88 ILLINOIS BIOLOGICAL MONOGRAPHS [186 



SUMMARY or SALT FORK CONDITIONS 



It has been shown in the previous pages that the sewage and other 

 wastes that drain into the Salt Fork from the Twin Cities have driven out 

 or killed all clean water life from the junction of the Boneyard with Salt 

 Fork to a point about four miles below St. Joseph, or fourteen miles below 

 Urbana. At this point a few living mussels are found and also a few cray- 

 fish. One must pass down the stream for a distance of twenty miles before 

 encountering a normal river fauna, comparable to that found in Spoon 

 River at a point less than a mile above the junction of that stream with 

 Salt Fork. The abundance of clean-water life in Spoon River is in marked 

 contrast with the total absence of this kind of life in Salt Fork, which 

 normally would have, in suitable habitats, a similar fauna in the barren 

 stretch of ten miles between the two localities compared. No better 

 example is known of the total annihilation of a fauna from so great a dis- 

 tance as the result of polluted conditions. 



Foul water algae and Protozoa, as well as some other animal life (slime 

 worms) characteristic of polluted water, are abundant in that portion of 

 the stream devoid of clean water life. The same relative conditions were 

 observed by Forbes and Richardson in their study of the Illinois River. 



Fish, especially young fish, have been made an index to the degree of 

 pollution of streams. It would seem from observations made during the 

 course of the present study, as well as from other occasions and in other 

 places, that bottom-inhabiting animals, such as river mussels and cray- 

 fish, provide a better index for this purpose. Fish are able to migrate 

 easily and swiftly from an unfavorable to a more favorable environment, 

 but these more sedentary animals, especially the mussels, cannot change 

 their eiivironment so easily and must either adapt themselves to the more 

 unfavorable conditions or perish. For example, young bullheads were 

 observed in Salt Fork about three miles above St. Joseph in the spring 

 when the water was comparatively high. But no mussels or crayfish 

 have been seen within five miles of this point. This indicates clearly 

 that fish are more flexible in this matter than the mussels and crayfish, 

 which are not as mobile. Ortmann (1909:93-94) believes that crayfish are 

 slightly more resistant than mussels to polluted conditions, and as scaven- 

 gers (they have been observed eating dead mussels) they could naturally 

 withstand a limited degree of unfavorable environment. Observations 

 made on the Salt Fork, however, indicate that the two groups appear at 

 about the same time. 



Forbes and Richardson (1913:498) distinguish three stages of impurity 

 of streams, which may apply equally well to either the stream itself or to 

 the organisms living in the stream. These terms are "given in the order 

 of diminishing impurity, namely, (1) septic or saprobic, (2) polluted or 



