A STUDY OF TWIN LAKES, COLORADO. 155 



days the lakes were covered with ice. I am indebted to Mr. Charles L. \Yillis for 



the data concerning Upper Lake, and to Mr. John J. Hartman for that pertaining to 



Lower Lake. 



Periods in 1901—1906 during which Twin Lakes were icebound. 



The records of the freezing over show only the dates on which the lakes became 

 completely covered with ice. For some time previous there was considerable ice on 

 both lakes, particularly around the edges and in the protected bays. Thus, it will be 

 seen that Upper Lake was icebound for 138 to 14'.i days, and Lower Lake for 1 tJ 

 to L55 days each winter during the period covered by these observations. While no 

 temperature observations were made during these periods, it can safely be said that 

 the temperature of the water must have been tolerably low for a period of at least 

 five months eaidi year. It will also be noted that Upper Lake was not covered with 

 icequiteso long as Lower. For the winter L902-3, Mr. Hartman reported the maximum 

 thickness of ice on Lower Lake as 34 inches (86 centimeters), and Mr. Willis found a 

 max i mum of 28 inches (71 centimeters) on Upper Lake. For the latter lake also. Mr. 

 Willis reported a maximum of 24 inches (60 centimeters) for the winter L903— 4. 



Several sets of temperature observations were made on these two lakes during 

 the months of July and August in 1902 and L903. In general the temperature condi- 

 tions during the summer were found to be similar to those that have been observed 

 in lakes of corresponding size and depth at much lower altitudes that is, the same 

 three regions were noticeable. There was an upper stratum of water, or superther- 

 mocline region, whose tempera! lire increased materially during the summer; a bottom 

 .stratum, or subthermocline, whose temperature changed very little during the sum- 

 mer; and a more or less distinct transition zone or thermocline between these two 

 strata. The thermocline region is always characterized by a considerable change in 

 the temperature of the water within a comparal ively thin stratum. This stratum was 

 found to be from 3 to 4 meters (10 to 13 feet) thick in these lakes, and the water in 

 the lower portion of it was about 5- C. ('•' F. ) colder than that in the upper por- 

 tion. The decrease in temperature with increasing depth both above ami below this 

 region, was much more gradual. This transition zone was not nearly so pronounced, 

 however, in these lakes in late summer as has been found by the writer in lakes in 

 southeastern Wisconsin and northern Indiana, but it agrees very closely with this 

 zone in the latter lakes when their upper stratum of water has a corresponding 

 temperature early in the summer. During these observations westerly winds blew 

 with considerable regularity, beginning usually about 10 a. m., and lasting till late 

 in the afternoon. As a result the water of the superthermocline region was kept 

 quite thoroughly stirred up, so that its temperature was tolerably uniform, thus 

 producing a fairly distinct thermocline. The superthermocline was considerably 

 thicker in Lower than in Upper Lake, on accountof the fact that the wind was more 

 effective in disturbing the upper water of the former becauseof its much larger size. 



