540 



stream at this point was carrying polluted water, and sewage or- 

 ganisms were abundant across the bottom and on both sides — a fact 

 to be attributed in part, according to our best information, to con- 

 tributions of sewage from the towns above. In August and Septem- 

 ber, 1912, however, the two sides of the Des Plaines were in marked 

 contrast. On the west side were minnows, sunfishes, green algae, 

 case-worms, sand-fly larv?e, Bntomostraca, Hyalella knickerbockeri, 

 and several kinds of mollusks, while on the east side Sphccrotilns, 

 Carchcsium, Vorticella microstoma, and numerous other sewage or- 

 ganisms predominated, with fishes and clean-water mollusks con- 

 spicuous by their absence. 



The Dcs Plaines River at Dresden Heights. — The Des Plaines at 

 its mouth was heavily loaded with putrescible sewage materials so 

 far advanced in decomposition that oxygen ratios were always very 

 low. They differed however, according to season, in comparison with 

 those of the next lower stations, being higher than these in the 

 hottest weather and lower at lower temperatures. In July, 191 1, for 

 example, when the water of the Des Plaines contained 1.2 1 parts per 

 million, that of the Illinois stood at 1.07 at Morris, and at .83 above 

 the Marseilles dam. In November, 1912, on the other hand, the 

 corresponding figures were 4.90 for the Des Plaines, 6.80 for the 

 Illinois at Morris, and 7.90 at the Marseilles dam. This seasonal 

 difference is again to be accounted for by the different effect of low 

 and high temperatures upon the beginning and rapidity of the de- 

 composition process in a polluted stream. 



The plants and animals of the Des Plaines being of a diverse 

 origin, were a mixture of clean-water forms from the lake and from 

 the upper river which had not yet been overcome by their septic 

 environment, and of species characteristic of polluted water, the 

 latter group strongly dominating. Saprobic organisms of the mid- 

 summer microplankton were more numerous here than at any point 

 on the river ; but mingled with these were many Lake Michigan dia- 

 toms, some of which presently disappeared down stream, two con- 

 tinuing, however, to the lower river, where they became very abun- 

 dant beyond the heavily polluted section. 



There were no fishes, mollusks, crustaceans, or insect larv?e (ex- 

 cept Chironornus) in the Des Plaines at Dresden Heights, although 

 the Kankakee, but a few rods away, contained the usual biological 

 population of a normal Illinois river. 



The Kankakee at its Mouth. — Notwithstanding the fact that the 

 Kankakee can hardly be called an uncontaminated river, its contrast 

 with the Des Plaines, especially in summer, was very marked. When 



