535 



bring the oxygen ratios down, doubtless fouling the stream by flush- 

 ing out sewers, scouring out tributary streams, stirring up the bottom 

 sediments, and washing off organic debris from the surface of the 

 country. The oxygen mean for x\ugust 3 and 4 was 4.21, and that 

 for August 17 was 2.35, the corresponding carbon dioxide ratios 

 being 5.8 for the first dates and 6.8 for the last. 



November 8, 191 1, the oxygen stood, at Chillicothe, at 10.15 

 parts per inillion, equal to 84 per cent, of saturation. Our lowest 

 reading at this place came in July, 191 2, on the 12th of which month 

 there were but 2.05 parts per million of oxygen at Chillicothe — less 

 than 24 per cent, of saturation. In the cooler weather of the fol- 

 lowing October and early November, with moderately high water, 

 the ratios rose to an average of 7.5 parts per million (64 per cent, 

 of saturation). 



Between Hennepin and Chillicothe the difference in dissolved 

 gases was but slight, and the plants and animals were virtually those 

 of the normal population at both places and on all our visits. Our 

 systematic collections here were made November 7, 191 1, Septem- 

 ber 18 and 19, 191 2, and October 22-25 ^^ ^^e same year. As the 

 collections of the second year were much more detailed and exten- 

 sive than those of the first, no especial account of the latter need 

 be given. 



September 18 and 19, the mussel-bar and dredges were used at 

 various points below Henry and above Chillicothe; and October 22- 

 25, work was done in the vicinity of Chillicothe only, with dip-nets, 

 dredges, and small seines, no large seines, set-nets, or dynamite be- 

 ing brought into use at these lower stations. The river gage above 

 the dam at Henry stood at 3.4 to 3.6. 



Four hundred and ten collections were made from this section, 

 of which 159 contained mollusks, 91 contained adult insects and in- 

 sect lar\^se, 42 contained Crustacea, and 18, fishes. Blue-green algje 

 were found in 10 of the Chillicothe collections — chiefly Oscillatoria 

 limosa and splendid a — usually, however, on boards and logs afloat 

 along the edge of the stream. Cladophora crispata and glomerata 

 were the most abundant filamentous green algae, Stigeoclonium tenue 

 and Spirogyra decimina var. triplicata coming next. Duckweed 

 (Lenina and Wolffia), hornwort (Ceratophylliun), and Blodea were 

 abundant higher plants. Sponges were found, as usual, on dead and 

 living mussel shells. Oligochaete worms were rare in the bottom 

 dredgings, as compared with those made above ; planarians were 

 occasional, and leeches occurred in fifteen collections. Of the crusta- 

 ceans taken, the little "side shrimp" {Hyalella kuickerbockcri) was 



