A LIMNOLOGICAL RECONNAISSANCE 11 



July 30, supplemented by those of July 31 for depths below 

 20 m. 



TABLE 8— TEMPERATURE OF OKOBOJI LAKE, JULY 30, 1919 



Depth, 



meters Temperaturp 



0-5 24.6° 



5-10 23.9° 



10-15 18.8° 



15-20 13.1° 



20-25 11.6° 



25-30 11.1' 



30-35 10.9° 



35-40.2 10.8° 



Mean 21.00° 



From the observations in table 7, a temperature curve 

 may be platted, as shown in Fig. 2 ; and from such a curve 

 the mean temperature of each one-meter stratum may easily 

 be derived. The figure shows a normal curve, with a 

 strongly marked thermocline, beginning at 11 m., and end- 

 ing at 15 m. There was a fall of 4.4° between 12 m., and 

 13 m., and only 1.9° between 13 m., and 15 m. The total 

 fall between 11 m. and 15 m. is 7.6°. In the observation 

 on the 29th there was a decline of 9.1° in the same distance. 

 Such variations are of constant occurrence in a lake; and 

 if the thermocline of lake Okoboji is defined as the zone 

 within which the temperature falls as much as 1.0° per 

 meter, it will be found to vary from 4 m. to 6 m. in thick- 

 ness. The irregularity shown at 14 m. to 16 m., on the 30th 

 should not influence us to limit the zone to 3 m. or extend 

 it to 5 m. for this occasion. 



The mean temperature in table 8 is computed as follows : 

 The mean temperature of the several 5 m. strata is multi- 

 plied by the per cent of volume of that stratum, as shown 

 in table 3. The sum of these products gives the mean tem- 

 perature of the lake as 20.996°, or, as stated, about 21.0*. 

 This is an exceptionally high mean temperature for a lake 

 of the area and depth of Okoboji. It is due to the high tem- 

 perature and small volume of the lower water ; and the first 

 of these causes is itself largely due to the second. 



