A STUDY OF TWIN LAKES, COLORADO. 



155 



dav^i the lakes were covered with ice. I am indebted to Mr. Chai'les L. \\'illis for 

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

 Lower Lake. 



Periods in 1901-1905 thirhitj u-hklt Ta-'in Lakes U'ere icebound. 



The record.s of the freezing over show only the dates on which the lake.s 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 l-±9 days, and Lower Lake for 142 

 to 155 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 each year. It will also be noted that Upper Lake was not covered with 

 ice quite so long as Lower. For the winter 190'2-3, Mr. Hartman reported the maximum 

 thickness of ice on Lower Lake as 3-4 inches (86 centimeters), and Mr. Willis found a 

 maximum 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 1903-4. 



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

 the months of Jul}' and August in 1902 and 1903. 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 temperature increai-ed 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 alwa3-s characterized by a considerable change in 

 tiie temperature of the water within a comparatively 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. (9^ F.) colder than that in the upper por- 

 tion. The decrease in temperature with increasing depth both above and 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 clo.sely with this 

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

 temperature early in the summer. During the.se 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 supertherniocline region was kept 

 ([uite tlioroughly 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 account of the fact that the wind was more 

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



