403 



upper and middle lakes, but in the channel itself, where both the fastest 

 and the average current are considerably greater than anything to be met 

 with in either the upi)cr or middle lake, if Spring Bay Narrows and pos- 

 sibly also a short stretch of channel below Al Fresco Park be excepted. 

 Also, when the lower lake collections of 192;} are sejiarated into hauls 

 from the open lake, or lake projier (U. S. Slips and Main Street cross- 

 sections), and hauls from Peoria Narrows, McKinley Bridge, and P. P. 

 U. Bridge — included with lower lake averages, for better comparison with 

 1922 in some of the tables following — it is found that there were 12 spe- 

 cies of the II group in the swifter water hauls to only one in the hauls 

 made in the wide portion of the lake. This is quite iu line with (he finding 

 of 1922 that about the only cleaner-preference forms that have survived 

 in or re-entered these waters since the mortality of the years shortly pre- 

 ceding 1920, are current-loving forms — as Pleuroceridae among snails; 

 caddis species of the family Hydropsychidae ; and a few sponges and 

 Bryozoa. The comparatively small representation both of group I and 

 group II species in the extra-channel zones of the lower lake proper sug- 

 gests, also, as did the data of 1922, that there continues here some bad 

 effect of wind-blown local pollution. 



Ptobi.\ L.\kk, 1922-1923 

 Increase or Decrease in Number of Collections 



Another method of testing the extent of change since 1922 in Peoria 

 Lake is to count the number of collections out of the total number taken 

 in which the less tolerant Sphaeriidae (species of Musculium other than 

 M. transversiiin; six-cies of Pisidium ; and si)ecies of Sphaeriuni) occurred 

 in 1923 and the ])receding year. While these little snails are known to be 

 more tolerant than the species we have referred to Group II, they are also 

 clearly less tolerant than the Tubificidac, than Mitsculiuiii traiiszrrsuiii, or 

 than most if not all of the Chironomidae and leeches recently taken, and 

 would be expected to spread over a greater area with any rapid improve- 



