446 



and the plankton accordingly follows similar lines of develop- 

 ment. This is noticeably prominent in the planktographs of 

 the river and Quiver and Thompson's lakes, as will be seen by 

 a comparison of Plates XII., XXIX., and XXXIX. The princi- 

 pal features of the common course of production are the coinci- 

 dence of the May and June pulses, the subsequent low level of 

 development throughout the summer and early autumn, and 

 the December rise. These three bodies of water were submerged 

 in the common flood of overflow in February-June, and the 

 succeeding minor flushes of summer and autumn caused re- 

 current ingress and egress of water from and to the channel. 

 The similarity in the course of production in these three local- 

 ities and the lessened differences in the amplitude of produc- 

 tion in this year are in no small measure the consequence of 

 this equalizing action of this interchange due to floods. 



Phelps Lake (PI. XLII. ) is the only one of our backwaters 

 which diverges from this marked agreement, and its divergen- 

 ces are increased by its intimate connection with Spoon River 

 during high levels and its isolation during the remainder of 

 the year. 



The vernal pulse of this year is noticeable for its amplitude, 

 its meteroric appearance and disappearance, and its coinci- 

 dence in different localities. It follows a prolonged period of 

 extreme overflow, and a very gradual and somewhat tardy rise 

 in vernal temperatures. It appears, moreover, at levels of 10- 

 11 ft., just when great stretches of bottom-lands are contribut- 

 ing their last run-off to the channel. The submergence of the 

 bottom-lands did not occur until late in February in this year, 

 so that the period of vernal increase in the plankton was not 

 preceded by a long interval of flood, as in 1896 and 1897, which 

 might carry away in suspension or solution those organic sub- 

 stances in the vast amount of vegetable detritus which covered 

 the bottom-land as a result of the low water of the preceding 

 autumn, and which may have been utilized by the plankton in 

 this extraordinary vernal development as a result of the juxta- 

 position of flood and vernal growing season. 



