314 CAMERON AND PRITCHARD [CHAP. 15 



criterion established by Stommel and Farmer (1953) in their simpler two- 

 layered model. 



From a consideration of the heat balance in such an inlet, Pickard and Trites 

 (1957) confirmed that the admixture of sea-water to the upper layer was from 

 a relatively shallow depth of approximately seven meters. 



Extensive observations oT velocity profiles in Knight Inlet, British Columbia, 

 by Pickard and Rodgers (1959) have shown a greater complexity of movement 

 than Tully and Cameron inferred. They demonstrated that not only were there 

 occasions when more than two opposing streams could be detected, but that 

 extensive non-tidal movements occurred in the deeper waters of the inlet. 



4. Some Variations on the Estuarine Sequence 



The estuarine types described above can be considered to represent discrete 

 band widths drawn from a continuous spectrum, or sequence, of possible 

 estuarine types. Specific actual estuaries may well exhibit intermediate charac- 

 teristics. In addition, there are variations, or offshoots, from this main sequence 

 which deserve some consideration. 



Bar-built estuaries are formed by the development of an offshore bar, or 

 low-lying elongated island chain, along a coastline of low relief. There is usually 

 a very restricted inlet between the sound and the sea. Where a river flows into 

 the sound thus formed, an estuary is established which differs from the typical 

 drowned-river-valley estuary as a result of this restricted communication. 

 While quite strong tidal currents may occur at the inlet, tidal currents and the 

 rise and fall of the astronomical tide are normally very small within the sound. 

 The relatively great width and small depth of such systems permit wind- 

 induced currents and wind tides to provide the major mechanism for move- 

 ment and mixing of these estuarine waters. No adequate dynamic description 

 of this type of estuary has yet been produced, and, in fact, this type of estuary 

 has probably received the least systematic attention. 



Another variation in estuarine circulation patterns occurs in certain tributary 

 embayments to large estuarine systems. Thus, along the Chesapeake Bay there 

 are several drowned valleys adjacent to and connected with the main estuary 

 and into which very little fresh water is discharged. A circulation pattern 

 develops in these tributary embayments which is definitely related to the 

 estuarine character of the adjacent water body. 



An example of such a tributary embay ment is Baltimore Harbor, which is 

 on the drowned valley of the Patapsco River, a small tributary to the Chesa- 

 peake Bay. So little fresh water is contributed to the harbor by the Patapsco 

 River, however, that it is ineffective in contributing to the density distribution. 

 The prime source of water in the tributary embay ment is the adjacent Chesa- 

 peake Bay. In the bay the dynamic processes associated with fresh-water 

 inflow at the head and salt-water inflow at the mouth continually maintain a 

 vertical salinity (and hence density) gradient. Within the tributary embayment, 

 however, these same processes do not occur, and vertical mixing tends to 



