170 
itself. In like manner the backwaters, which are usually much 
shoaler than the river, are subject to greater changes, exhibit- 
ing in warm days greater extremes of heat, as high, for example, 
as 96° having been found in the margins of bottom-land ponds. 
On the other hand, the flood waters in the forests and marshes, 
where the vegetation protects the water from the direct rays of 
the sun, remain at lower temperatures than those of more open 
tracts. The lakes and bayous with aquatic vegetation also re- 
main cooler in their deeper waters, as, for example, Thompson’s 
Lake, where, among the Ceratophyllum, the temperature at 
the surface on the fifteenth day of July was 88.2°, while only 
six inches below, in the vegetation, it was 80°, the difference 
being due to the protection from sun and wind which the veg- 
etation afforded. 
Another factor tending to modify the temperature is the 
earth temperature, which in the very shallow waters of our 
environment becomes relatively important in both summer and 
winter. In the low temperature of winter this is heightened by 
the fact that most of the bottom of the backwaters is strewn 
with a mass of vegetation whose decay must produce some 
heat. This probably accounts for the higher bottom tempera- 
tures sometimes observed in winter (cf. Tables III. and VIII.) 
in Flag Lake, where such detritus was more abundant than in 
the river, where but little is found. For example, on February 
26, 1897, the bottom temperature in Flag Lake was 36°, while 
in the river, with about the same surface temperature (32°) and 
greater depth, it was only 32.5°. This difference may also be 
due to the effect of the current in the river in mingling more 
quickly the surface and bottom waters and thus equalizing their 
temperatures more rapidly. 
The temperatures recorded in the Illinois River, Spoon River, 
and in Thompson’s, Quiver, Dogfish, Flag, and Phelps lakes are 
to be found in Tables III.-IX. respectively, and they appear on 
the plates with the hydrographs and plankton data of the re- 
spective years and stations. The extreme range of temperature 
observed by us in the river and its adjacent waters at Havana 
