchard, 1950, 1952) and the regions of the continental shelf in which 

 the circulation is estuarine in character (Ketchum and others, 1951). The fact 

 that this wide variety of systems yields a study from very nearly a single point 

 of view is significant. It suggests that while the circulation systems may be 

 quite different, the salinity distribution is achieved in much the same way in 

 each case. 



This has been explained through consideration of the combined tidal and 

 wind-driven circulations as only a means by which the river and sea water mass- 

 es are mixed within the estuary. River water tends to float stably on the salt 

 water from the sea. As the river water moves down the estuary it becomes 

 more saline due to salt from the sea water mixing upward from below. The net 

 flow through the mouth of the estuary is seaward and equal to the river dis- 

 charge. But since both the horizontal and vertical salinity gradients in the estu- 

 ary tend to establish a more or less steady configuration, and salt is being car- 

 ried out with the river water, there is a corresponding inward flux of salt water 

 from the sea. The volumes of water exchanged to maintain the salt balance in 

 an estuary may far exceed the river discharge during corresponding periods of 

 time. This important mechanism seems to be present in estuaries of all kinds, 

 though it may be obscured by the stronger action of tides, winds, branches of the 

 offshore circulation and other effects. 



A description of the manner in which mixing actually occurs and the paths 

 followed by various parcels of water within an estuary is difficult to give fully, 

 and is usually treated as a number of separate processes acting together. The 

 importance of the several processes depends on the climate, the geometry of the 

 wetted basin and the relief of the surrounding land, the tidal signature, river 

 flow, permeability of the ground and other factors. For example, the circula- 

 tion in fjord type estuaries which are deep, sheltered from the wind and often 

 cut in hard impermeable rocks differs from the coastal plane estuaries which are 

 generally broad and shallow, relatively unprotected from the wind, and often 

 surrounded by soft and permeable sedimentary rocks. In fjords, wind and 

 waves are less likely to be important influences on mixing, but the regular 

 march of tides and changes of river discharge may control the circulation. In 

 the coastal plane estuaries, wind, waves and tides may dominate the water mo- 

 tions and obscure the effects of river flow. In regions of the continental shelf 

 where estuarine conditions exist, mixing may be strongly influenced by branches 

 of the deep water circulation. It may be helpful to describe a few of these sep- 

 arate effects in turn. The current systems and mixing processes due to tidal 

 action are often most conspicuous. 



The tidal signature at the mouth of an estuary is but the beginning of 

 what may be a complex sequence of wave motions within the embayment. De- 

 pending on the depth of water and the regularity of the boundaries, the wave may 

 enter and travel as a single progressive wave slowly losing energy until it is 

 damped completely, or it may be reflected back on itself. If the estuary is nar- 

 row and long the primary reflected waves are subject to analysis by the method 

 developed by Redfield (1950). If the estuary is broad or dendritic, pairs of in- 



