88 ILLINOIS BIOLOGICAL MONOGRAPHS [186 
SUMMARY OF 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 environment 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 
