782 Wisconsin Academy of Sciences, Arts, and Letters. 
terminal moraines were formed, one of which maintains the 
water in Lower Twin lake, and another, about 200 meters 
wide in its narrowest place, which maintains the water in Up¬ 
per Twin lake. The lakes are entirely surrounded by morain¬ 
al detritus with no bed rock exposed except for a short distance 
along the north side of Lower lake. 
Upper lake at its usual stage about midsummer has an area 
of about 192 hectares, and Lower lake about 582. The max¬ 
imum depth of the former was found to be 25 meters, and of 
the latter 22.5 meters. Both size and depth, however, are sub¬ 
ject to considerable variations, as the lakes are now used as a 
storage reservoir by the Twin Lakes Reservoir company. The 
dam maintained in the outlet makes it possible to raise the 
water of the lakes about 7.8 meters above extreme low water 
mark. While the maximum depth of Lower lake is only 2.5 
meters less than that of Upper, its average depth is much less, 
since most of the eastern portion of Lower lake is comparative¬ 
ly shallow (see Plate L). In the shallower portions of both 
lakes, the bottom is sandy and gravelly for the most part, with 
pebbles and boulders of various sizes in some places. In the 
deeper water, the bottom consists of a marly deposit. 
The principal affluent is Lake creek, which flows into the 
west end of Upper lake from Lake Creek canyon. About a 
dozen other streams of various sizes contribute their quota of 
water to the lakes. 
As might be expected, the climatic conditions at the altitude 
of the lakes, 2,800 meters, are such as to give them an alpine 
character. Records obtained from Mr. Charles L. Willis 
show that for four years Upper lake was entirely covered with 
ice for 138 to 149 days each winter. Mr. John J. Hartman 
reported that Lower lake was completely covered with ice for 
142 to 155 days each winter during the five years that he had 
made observations on it. The maximum thickness of ice on 
Upper lake for the winter 1902-3 was 71 centimeters, and on 
Lower lake 86 centimeters. 
Several sets of temperature observations were made on these 
two lakes during the months of July and August, both in 1902 
and in 1903. In general, the temperature conditions during 
