418 



stages above 8 ft., 90 from Jan. 1 to Mar. 30, and 14 and 10 re- 

 spectively in the floods of May-Jnne, and August. Of the 114 

 days there were 28 of levels above 10 ft., when, owing to run-off 

 from Spoon River, a current passes through the lake to the 

 river regardless, as a rule, of rising or falling water. In addi- 

 tion there were 43 days of falling water when a run-off might 

 be expected, making a total of only 71 days in this year in 

 which there was any run-off to channel waters from this lake. 

 The remaining 43 days of levels above 8 ft. w^ere times of sta- 

 tionary (20 ) or rising (23) water, when discharge from Phelps 

 Lake was diminished or cut oft'. 



Of the 71 days of discharge, 61 fall in the winter, in Janua- 

 ry-March, and 5 each in June and August, at times of depres- 

 sion in production (PI. XL.). Nevertheless, the plankton con- 

 tent in Phelps Lake at all of these times greatly exceeds that in 

 channel waters. The ratio of Illinois River and Phelps Lake 

 plankton in January is 1 to 189, in February, 1 to 607, in March, 

 1 to 274, in the June Hood, 1 to 4, and in the August rise, 1 to 7. 

 These latter ratios are somewhat exceeded by those of the av- 

 erage production for the year, 1.16 to 13.17 cm.^ or 1 to 11. 

 During the months of little or no discharge, April-December, 

 production in the lake as shown in monthly averages is 4- to 

 1600-fold greater in Phelps than in the Illinois, the latter figure 

 being reached in November and the other months averaging 

 only 11-fold. 



Thus, this lake contributed to the enrichment of channel 

 plankton for a relatively brief part of the year, and at all times 

 produced a plankton greatly in excess of that in channel wa- 

 ters. The sharp contrast between the poverty of channel wa- 

 ters and the wealth of this lake is due to the impounding func- 

 tion in the latter, and to the repeated flushings by storui w^aters 

 of recent origin in the former. Full time for the normal utili- 

 zation of the resources for growth of the plankton is permitted 

 in the lake but not realized in the constantly replaced river 

 water. 



This is the only year in which collections were made in 



