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 foi'ests 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 CeyaiophnUuui, 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 l)ottom 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 



