411 



•greater emphasis on the area embraced under the head of Peoria Lake, 

 which, inclusive of ChilHcothe and the P. P. U. Bridge, amounts to an 

 even twenty miles; and where 181 of the total number of 260 hauls were 

 made. In the three short reaches recognized below Peoria collections 

 ran only 27, 23, and 35, compared with 50, 55, and 59 in the same 

 sections in the period 1913-1915. In both periods there was an essen- 

 tially similar distribution of collections as between channel and shore, 

 or shallower water zones ; with the dift'erence, itself apparently attribut- 

 able to the same influence that destroyed the bottom organisms, that 

 more vegetation was encountered at the shallower stations in the earlier 

 than in the more recent collecting. In making the comparisons many 

 of the more distinctly weed-forms have been thrown out entirely, .though 

 some species of both periods that live a part of the time in the bottom and 

 a part on vegetation are necessarily included. To equalize matters 

 further, some of the 1913-1915 material that had been earlier rather 

 incompletely determined for a mass-valuation calculation, was gone over 

 again in 1923 for additional species, at the expense of several weeks' time. 



The results of the comparisons are shown in the table on page 413, 

 following. Briefly, the figures show around a dozen kinds of formerly 

 common cleaner-preference species of small bottom animals that have 

 disappeared from Peoria Lake in ten years ; a similar number for the 

 twenty odd miles between Peoria and the Copperas Creek Dam ; nearly 

 seventy kinds that have recently failed to appear in the first ten to nine- 

 teen miles above Havana ; and twenty-four or more kinds that are miss- 

 ing in the more than thirty miles between Havana and Beardstown. 

 These numbers include no fresh-water mussels except a very few scat- 

 tering occurrences of very young or dwarf individuals, taken with the 

 mud dipper, small Blake dredge, or Petersen dredge ; and would be very 

 much larger, particularly in Peoria Lake, had it been possible recently 

 to cover the area thoroughly with a commercial mussel bar, for com- 

 parison with Danglade's 1911-1912* lists of nuissels from the Illinois 

 River. 



Despite the inequalities and deficiencies mentioned, the results of the 

 comparisons seem for the most part understandable, as they are con- 

 sistent with each other and with otherwise ascertained facts of the biology 

 and hydrography. First, with regard to the relatively enormous losses 

 of cleaner-preference species in the formerly exceedingly rich reach 

 of river just above Havana: Here is where the bottom fauna formerly 

 had both its greatest variety and its greatest abundance ; favored both 

 by the slacker current above the Havana or Spoon River bar and the 

 rich soft mud bottom resulting therefrom. Here also because of the 

 same factor of more abundant sedimentation was where the more sensi- 

 tive bottom organisms might be expected to be put in the greatest jeopardy 

 as the stability of the suspended matters carried this far down the river 

 became lessened. 



• Report. U. S. Bureau of Fl.xherles. 1913. 



