70 



LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



A more complicated picture is found in polluted estuaries where the 

 circulation of the water masses play a more important role. The water 

 exchange in an estuary includes a net outward movement of surface water 

 contributed by rivers at the head of the bay and a net inward movement 

 of the denser more saline sea water at mid-depth or near the bottom. 

 Along the length of the estuary vertical mixing tends to increase the 

 salinity and the volume of the outflowing surface waters with the result 

 that the total surface outflow at the mouth of the estuary is greater than 

 the flow into the estuary from the rivers. An imporant corollary of this 

 generalization is that introduced pollution can be removed only in the sur- 

 face waters, since any material which sinks to the deeper water moves in a 

 net "upstream" direction. 



It may be pointed out that the float tests, which have long been 

 standard techniques in the study of such situations, give only a small part 



LONGITUDINAL SALINITY SECTION Of MOUNT HOPE SAT 



NAUTICAL MILES 



Fig. 6. The distribution of salinity in 

 a longitudinal section of Mount Hope 

 Bay at the end of the ebb and flood 

 tides. 



of this complicated picture. The floats show only the net flow of the sur- 

 face waters, and give no information concerning the rate of vertical mixing. 

 In studying the effects of various wind conditions the use of floats is 

 especially deceptive. The wind drives the float more rapidly than it drives 

 the water. The same wind, furthermore, increases vertical turbulence to 

 such an extent that the pollution is dissipated more rapidly instead of being 

 carried farther as suggested by the float results. Measurements of the 

 salinity of the water provide the most useful tool in studying the exchanges 

 and dilution of various water masses. 



Typical salinity contours in Mount Hope Bay, below Fall River, 

 Massachusetts, at the end of the ebb and flood tides are given in Figure 

 6. These data, collected by the continuous salinity-temperature recorder, 

 illustrate the general principles of estuarine circulation described in the 

 Survey of the River Tees (1931, 1935, 1936). The picture at low tide 

 shows relatively flat, elongated salinity contours demonstrating the greater 

 surface flow of the less saline and consequently lighter river water. The 

 deep water retains much of its dense, high salinity character. Follow- 



