143 
days being sufficient for the passage from Utica to Grafton (230 miles) 
in the month of August in the years 1910-18, and an average of twenty- 
nine. days in the same month of the years 1890-99. 
This fact has many interesting consequences. , Decomposition and 
assimilation processes set up at a given point and proceeding at a given 
rate are now extended farther down stream than formerly, and those 
which were usually completed at the mouth of the river before may now 
be only about half finished at that point. A plankton organism, if car- 
ried continuously by the main current, has much less time to multiply in 
the Illinois than formerly and reaches smaller numbers, and, other things 
being equal, it is proportionately less likely to be devoured on the way, 
the difference in this respect being the same as if the river were shortened 
by about one-half. Disease germs brought into the stream with sewage 
may now be carried as far again downward before they perish, and the 
sewage itself, imperfectly transformed before, as shown by chemical 
tests, now escapes from the river in still earlier stages of average trans- 
formation. 
Of course neither plankton organisms nor any part of the water of 
the stream are actually carried the full distance down the main current, 
but everything is retarded more or less as it approaches the bottom or 
the shores. We have no data to show what the general average move- 
ment of the whole content of the river now is or how it compares with 
the former average rate of progress, but there can be no doubt of the 
general truth of the foregoing statement. 
6. The sewage load of the river has greatly increased since 1899. 
' This is a necessary inference from the growth of Chicago and the river 
towns and the completion of the intercepting sewer along the lake front 
turning into the canal all sewage formerly sent into Lake Michigan, and 
it is shown also by a comparison of chemical data. The chlorine content 
of a polluted water (mainly chloride of sodium or common salt) is the 
best chemical index to the degree of its contamination by sewage, since 
the chlorine compounds pass down stream virtually undiminished and 
unchanged, while the nitrogen is rapidly transformed, and presently 
drawn upon as food for plants. A comparison of the amounts of chlorine 
passing Averyville, just above Peoria, before 1900 and in 1914, gives us 
214 times as much per second for the latter period.* 
7. Notwithstanding the increase in the quantities of sewage coming, 
into the river from Chicago, the flow of the stream has been increased 
in so much larger a ratio that the water of the river contained in 1914 
a smaller percentage of sewage than before 1900. The sewage of the 
stream had become much more dilute than formerly. 
The ratio of chlorine (parts per million) in the water passing Lock- 
port, 35 miles from Lake Michigan, in 1897-99 compares with that of 
1914 as 7.3 to 1, and the corresponding chlorine ratios for these periods 
at Averyville, 126 miles farther down, were as 2.3 to 1. 
* The chlorine average at Averyville July to October, in 1897-1899, was 7.69 kilo- 
grams per second; in 1914 it was 17.31 kilograms per second. 
