occupied, and on the preceding day, the wind was 

 northerly and nrioderate, but not of sufficient duration 

 to cause a complete reversal of the horizontal gradi- 

 ent on a body of water the size of Lake Erie. Fur- 

 thermore, on cruise 6, when this area was observed 

 immediately after two days of strong south and west 

 winds, the colder water remained in the Port Stanley 

 vicinity, as it did on cruise 4. Kettle Creek empties 

 into the lake at Port Stanley, but on account of the 

 small effluent, it is not a factor in regulating the 

 lake temperature except in the local marginal zone. 

 It is possible that south and southwest winds set up a 

 greater wave action in this area than in other parts 

 of the Great Plain, causing a more marked lowering 

 of the surface temperature by deeper mixing. 



Figure 8 shows graphically the temperature dis- 

 tribution on cruise 4 in the vertical section of the 

 Great Plain from Port Stanley, Ontario, to Ashtabula, 

 Ohio (stations 04-32 to 04-37). Both the lower sur- 

 face temperature of the Port Stanley region and the 

 layer of cold bottom water are clearly pictured. 

 Figure 1 shows the location of this section, and table 

 6-K, station 35, gives the seasonal changes in the 

 serial temperatures at that station. 



The cold bottom layer, which covered the floor 

 of the Great Plain included within the 20 -meter depth 

 curve, now had an average thickness of 4. 5 meters 

 and ranged from 10. 1° C. in the deeper parts to 12° 

 at the 20 -meter curve. This cold water was insu- 

 lated from the warmer upper layers by a thermocline 

 of 4° change in a meter. 



As may be seen from figure 7, in July about 45 

 percent of the total lake area was covered with bot- 

 tom water having a temperature less than 12°. The 

 thermocline was now established over the lake from 

 Erie to Cleveland. Its average depth was 18 meters 

 in the western part of this area and 14 meters in the 

 eastern part. The greatest temperature difference in 

 1 m.eter was at station 04-26, where readings were 

 15. 2° at 13 meters and 7. 5° at 14 meters--a differ- 

 ence of 7. 7°. Two weeks later, on July 22, the 

 thermocline at this same station was between 14 and 

 15 meters, where there was a difference of 6. 7°. 



The surface temperatures in the Deep Hole area 

 were comparatively low and the thermocline there- 

 fore was not so definitely established. At station 

 04-15, at the deepest part of the lake, the temper- 



ature dropped 5°C. from the 10-meter level to the 

 20 -meter level, but at no depth was the difference 

 as great as 1° to the meter. 



Cruisers. --The interval between regular 

 cruises 4 and 6 was utilized in determining again 

 the limits of the 6° C. water of the Deep Hole area. 

 The mean surface temperature for 14 stations on this 

 cruise was 20. 9°, as compared with 20. 8° for the mean 

 of the same stations on cruise 6. This section of the 

 lake is unique in that the thermocline occurs at wide- 

 ly different depths due either to oscillatory movements 

 or upwelling of the cold Deep Hole water. Thus at 

 station 05-17 (Long Point) there was a difference of 

 7.2° between 10 and 11 meters; at station 05-14, 

 there was a temperature drop of 2. 6° from 26 to 28 

 meters. 



Cruise 6. --The mean of the observed surface tem- 

 peratures for the lake reached a maximum in August 

 at 20. 6° C. The mean bottom temperature for this 

 cruise was 14. 7° (table 4) . The north shore of the 

 eastern area again had the lowest surface-water temper- 

 atures. As may be seen from the isotherms of figure 

 9, a horizontal gradient ranged from 17° along the 

 north shore to 21° at midlake; the southern portion of 

 the area was around 21°. Water temperature of 22° 

 was found from Erie out to Northwest Shoal (station 

 50), and Port Stanley was again the region of the cold- 

 est surface water in the Great Plain. Here the water was 

 between 18° and 19°, whereas the southern marginal 

 zone was from 20° to 21°. An even horizontal gradient 

 radiated from Port Stanley. West of the Fairport- 

 Rondeau line the surface temperature did not vary more 

 than 0.3° from 21.5° 



The cold bottom waters of the Great Plain had 

 shrunk to the e.xtent that the 14° C. isotherm (fig. 10) 

 covered only half the area that the 12° isotherm had 

 included a month previous. The lowest temperature 

 for this area was now 11. 2° as compared with a low 

 of 10. 1° on cruise 4. 



An excellent example of the role that the lake 

 bottom contours play in the regulation of the thermo- 

 cline depth was observed at stations 06-12 and 06-61, 

 which are only 5 miles apart (fig. 1). At station 12, 

 less than 2 miles off the southern shore at Westfield, 

 the depth is 22 meters. Here the water was practical- 

 ly homothermous vertically from the surface to 15 

 meters, but dropped 1° per meter from that level to 



44 



