LEVELS AND TEMPERATURE OF LAKE OKOBOJI 97 



ference in temperature between the surface at the end of the 

 pier and the bottom at the same place (six and a half feet deep) 

 was sometimes as much as two degrees, at one time after a day 

 of bright sunshine with little wind amounting to five degrees 

 (July 12). Even this large difference in temperature was 

 nearly equalized by circulation during the night. A little wind 

 was commonly enough to bring in and down the warm surface 

 Avater of the lake, or to blow out and away the warm surface 

 water, causing the colder water below the surface to rise. The 

 morning observations often gave the same temperature at the 

 surface as at the bottom at the end of the pier, and but three 

 times (July 5, 14 and 30) giving a greater difference than one 

 degree. These were days of bright sunshine and little wind. 



The data for the temperature curve of the lake were ob- 

 tained the fifth of August, as late as it was convenient to gather 

 the data. Unfortunately the entire week preceding that date 

 was characterized by clouds, strong wind and somewhat of rain- 

 fall, which condition accounts for the irregularity noticeable 

 in the curve of temperature. Even in this irregularity the 

 planes of demarcation of the three zones are pronounced. The 

 area of the hypolimnion extends from near Terrace Park north- 

 ward through the central portion of the lake to opposite the 

 center of Omaha Beach. The thermocline extends over this 

 area and a little to each side of it from Terrace Park to Omaha 

 Beach and then extends northward to opposite Pikes Point. 

 It is to be noted that within the epilimnion (where the water is 

 forty feet or less in depth) is included the waters of all the 

 bays of "West Okoboji, all of Lower, Middle and Upper Gar 

 Lakes, and all of East Okoboji for which data on depth are 

 recorded. The volume of water of West Okoboji included in 

 the epilimnion at the time of observation, which was very nearly 

 the maximum for the year,* is computed as approximately 

 171,540,503 cubic yards. The volume in the thermocline, twenty- 

 five feet thick, is approximately 72,709,309 cubic yards, and the 

 volume in the hypolimnion approximately 38,713,961 cubic 

 yards. The above figures are based on the soundings made in 

 1905 by the engineering students of Iowa State College. 



*Edward A. Birge and Chancey Juday, "A Limnological Study of the 

 Finger Lakes of New York," Bulletin of the Bureau of Fisheries, Vol. 32, 

 1912, Document No. 791, page 546. 



