28 IOWA STUDIES IN NATURAL HISTORY 



high temperatures near the bottom. In 1919 the bottom 

 temperature at 34 m. was 10.8° and was probably 10.7° or 

 10.6° at 40 m. This must be an exceptionally low tempera- 

 ture for the lake since lakes in the same general region had 

 unusually low bottom temperatures in 1919. This situation 

 was due to a period of hot calm weather in June, which heat- 

 ed the surface stratum of the lakes and thereby checked the 

 distribution of heat to the lower water. The bottom tem- 

 perature of lake Mendota on August 1 at 23.5 m. was 9.5°, 

 the lowest recorded ; and that of Green lake was 4.9° at 72 

 m., the only temperature below 5.0° found by the Wisconsin 

 survey in this lake. That of lake Geneva was 7.0°, a low 

 temperature, but not the lowest on record. The general 

 temperature, however, of the water below 10 m. was the 

 lowest recorded. 



Temperature observations on lake Okoboji were made by 

 Professor John L. Tilton, of Simpson College in 1915 and 

 1916 (Tilton '16, '17). The record of August 5, 1915, 

 showed a temperature of 15.5°C. (59.8°F.) at a depth of 

 about 35 m. (115 ft.). On the same date the surface tem- 

 perature was 20.0°C. (68°F.). On July 13 the temperature 

 at 35 m. was 15.0°C. and at about 41 m. (135 ft.) 14.7°C. 

 These observations were made with a minimum thermo- 

 meter weighted and sunk into place. 



In 1916 Professor Tilton took readings in June and July. 

 On July 18 he found a temperature of 13.1°C. at 32 m. This 

 series was read with a Leeds and Northrop electrical re- 

 sistance thermometer. This instrument became disabled and 

 on July 26 readings were made with the minimum thermo- 

 meter, showing a temperature of 15.6° at a depth of 27 m. ; 

 the reading at this depth on July 18 was 13.3°. 



The attention of the authors of the present paper was 

 called to Okoboji lake by this work of Professor Tilton. The 

 readings at first seemed incredible. Nothing in our pre- 

 vious experience warranted the idea that a lake could have 

 a midsummer temperature greater than 15° at a depth of 

 40 m., or even a temperature approximating 15°. Lake 

 Geneva, the nearest parallel in depth, had shown in 18 sea- 

 sons a mean bottom temperature of 8.0° with a minimum 



