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. 
> 
> 1D) Z ue ct D o 
iS ao | a | = a |é o |& & |v 
ee) E/E) 2/22] oes] sea dles 
3) (e) . 
2) ele Sea ee ee eee 
Illinois River..] .213| .23] .27| 4.59] 6.08] 7.22] 4.23] 3.88] 2.56] 1.70] .88) .71| 2.71 
Thomp. Lake. .|3.79 | 1.27] 2.96]14.49|29.59|10.66) 4.74| 6.19] 5.37|10.64] 6.39) 3.08] 8.26 
Ratio........../1:18 | 0:5 | r:tt] 1:3 | 1:5 [:1.5)1:1.1) 1:1.6) 1:2 | 1:6 | 0:7 | 134 | 133 
Average height 
of river in ft..17.77 | 7.89112.34!10.02! 9.181 6.19! 4.36! 2.26! 3.78! 3.44! 4.26! 4.97 
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 thelake is from 3-to 18-foldthatin the river and averages 8.4, 
