574 
of monthly averages, being from 1.3 to 17 times as great. At all 
levels, waters from impounding areas in the bottom-lands are 
drawn into the channel, mingled with the plankton-poor contri- 
butions of tributaries, and further enriched by the growth of in- 
digenous channel plankton. The reservoir backwaters are thus 
of great importance both as a source of the channel plankton. 
of the Illinois River and in its maintenance. 
24. The total annual production of plankton in the [lli- 
nois River, on the basis of normal discharge and a plankton 
content at the mouth of the river equal to that of our average 
record at Havana, is 67,750 cubic meters. 
25. Filter-paper catches indicate the presence, on an av- 
erage, of a plankton 3.3 times the volume of that taken by the 
silk net. Leakage through the silk is therefore a matter of 
some volumetric importance. 
26. The annual production of plankton and of the fisher- 
ies of the Illinois River show some correlation in their changes 
from year to year. 
University of California, 
August 23, 1903. 
