331 



waters of tributary streams such as Spoon River, the paucity of 

 whose plankton content (.023 cm.^) may be seen in Table IV. and 

 Plate XXIV. It is the result of the relatively greater volume 

 of impounded backwaters into which, by reason of the ease of 

 entrance in overflow stages, the flood waters of tributaries in 

 part make their way. Their diluent action is thus less than at 

 lower levels, and owing to river levels their contributions go 

 into backwaters quite as much as into the channel. Thus, at 

 such levels Spoon River flood water invades the Thompson's 

 Lake region to the north and rushes southward through the 

 bottoms about Phelps Lake (PI. II.). At levels only a trifle be- 

 low that of this flood all of the storm waters are carried with- 

 in the banks of Spoon River and enter the channel directly. 

 Conditions such as these at the mouths of tributaries tend to 

 render floods which occur during overflow stages less destruc- 

 tive to the channel plankton. 



The June pulse has a duration of 35 days,— from May 31 to 

 July 5, — with a maximum amplitude of 6.99 cm.'* per m.^ on 

 June 14. The mean also falls on the 14th, 40 days after that 

 of the preceding pulse. This is a month of falling levels, the 

 "June rise" ( p. 161 ) declining from 13.6 to 9.6 ft. above low water 

 — a total movement of 4 ft. The decline was checked thrice by 

 silt-bearing flood waters of slight proportions, the results of 

 which are seen in the increased silt content of the catches. The 

 chemical conditions are relatively stable, the only marked 

 changes being the usual summer rise in nitrites and chlorine 

 (PI. XLV.). The nitrates fall somewhat, while the other forms 

 of nitrogen remain somewhat uniform, exhibiting no move- 

 ment in correlation with, or proportionate to, that in the plank- 

 ton production. Temperatures exhibit an upward pulse whose 

 apex coincides with that of plankton production. Flood waters 

 may contribute to the depressions which delimit this heatwave. 

 The average production for this month (see table on p. 292) 3.96 

 cm.^ per m.^ is larger than that found in any other year save 

 1895, when abnormally low water prevailed. It exceeds that 



