410 



2.19 cm.^ for channel waters. The relative fertility of each is 

 perhaps better expressed by the average of the monthly aver- 

 ages, 8.26 and 2.71 respectively. The run-off of the impounded 

 lake waters would thus tend to enrich the plankton content of 

 the channel in some ratio dependent upon the relative vol- 

 umes and plankton contents of the mingling waters. We have 

 also seen that the enriching function of the contributions of this 

 lake is continuous throughout a large part of the year, with a 

 few interruptions dependent upon cessation of run-off in rising 

 levels in low-water periods, and, rarely, to a lower plankton 

 content in lake waters, due generally to increase of plankton 

 in channel waters as the current slackens in low river stages. 



The following comparison of the averages of the monthly 

 averages for the years of our operations, taken from the table 

 between pages 342 and 343, is instructive in indicating the vary- 

 ing relation of production in lake and channel waters. 



COMPARISON OF MONTHLY PRODUCTION IN THOMPSON'S LAKE AND ILLINOIS RIVER. 



The average height of the river for the years represented 

 in the several monthly grand averages is also given in the above 

 table, and exhibits some relations to the relative plankton pro- 

 duction in Thompson's Lake and the Illinois River, which, 

 however, are so combined with other factors — such as seasonal 

 temperature changes, the period of dominance of vegetation, 

 and qualitative seasonal changes in the plankton itself — as not 

 to be readily analyzed. We find in January-May a period of 

 high levels and low temperatures, of flood invasion everywhere — 

 but most potent in channel waters, a period in which production 

 in the lake is from 8- to 18-fold that in the river and averages 8.4, 



