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 Illi- 

 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. 



