519 



negligible in amount or due to local conditions. In November, 191 1, 

 however, after a flooding rain which brought the river up some three 

 feet, midstream water above the dam contained thirty per cent, more 

 oxygen than north-shore water below the dam — a relation the re- 

 verse of that found at this point at any other time. This can only 

 be understood as due to the fact, already commented upon under 

 Morris, that, at this stage of the river, Des Plaines and Kankakee 

 waters were still imperfectly mixed even at Marseilles. The mid- 

 stream samples, taken nearer the south shore than the north, were 

 evidently Kankakee water, and the north-shore water, on the other 

 hand, was still essentially that of the Des Plaines. 



In July and August, 191 1, when the midstream microplankton of 

 the river was collected at several stations between Dresden Heights 

 and Chillicothe, specimens of septic species were nearly all Sphcu- 

 rotiliis natans. In the first collections, made during the last days of 

 July, this species was the most abundant at Dresden Heights, con- 

 siderably less so at Morris, and almost insignificant in number at 

 Marseilles, the actual figures of individual specimens per cubic centi- 

 meter of water for those three points being 186, 117, and 9 respec- 

 tively. Counts of Carchesiuin and Bpistylis, also saprobic species, 

 were likewise much below those at Morris. A like difference in the 

 yields of septic species between Morris and Marseilles was found in 

 the collections of August 11 and T2 and August 23 and 24, when 

 the average numbers per cubic centimeter were 62 and 12 for these 

 two points respectively. This reduction in numbers down stream 

 is probably to be understood, however, as mainly due to a mere 

 settling out of particles carrying these organisms, and not to a change 

 in the character of the water. 



We have no exactly comparable chemical data for July; but 

 analyses for August show oxygen ratios of 20.4 parts per million 

 at Morris on the nth and 11 parts at Marseilles on the 12th, and 

 of 16.35 parts per million at Morris on the 22d and 23d and 7.4 

 parts per million at Marseilles on the 24th and 25th. The carbon 

 dioxide ratios, on the other hand, were much larger at Marseilles 

 than at Morris on these dates. Active decomposition of organic mat- 

 ter was thus clearly evident in this midsummer weather, at the low 

 stage of water then prevailing. With higher water and cooler weather 

 the dififerences between Morris and Marseilles were greatly dimin- 

 ished, the percentages of saturation February 16 to 20 standing at 

 48.76 for Morris and at 43.70 for Marseilles. In the fall of the 

 following year these ratios were, in fact, reversed, the Marseilles 

 determinations being 5 per cent, higher than at Morris September 

 27 and 9 per cent, higher November 14. 



