495 



by a mere increase of the waters themselves, which, in so sluggish 

 a stream as the Illinois, with bottom-lands so extensive and so widely 

 overflowed by so small a rise of the river levels, will take effect 

 mainly in great expansions of shallow water, long continued or per- 

 manently maintained, with muddy bottoms and more or less weedy 

 shores — situations c]uite capable of producing a relatively enormous 

 plankton as well as an abundant supply of shore and bottom animals 

 and plants; (b) by the addition of increased quantities of organic 

 matter to the contents of the stream in the form of a larger inflow 

 of sewage from Chicago and its suburbs, in condition to increase the 

 plankton by increasing the supply of food available to the minute 

 organisms which compose it; and (c) by the addition to the plank- 

 ton of the river, of that of Lake Michigan brought down in the 

 waters of the canal. 



The efiicacy of the first of these conditions is undoubted, as 

 has been already shown, and that of the second is, generally speak- 

 ing, quite possible. The importance of an abundance of organic 

 matter in the water as a means of producing a rich plankton is, in 

 fact, so well known that growers of pond fishes in Europe delib- 

 erately manure their ponds to increase the supply of food for their 

 fish ; and there is considerable evidence, also, that the plankton of 

 the Elbe is largely increased by the sewage of Hamburg and Altona 

 poured directly into that stream. Whether Chicago sewage has a 

 like effect in the Illinois, and whether, if it has, that effect may not 

 have disappeared before the waters of the stream have reached Ha- 

 vana, are problems which will be taken up in a later paper. The 

 supposition that the waters of the canal may bring to the Illinois a 

 richer plankton than that of the river itself is negatived at once by 

 the fact that Lake Michigan water is relatively poor in microscopic 

 life, and hence must dilute the river plankton instead of increasing 

 it ; and by the further fact that most of the characteristic minute 

 organisms of the lake have died out in the river before they reach 

 Havana.* 



ILLINOIS RIVER WORK OF 1911 AND 1912 



The Illinois River, it will be remembered, is fonned by a union, 

 near Dresden Heights, of the uncontaminated Kankakee and the 

 heavily polluted Des Plaines... The waters of these streams flow down 



*A partial exception to this statement is found in the survival in the river of 

 the lake diatom TabeUaria Hloccitlosa, its increase after it passes the Morris-Mar- 

 seilles ordeal, and its immense multiplication in Peoria Lake. This species was 

 not found in the Illinois before the opening of the drainage canal. 



