435 



by the low levels, the cutting off of breeding backwaters, and 

 the relative occupancy by vegetation, and consequently a ver- 

 nal pulse of unusual dimensions reaches a culmination in its 

 area. 



The June- July pulse of the river, abnormal in its location 

 and relative size and apparently without equivalent elsewhere, 

 is due to the unusual development of a stagnation plankton in 

 the sewage-laden river in a period when rising temperatures 

 hasten the decay of its unusual load of organic matter. 



The causes above enumerated render this the year of great- 

 est fertility, in so far as our records reveal production, in the 

 river and in Thompson's and Flag lakes, the next to the greatest 

 in Dogfish Lake, and the least in Quiver Lake. 



The low levels preclude any extensive impounding of flood 

 waters and, moreover, the period of run-off is of slight extent. 

 The rise in plankton content (PI. IX.) in the river following 

 the April, July, and September floods is suggestive of the effect 

 of impounding, the plankton content (cf. PI. XXVL, XXX., and 

 XXXVL) being generally greater in the discharging backwaters 

 examined than in the recipient channel during these run-offs. 

 At all other seasons in our records for this year there is scant 

 opportunity for enrichment by tributary backwaters, and but 

 little suggestion of it. 



1896. 

 (PI. X., XXII, xxvn., XXXI., xxxiii., xxxvii., xl.) 



This is the most fully represented year in our series in the 

 number of stations examined. There are 76 collections in the 

 Illinois, 9 in Spoon River, and 31, 30, 27, 27, and 29 respectively 

 in Quiver, Dogfish, Flag, Thompson's, and Phelps lakes — a total 

 of 229. It was a year of higher levels, averaging 6.98 ft. above 

 low water, — almost twice the record of the preceding year, — and 

 witnessed a series of recurrent floods approaching or surpassing 

 bank height of the stream. 



The accompanying table gives a summary of the data of 

 production. 



