999 
and bottom fauna associated with growths 
of rooted aquatic vegetation. 
Throughout the periods when biologists 
were stationed at the laboratory (sum- 
mers, 1941-1951), recording thermographs 
were operated to measure fluctuations of 
air temperatures and of water tempera- 
tures at various depths, fig. 4+. Daily re- 
cords were made of rainfall; these were 
compared with the fluctuations in water 
levels recorded on a continuous water level 
recorder and with changes in the trans- 
parency of the water as determined by 
Secchi disc readings. In 1942, when a 
biologist was stationed at the laboratory 
throughout the greater part of the year, 
analyses for dissolved oxygen were made 
on waters collected at frequent intervals 
from late February to the third week in 
October at depths of 0, 3, 6, 9, 12, 15, 
and 18 feet. Because physical and chem- 
ical data were collected for a longer con- 
tinuous period of time in 1942 than in any 
I~tinois NATuRAL History SurvEY BULLETIN 
Vol. 26, Art. 2 
other year, the records for 1942 have been 
used to illustrate the relationships be- 
tween these data, figs. 5 and 6. Fig. 5 
shows the average weekly air temperatures 
and the average weekly water tempera- 
tures at a depth of + feet (by recording 
thermographs) from the third week in 
March through the greater part of Octo- 
ber; also, water levels, transparencies of 
water, and rainfall for approximately this 
same period. 
Rainfall, Water Levels, and Trans- 
parency.—The drainage basin of Ridge 
Lake is large in proportion to the capacity 
of the lake basin. This fact, coupled with 
the fact that most slopes in the drainage 
basin are steep and that some of them are 
overgrazed, insures a rapid runoff of rain- 
fall carrying a relatively large load of silt, 
most of which passes into the upper end 
of the lake basin, where the heavier par- 
ticles of silt are dropped. Fine clay par- 
ticles disperse throughout the waters of 
Fig. 4.—Recording thermographs on instrument tower in Ridge Lake opposite the labora- 
tory. These thermographs were used to record air temperatures and water temperatures at 
various depths. The bulbs of water temperature recorders were suspended from a float built 
around the concrete tower so that they would rise and fall with changing lake levels. The rain 
gauge shown here, when in use, was placed on a second tower in the upper part of the lake. 
A water-level recorder was located on the boat pier in front of the laboratory. 
