important role in the circulation of the waters of 

 the lake. 



In February 1926 the monthly mean lake level 

 reached the minimum for the past 50 years. The 

 level was then 3 feet lower than it was during the 

 period of the survey. The average lake levels in 

 June and September 1929 were 1. 9 feet and 1. 4 

 feet, respectively, higher than the average stages 

 of those months for the past 10 years. These vari- 

 ations in mean lake levels are principally the con- 

 cern of the navigator and the city engineer, and 

 only indirectly enter into the problems of this survey. 



The maximum daily range of 4. 9 feet at the 

 Buffalo gauge during the period of the survey oc- 

 curred on May 16. The daily range of water level 

 sometimes exceeds 8 feet in severe storms. 



TEMPERATURES 



Conditions at Time of Different 1929 Cruises 



Cruise 1. --Field ice left Lake Erie earlier than 

 usual in 1929. The last of the ice passed down the 

 Niagara River May 2, 1929, although shipping 

 opened at Buffalo on April 12. Thus the lake was 

 free from ice at least 3 weeks earlier than in 1928, 

 when ". . . . there was closely packed field ice ex- 

 tending from Buffalo as far as the eye could reach 

 on May 15, and for several days thereafter" (New 

 York Conservation Dept. , 1929, p. 46). Ice is 

 nearly always present during April, and spring 

 storms keep the water well mixed. Vernal warm- 

 ing becomes evident in May; and, although the 

 regular cruises did not begin until the following 

 month, 19 stations were occupied between May 17 

 and 28. The best values obtainable from these 

 May observations for the temperatures of the whole 

 lake are: surface, 8. 2°; bottom, 6. 0°. The dis- 

 tribution by sections was as follows: the Great 

 Plain area, surface 8. 2°, and bottom 7. 2°; Deep 

 Hole area, surface 7. 3°, and bottom 4. 8°; east- 

 ern area, surface 10. 9°, and bottom 7. 0°; Long 

 Point Bay, surface 7. 7°, and bottom 4. 8°. 



The coldest column of water was observed at 

 station 01-27, at the southwestern edge of the Deep 

 Hole. Here the surface was only 4. 8° and the bot- 

 tom 4. 3°. The air temperature at this station was 

 6. 0°, and the wind northeast, force 4 (table 5). 

 The little surface warming that had taken place 



15 



here was destroyed by the wave action from this 

 cold wind, blowing as it was from the cold waters 

 of the Deep Hole. 



During the last half of May, then, the lake 

 was still cold, with but slight vertical temperature 

 gradients and no thermocline. The last two con- 

 ditions are conducive to vertical mixing; such a 

 situation did not exist again until the middle of 

 September, when the waters of the Great Plain were 

 once more homothermous. 



Surface -temperature changes of several de- 

 grees in 24 hours may be expected in the spring. 

 During the afternoon of May 24, the sea was chop- 

 py and the air temperature was 8° C. at station 15 

 (Deep Hole), while the surface temperature was 

 5. 6°. The next day was bright and warm and the 

 sea smooth. The afternoon surface temperature at 

 station 15 was now 8. 3°--an increase of nearly 3° 

 in 22 hours. On the second day, 5. 6° water was 

 found at a depth of 6 meters. Such sudden changes 

 were not observed after the thermocline was well 

 established. 



Cruise 2. --During the first half of June, the 

 surface water was warmed at a considerable rate. 

 The western end reached 20° C. by the 18th, where- 

 as a month previous this area was only 8°. Cold 

 surface water was confined to the area of the Deep 

 Hole. 



The mean temperatures for the lake on cruise 

 2 were: surface 15. 5° C. and bottom 9.4° (table 4). 

 The top and bottom isotherms for the lake are shown 

 in 2° intervals in figures 4 and 5. The steep hori- 

 zontal thermal gradient to the westward is partly due 

 to the fact that the cruise commenced at the Buf- 

 falo end on June 7 and ended 12 days later at the 

 western end, and partly due to the greater rapidity 

 of warming in the shoaler western section of the lake. 

 The lowest temperatures on this cruise were in the 

 Deep Hole region where the surface was 10. 0° and 

 the bottom 4. 4°. Temperatures were only slightly 

 higher in the adjacent waters of Long Point Bay. Al- 

 though surface marginal -zone temperatures of the 

 Eastern area were uniformly around 13°, the bottom 

 waters were slightly warmer along the southeastern 

 shore. Station 02-12, which is only 1 1/2 miles off 

 the south shore, proved an exception, with 6. 1° 

 bottom water. The first thermocline was also en- 

 countered at this station, where the temperature 

 dropped 3. 4° between 13 and 14 meters (table 6A). 



