19 meters, where a thermocline of 8. 6° was encoun- 

 tered between the 19- and 21 -meter levels (table 6 -A, 

 station 12). 



Station Gl is directly offshore from station 12, 

 and the depth is 41 meters. Here the vertical gradi- 

 ent was very slight down to the 26 -meter level, where 

 the temperature dropped rapidly to the 30 -meter 

 level. The temperatures in this discontinuity layer 

 were as follows: 26 meters, 17. 7° C. ; 28 meters, 

 14. 2°; 29 meters, 10. 9°; 30 meters, 8. 7°. As these 

 two stations were occupied only 1 hour apart, they 

 may be considered as being observed simultaneous- 

 ly, and we find the thermocline rising from the 28- 

 meter level to the 19-meter level in conformity with 

 the bottom slope. It is of interest that the thermo- 

 cline level changed but 9 meters, although the dif- 

 ference in depth of water was 19 meters. In other 

 words, position of the thermocline only tends to 

 conform with the bottom. The bottom temperature 

 of 17. 1° (table 5) at station 06-13, only 1 mile off 

 the south shore, indicates that the thermocline is 

 limited at this season of the year to areas of greater 

 depth than 16 meters, except where upwelling produces 

 abnormal conditions. 



The thermocline at station 06-26 was between 

 16 and 18 meters, where the temperature difference 

 was 9. 1° as compared with a depth of 13 meters on 

 cruise 4 and 14 meters on cruise 5 at this station. 

 The mean depth of the thermocline in the Deep Hole 

 region, as obtained from 6 stations, was now 25 

 meters. In the Great Plain, the average thermocline 

 depth was 20 meters --about 2 meters lower than in 

 July. 



Cruise 7 . --Only 6 stations were occupied on 

 cruise 7 as this interval was utilized in current 

 observations. 



At 2:00 o'clock on August 26 at station 07-17, 

 there was an even vertical gradient from the surface 

 down to 6 meters, where the temperature was 18.8° 

 C. The temperature was 15. 8° at 7 meters and the 

 gradient was fairly even from that depth to the bot- 

 tom, 20 meters (table 6-D, station 17). To collect 

 more data on the shifting of the thermocline depth, 

 the Shearwater was anchored on this station (17) from 

 9:00 a. m. to 7:00 p. m. on August 27. The record 

 of the serial temperatures taken throughout the day 



is shown in table 6-D, station 17. The thermocline 

 depth steadily rose from 17 meters to 14 meters be- 

 tween 9:30 a. m. and 6:30 p. m. The next day 

 (August 28) at 9:00 a.m. the thermocline was be- 

 tween 16 and 17 meters, and was not nearly so well 

 marked. The rise of 4° in bottom temperature in the 

 past 15 hours indicated that the cold bottom waters 

 were moving out of Long Point Bay and into the Deep 

 Hole. 



Thus the serial temperatures at this station es- 

 tablish beyond doubt that cold water wells up from 

 the Deep Hole and flows into the comparatively 

 shallow Long Point Bay. No other source can ac- 

 count for the temporary presence of cold bottom 

 water in Long Point Bay. The bottom temperatures 

 at station 52, which is in the center of the bay, on 

 the successive cruises 4, 5, 6, and 8, were 11.2°, 

 16.0°, 8.0°, and 16. 7° respectively-values that 

 indicate that the bottom waters are in movement. 

 The upwelling of cold water out of the Deep Hole 

 and into Long Point Bay is depicted by the isotherms 

 of figure 11. 



Cruise 8 . --The mean temperatures for the Sep- 

 temper cruise - -surface 20.0° C. and bottom 16.8°-- 

 indicate that the maximum surface temperature for 

 the whole lake was reached during the latter part of 

 August. The additional surface warming that took 

 place during September was more rapidly transferred 

 to the deeper layers by storms of greater intensities. 

 Surface temperatures of the lake from Erie eastward 

 were now between 20. 2° and 21. 7°, whereas those of 

 the Great Plain ranged from 18. 2° to 20. 2° (fig. 12). 

 Stations east of Erie were occupied prior to the storm 

 of September 10 and 11, and those west of Erie after 

 the storm. This timing accounts for the difference 

 in the mean surface temperatures for the two sections 

 of the lake on this cruise. The still fractionally 

 colder surface water off Port Stanley was matched by 

 a sUghtly colder area between Point Pelee and Lorain. 



Cold bottom water was now confined to the Deep 

 Hole region, where 12° C. water covered about 10 

 percent of the total lake area. The bottom waters of 

 the entire Great Plain were close to 19°, and at no sta- 

 tion in this area was the vertical differential over 0.6°. 



It will be noticed &om figure 13 that the cold 

 bottom isotherms are crowded over to the south shore 



48 



