388 



slight pulse of production which, however, declines with the 

 culmination of the rise and increase in current attendant 

 thereon. The nitrites rise again (PI. XLV.) to twofold the 

 quantity present in December, when plankton production was 

 fourfold as great as in the present month. The movement of 

 free ammonia and oxygen consumed suggests varying degrees 

 of sewage decay, while the chlorine falls with flood dilution. 

 The ice blockade continues till the close of the month, but gives 

 way with rising temperatures and the culmination of the flood 

 on the 24th. The average production for the month (.18 cm. 3 ) 

 is much less than that of January, 1898 (.45), when the direc- 

 tion and extent of movement in levels were very similar, but the 

 levels lower by 2.5 ft. on the average, and hence the current less 

 rapid. Less time for breeding is thus afforded in 1899, and 

 production is less than in 1898. 



The February pulse has a duration of 28 days, — from Jan. 

 31 to Feb. 28, — with a maximum amplitude of 1.92 cm. 3 per 

 m. 3 on the 21st. Its mean falls on the 20th, 33 days after that 

 of the preceding pulse. During the first three weeks of this 

 month there is a steady decline in levels under an ice-sheet of 

 unusual thickness (31 cm.), which was carried out by the sud- 

 den rise from a level of 5.5 ft. to one of 10.2 ft. in the last week. 

 The temperature beneath the ice remains at the winter mini- 

 mum of 32°+ with a variation of less than 1°. Chemical con- 

 ditions (PL XLV.) are subject to abrupt and great change with 

 the rising flood of the last week. This is due in large part to 

 the sudden increase in industrial refuse from the accumulations 

 on the banks above previous water levels at Peoria and Pekin. 

 These are carried into the stream by the flood and cause the 

 fivefold increase in oxygen consumed and the rise in albumi- 

 noid ammonia and organic nitrogen. Prior to this flood there 

 had been a rise in nitrates, and on the 21st the chlorine, nitrites, 

 organic nitrogen, and free ammonia all exhibit a very marked 

 upward movement suggestive of the approach of stagnation 

 conditions. Stagnation is not reached, however, owing to the 

 higher levels, to the break in the ice in the blockade late in Jan- 



