Prior to 1950, studies of estuaries were primarily observational. Rhodes ( 1950), for instance, described velocities and salinities in several estuaries along the eastern coast of the United States. Stommel & Farmer (1952) compiled data from 20 world wide estuaries, ranging in size from the Moros in France (2.3 km long and a few meters deep) to the Straits of Juan de Fuca (100 km long and 350 m deep). Their report, although more than 20 years old, remains one of the more extensive compilations of salinity and velocity data in existence and, in addition, shows the wide range of water bqdies we call estuaries. Since the appearance in the early 1950s of Ketchum's work (1951a,b, 1955), of the unpublished manuscript On the Nature of Estuarine Circulation by Stommel & Farmer (1952), and of Pritchard's analyses of the salt balance in the James (1952 and later), researchers have used more analytic techniques to try to understand the process of mixing in estuaries and to quantify such aspects as residence times and pollutant concentrations. It is not easy to agree on what an estuary is. Schubel (1971) listed 10 earlier definitions each of which he found unacceptable for some reason and settled on an eleventh definition by Pritchard (1967): An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage. Even this definition is inadequate for our purpose, because it excludes such estuaries as San Diego Bay where the fresh-water flow is less than the evaporation, but which can be treated like other estuaries with respect to mixing problems. For our purposes it may be more appropriate to say that estuaries are something like pornography hard to define exactly, but we know one when we see one. A number of writers (Stommel & Farmer 1952, Bowden 1967, Pritchard 1967, Schubel 1971, Dyer 1973) have given classification schemes for estuaries. In hydro dynamic terms all the schemes distinguish three major categories: sharply stratified estuaries, such as fjords and salt-wedge estuaries; partially stratified estuaries, in which there is a significant vertical-density gradient and vertical mixing is inhibited;