543 



siderable progressive improvement of river conditions was manifest, 

 due to sedimentation, to self -purification, and to dilution with cleaner 

 tributary waters. Oxygenation increased between Marseilles below 

 the dam and Starved Rock by 36 per cent, in August and by 14 per 

 cent, in September, but in February it was the same at these two 

 stations. Other evidences of sewage contamination were similarly 

 diminished — the color of the water, the number of foul-water or- 

 ganisms either suspended in the water or growing at the edges, the 

 scarcity and small variety of fishes resorting to the middle of the 

 stream — until at Starved Rock we got our first somewhat represen- 

 tative collections of distinctly main-stream fishes — gizzard-shad, red- 

 horse, carp, bullheads, and black bass. Although the slack-water 

 sludges were scarcely changed in character from those farther up the 

 stream, living mussels began to appear in this section in small variety, 

 but mainly in the cleaner water of the Fox; decapod and isopod 

 crustaceans (crawfishes and Asellus) came in here in some numbers; 

 and sponges and Bryozoa put in an appearance in especially favorable 

 spots. The contaminational blue-green algae, common along shore 

 above, practically vanished ; and the chlorophyll-greens changed from 

 Stigeoclonium teiiue as the dominant form to species of Cladophora, 

 significant of cleaner water. The degrees of this improvement va- 

 ried, of course, as usual, with the temperature and the stage of water. 



Spring Valley, — It was at Spring Valley, fifty-seven miles below 

 the mouth of the Des Plaines, that, in summer time, the last visible 

 symptoms of water pollution were to be seen. The water here had 

 not yet recovered its normal slightly greenish tint, but was still 

 grayish with suspended specks of septic and pollutional plankton ; 

 and it still smelled slightly of sewage, in part no doubt because of 

 local contamination from Peru and La Salle, a few miles above. It 

 was here that the little amphipod crustacean, HyalcUa knickcrbockcyi, 

 made its first appearance below the Kankakee, and here that the 

 first specimens of the river shrimp, PalcBmonetes exilipes, were col- 

 lected. A considerable variety of aciuatic insect larvse, of living mus- 

 sels, and of gastropod mollusks, and a much smaller proportion of 

 dead shells, testified further to an improved environment. This was 

 also the first place on the river in which commercial fishing opera- 

 tions were being carried on at any time. 



These biological tests were more favorable to the Spring Valley 

 situation than the chemical ; but they are, on the whole, more reli- 

 able, if they are used with intelligence and discretion, because they 

 show the accumulated general consequences of local conditions, favor- 

 able and unfavorable, while the chemical determination applies only 



