at Station l58 on May 6. Reference to Table 10 vd.ll show that, if there was 

 a thermocline at Station 1$8 on June 25, corresponding to the one at Station 

 8F, it was obliterated some time before the temperatures were taken on the 

 following day. 



The cases of thermal stratification in 1930 cited above indicate that 

 there were at least three distinct periods of thermocline formation, the 

 first in early May, the second in early June, and the third in late June. 

 The record for Station 37A, given in Table 12, and partially in Fig. 7 shows 

 the ores^nce of thermoclines at corresponding times, and another in early 

 August which is absent from the records of Stations 1$8 and 8F. It is of in- 

 terest to note that the thermoclines of May 7 and June $ were located nearer 

 the surface than thDse at Station l58 on comparable dates. On May 7 it was 

 found between 2 and h meters, and on June 5 between U and 6 meters, whereas 

 at Static ii 158 it was found between 7 and 8 meters on May 6, and between 5 

 and 6 meters on June 3. The thermocline of June 20 at Station 37A was located 

 at the same depth as the one at Station 8F on June 25, but it had a much 

 steeper gradient than did the latter. That the thermoclines of June 5 and 

 June 20 at Station 37A represent distinct periods of thermocline formation is 

 indicated by the fact that the bottom temperature was higher on the latter 

 date than on the former, and by the fact that at Stations l58 and 6F on June 11 

 and 12 respectively, there was no evidence of stratification. 



An unusual condition ecisted at this station in early August. On 

 August 9 the temperature was uniform in the upper 10 meters, but between 10 

 and 13 meters there was a gradient of 6.6*. The bottom temperature (18.2°) 

 was lower than it had been on the three preceding dates. Three possible 

 explanations for this condition may be suggested: (1) between July 18 and 

 August 9, the water cooled to near 18° and subsequently warmed to near 25''in 

 the upper 10 meters, (2) a layer of cold bottom water was forced westward 

 from the deeper central basin as a result of disturbed hydrostatic equilbrium, 

 (3) the bottom layer of water decreased in temperature about 3° through loss 

 of heat to the cold bottom mud during a period of thermal stratification. The 

 first explanation is not valid because the period in question was character- 

 ized by unusually high air temperatures. It is not possible to state defi- 

 nitely which of the other two explanations is the real one. The second seems 

 improbable from the fact that on August 9 and on several preceding days the 

 winds were light. If the western limit of the cold layer of the central basin 

 lay only a short distance east of Station 37A, a strong wind would not be 

 necessary to cause sufficient westward displacement, but since we have no data 

 on this point, preference should be feiven to the third explanation. The minor 

 cases of June 5 and July 9 (Fig. 7) might readily be explained by loss of 

 heat to the mud, and with a more protracted period of stratification, it is 

 not unlikely that the more pronounced reduction indicated in the present case 

 could have taken place. On August 10 and 11 there were brisk winds and it is 

 probable that the water was mixed from top to bottom. 



The data which havB been presented are considered typical of Western 

 Lake Erie, although they have been taken from only a few stations. A review 

 of the data shows clearly that thermal stratification is the exception rather 



h9 



