SECT. 3] ESTUAMES 307 



and fresh water occurs, and where dynamic conditions are established which 

 bring about the eventual discharge of the river water to the sea. 



An important feature of estuaries is that although marked variations in the 

 fresh-water inflow do occur, the system tends to be buffered against equivalent 

 variations in the accumulation of fresh water in the estuary. Thus the changes 

 in the total salt concentration are not in direct proportion to changes in the rate 

 of fresh-water inflow. While an increase in river discharge will lead to a transient 

 downstream displacement of the surface isohalines and some freshening at all 

 depths, there is a counter response resulting in an increased inflow of salt water 

 from the sea in the bottom layers, with a marked increase in vertical stratifica- 

 tion. 



Thus we are led to the concept of a quasi-steady dynamic balance tending 

 to prevail in the system. On the one hand there is the continual discharge of 

 river water into the estuary ; at the same time, the salt water presses inward 

 from the sea. These two influences interact, and the characteristics of the 

 estuary reflect the balance of forces associated with each. 



Early studies of estuaries have tended to emphasize the distribution of 

 properties within an estuary. This emphasis reflected both the interests and 

 specialties of the observers. Biological studies attempted to correlate the 

 distribution of sessile organisms with the average distribution of salinity, of 

 dissolved oxygen or of nutrient salts. The sampling of these scalar proper- 

 ties was a traditional and practical approach, while the measurement of 

 velocities received much less attention, owing in part to technical diffi- 

 culties. 



The more recent appreciation of estuaries as dynamic systems has been 

 associated with increased emphasis on the movement of waters within those 

 systems. Studies of the circulation in estuaries and of the degree and extent of 

 turbulent mixing have received appropriate attention. It is from this viewpoint 

 that we propose to discuss estuarine oceanography. 



3. Estuarine Circulation Patterns 



Pritchard (1955) has described the various circulation patterns that are 

 observed in estuaries, ranging from those occurring under highly stratified 

 conditions to the patterns present in well-mixed vertically homogeneous types. 

 In his treatment he has emphasized the kinematic processes which control the 

 distribution of salt in the estuary, indicating the conditions under which 

 advection and turbulent diffusion are important. 



In this discussion we shall follow Pritchard's development of the estuarine 

 sequences, emphasizing present understanding of the dynamic processes that 

 are in control of the circulations. In what immediately follows we shall avoid 

 formal reference to the pertinent equations of motion, deferring this considera- 

 tion until the essential concepts of estuarine dynamics have been pre- 

 sented. 



