ArticLe 1V.—Changes in the Bottom and Shore Fauna of the 
iddle Illinois River and its Connecting Lakes since 1913—I9I5 as a 
Result of the Increase, Southward, of Sewage Pollution. By Rosert E. 
RICHARDSON. 
The following short account of the principal changes that have oc- 
curred both in the composition and abundance of the small bottom-inver- 
tebrate fauna of the Illinois River system between Chillicothe and Brown- 
ing since 1913—1915—these points being respectively 146.5 and 229.5 
‘miles below the mouth of the Chicago River—is based on hauls made in 
July, August, and September, 1920, with the Petersen quantitative bot- 
tom-sampler at seventy-one stations in the river and Peoria Lake between 
Chillicothe and the head of Grand Island, and at twenty-five stations in 
five of the more important bottom-land lakes in the neighborhood of 
Havana. While the composition of the small bottom-fauna was apparent- 
ly normal as far north as Chillicothe in 1915,* the 1920 data herein pre- 
sented disclose conspicuous changes since that time both throughout the 
northernmost and southernmost portions of the section of river system 
studied, these including sweeping reductions in numbers and poundage of 
the still surviving groups of small bottom-animals, total obliteration of 
numerous families and species, and, in the range covered above Havana, 
the intrusion in large numbers of new pollutional or tolerant forms. If, 
as is approximately correct, we assume that Chillicothe marked about the 
upper limit of wholesome conditions in the bottom muds in 1915, then the 
added river mileage that is shown to have been more or less seriously 
damaged since that time figures out at not less than eighty, and represents 
an average southward encroachment of pollution amounting to fully six- 
teen miles per year for each year of the five. : 
Valuations of the small bottom-animals have been made in the same 
manner as for the 1915 material reported upon in the paper just cited. 
Valuable assistance in working up the Chironomidae of the collections 
of this year, as well as in the elucidation of older data for comparison, has 
been given by Dr. C. P. Alexander and Mr. J. R. Malloch. 
*The Small Bottom and Shore Fauna of the Middle and Lower Illinois River 
and its Connecting Lakes, Chillicothe to Grafton: its valuation; its Sources of Food 
Supply; and its Relation to the Fishery. By R. E. Richardson. Bul. Nat. Hist. Survey, 
Wol. 13, Art. XVI. 1921. 
