177] FAUNA OF BIG VERMILION RIVER—BAKER 79 
which coincides with the absence of mollusks and crayfish from this part of 
the stream. The rise of the oxygen content 200 feet upstream in Spoon 
River shows the effect of the cleaner water, and this also coincides with 
the presence of clean water life in this part of Spoon River. The lower 
temperatures were probably responsible for the rise in oxygen content at 
the stations four and eight miles below Urbana. In the summer months, 
during low water and high temperature conditions, the percentage would 
probably be much lower. This difference was noted by Forbes and 
Richardson in their study of the Illinois River. The percentages of dis- 
solved oxygen are relatively higher in these two stations of the Salt Fork, 
four and eight miles from the source of pollution, than in the Illinois River 
at Starved Rock, Hennepin, and Chillicothe, where samples taken in Octo- 
ber showed smaller percentages, though many miles from the chief source 
of pollution at Lockport (Forbes and Richardson, 1913:565). 
The rise and fall in the number of bacteria and the presence of many 
gas-forming organisms in the Salt Fork is well shown in the table. The 
sudden rise from Cunningham Avenue to the point one and a quarter 
miles below the Champaign sewer outlet is very striking and indicates 
in a graphic manner the difference between clean water and that heavily 
polluted by sewage wastes. The high number 200 feet up from the mouth 
of Spoon River indicates that there is some pollution in this stream, al- 
though not far above this point living mollusks (Naiades) were found in 
abundance. 
In Table XI, the relative high amounts of albuminoid nitrogen, as well 
as ammonia nitrogen, supplied by the sewage material, in contrast with the 
small amounts in the purer water of the stream, are strikingly shown. 
All of these agencies are inimical to clean water life, especially fish, mussels, 
and crayfish, which by their relative abundance or absence, indicate in a 
most satisfactory manner the quality of the environment, and its fitness 
for the well-being of its inhabitants. 
The sewage in the Salt Fork has greatly increased in recent years, fol- 
lowing the increase in population, and, as in the case of the sewage from 
Chicago in the Illinois River, the polluted condition is gradually creeping 
down the stream below St. Joseph, changing the pure water inhabitants 
to those that can live under septic conditions. As no collections were made 
during previous years from the stream below St. Joseph it is not now pos- 
sible to make comparisons of the present with previous conditions. Such 
lists as have been available have not indicated precisely the location from 
which they were collected (as in the neighborhood of St.. Joseph, which 
might mean in the Salt Fork or in Spoon River) and they cannot be used 
for this reason. The value of exact local lists is emphasized in studies of this 
kind, showing that strictly technical (or pure science) information is often 
of the greatest value in the study of economic probems. 
