18 IOWA STUDIES IN NATURAL HISTORY 



1,770,000 cu. m. ; its maximum depth 10.2 m. ; and its mean 

 depth |-v-| is 357 cm. Its mean temperature is nearly 



10.9°, so that T-4 nearly equals 6.9° ; and this temperature 

 multiplied by the mean depth equals 2463 cal., or substan- 

 tially the result shown in table 12 for the heat-income. 



The Transportation of Heat. All of the heat accumulated 

 by the lake conies from its surface. Even that which is 

 brought in by ground water enters ordinarily at the sur- 

 face or very close to it and in a lake so large as Okoboji 

 the contribution of heat from the ground water — which is 

 plus or minus, according to the time of year — is so small 

 as to be negligible. The temperature of the water in the 

 well at the Lakeside Laboratory was 11.3° on July 30. 



There are two agents by which the heat of the summer 

 heat-income may be conveyed from the surface to the deep- 

 er water of the lake. It may be (a) delivered directly by 

 the sun to the deeper strata of water, since the water is 

 not opaque; or (b) it may be conveyed by the action of the 

 w^ind, causing waves and currents by which the water 

 warmed at the surface of the lake is carried downward and 

 mingled with the colder water. It will be seen that in a 

 lake of the type of Okoboji the latter method is by far the 

 more important. 



It will be well to assume at first that the distribution of 

 heat is wholly due to wind and later to estimate the contri- 

 bution of the sun to this task. 



So long as the temperature of the water is below 4° there 

 is no trouble in conveying the warmed surface water down- 

 ward, for it becomes heavier as it warms and it consequent- 

 ly sinks. The excess weight is very small and unless aided 

 by wind the downward movement and consequent mixture 

 would be slow. But wind is not lacking in early spring and 

 even a light breeze is efficient in causing mixture of the 

 water if gravity aids it. 



But when the temperature of the surface water passes 4° 

 it becomes lighter as it warms and apart from the action 

 of the wind it would remain floating on the colder and there- 



