171 



was 32°-96°. The highest temperature recorded in the river 

 was 89°, on the afternoon of August 3, 1897, and again, at the 

 same time of day, July 26, 1898. 



The diurnal range in temperature is considerable at times, 

 depending naturally upon that of the air. On August 3-5, 1898, 

 in connection with a test of the diurnal movements of the 

 plankton and accompanying analysis of the gases dissolved in 

 the water, the temperatures recorded indicate in the surface 

 waters a range of 5,5°, with a maximum of 79,5° at 5:00 p, m, 

 and a minimum of 74° at 2:00 a, m. The bottom water (depth 

 2.44 meters) showed a range of but 2°, from 74° at 8:00 a, m. 

 to 76° at 11:00 a. m. The air temperatures on the days in ques- 

 tion ranged from 83° at 5:00 p, m. to 58° at 5:00 a. m. A diur- 

 nal variation of 5,5° in surface waters and 2° in bottom waters 

 is thus indicated at this time. Other conditions will probably 

 show a slightly greater range. 



Monthly and Yearly Averages of Surface Temperatures, 1894-1899, 



Illinois River. 



Year 



1894. 

 1895. 

 1896. 

 1897. 

 1898. 

 1899. 



Monthly 

 average 



Jan. 



32-75 



32-7 

 32.9 



32.78 



Feb. 



32 



33-7 

 32.25 



Mar. 



39-52 

 43-8 



32-1243.3 

 32-6 35.2 



32.73I40.45 



April 



58 



64.54 

 60 

 53-32 



60.46 



May 



72.7 

 66.3 

 65.8 



68.27 



June 



80.25 

 80 



74-7 



75 



78.8 



77-75 



July 



Aug. 



82.25 



79 



80.7 

 81.02 

 82.87 



81.03 



83-5 



80.51 



82 



80.9 



80.56 



81.49 



Sept. 



77-5 

 78.87 



65-75 

 77.07 

 71-87 



74.21 



Oct. Nov. 



58 I41 



54.26:42.5 



56 44 



65-1 i45-7 133-02 



54-37141-4232--'' 



Dec. 



39 



37-5 

 33-6 





57-55I43-00 35.22 



56.66 

 55-84' 



57.08* 



*Average of monthly averages. 



The temperature records are too isolated to plot complete 

 thermographs of the river and its backwaters, though they do 

 give a very fair idea of the seasonal fluctuations, especially in 

 the later years, when they were more evenly distributed, and 

 in the midsummer, when they were more numerous. . They were 

 usually taken between 7:00 a.m. and 5:00 p. m., and may be 

 regarded as day temperatures. A comparison of the records in 

 Table III,, the plottings (PL VIIL-XIII,), and the above table, 

 giving the monthly averages of surface temperatures of our 

 records, shows the following seasonal routine in the river: 



