202 



also indicate a somewhat similar wave of bacterial develop- 

 ment, which is to be correlated with the wave of nitrification 

 detected by the chemical analyses. By the time the sewage of 

 Chicago entered the Illinois River at La Salle it was thus al- 

 ready in the advanced stages of decay and available for the sup- 

 port of the phytoplankton or other vegetation, if, indeed, it 

 was not already used to some extent by these agencies. The 

 progressive nitrification of the sewage in the canal is shown by 

 the average nitrates found by Palmer ('96) at Lockport (.84), 

 Morris (1.44), and La Salle (2.51 parts per million). The average 

 at Havana, about one hundred miles below La Salle, in the same 

 year, was only 2.34 with the added amount from Peoria's con- 

 tribution. At Kampsville, about 190 miles from La Salle, the 

 amount falls to 1.39. 



The sewage of Chicago under conditions prior to the open- 

 ing of the drainage canal in 1899 thus enters largely into the 

 sources of fertility of the river water. It reaches the maximum 

 of decomposition before mingling with the channel waters at 

 La Salle, and is reinforced by the sewage and wastes of Peoria. 

 The products of decomposition ( nitrates ) continue in dimin- 

 ishing quantity, diluted by tributary streams — as, for example, 

 by Spoon River, where the average amount of nitrates ( 1.01 ) 

 is somewhat less than that of the river at that point ( 1.58, for 

 1894-99 ) — and utilized by the developing phytoplankton and 

 other aquatic vegetation. Entering practically at the head- 

 waters of the Illinois, it becomes one of the most potent fac- 

 tors in the maintenance of the abundant plankton found in the 

 river and its backwaters. 



The sewage of Peoria, as represented by the pumpage of the 

 water-works, is but a small fraction of the total amount re- 

 ceived, being less than one per cent, if industrial waters are 

 included. For two reasons its effect upon the plankton in the 

 river at Havana is proportionally much greater than the fig- 

 ures indicate. The first is the proximity of Peoria, it being 55.7 

 miles above Havana. The maximum stages of decomposition 

 are usually passed, even in the coldest weather, before the sew- 



