338 
slight pulse of production which, however, declines with the 
culmination of the rise and increase’ in current attendant 
thereon. The nitrites rise again (Pl. 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 em.’) 
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.* per 
m.* on the 2Ist. 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 e¢m.), 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 inthe blockade late in Jan- 
