424 



and averages 7.3. In three instances, March Sand 1, (.01 and 

 .02), March 29 (.20 and .43), and April 26 (10.72 and 15.81) the 

 lake contains less than the river. All of these instances fall at 

 'times of high levels, exceeding 11 ft., when Spoon River floods 

 invade this territory, and this deficiency in Phelps Lake is doubt- 

 less due to their diluent effect. Since our station for collections 

 was located in the upper end of the lake (PI. XL), the full effect 

 of the flood would be detected at this point, but would be di- 

 minished by mingling with the lake waters and the adjacent 

 impounded backwaters before it joined the channel. The first 

 of these exceptions, on March 3, is not accompanied by increased 

 turbidity (.45) in the lake (Table IX.), but the other two, March 

 29 and April 26, are attended by a marked rise in turbidity (.05 

 and .16). During this period of maximum spring flood in March 

 and April, owing doubtless to this diluent action of Spoon River, 

 the run-off from this area, as indicated by plankton content at 

 the Hpper end of the lake, dilutes, or but slightly enriches, the 

 channel plankton. This appears in the monthy averages (table 

 following p. 342), which for March are .33 cm.' for the river and 

 but .25 for the lake. In April they are 4.4 and 5.6 respectively. 

 In later months, during the declines of the spring flood, and 

 owing to absence of the flushing action of Spoon River floods 

 and to the rise in impounding function with decline in levels 

 and delimitation of the lake, we find a rapid rise in the relative 

 plankton content in lake waters. The production in coincident 

 collections is greater in the lake than in the river by 3- to 15- 

 fold, and the monthly averages for lake and river respectively 

 rise to 40.44 cm.'^ per m.' and 11.30 in May; to 27.67 and 3.96 in 

 June; and to 6.97 and .58 in July; that is, the production is 

 from 3 + - to 12- fold greater at this season in the lake than in 

 the river. During the run-off in these months this lake and its 

 contributing adjacent bottom-lands serve to increase, in 

 some unknown ratio dependent on their relative volumes, 

 the plankton content of the channel waters with which they 

 mingle. 



Although the frequency of the plankton collections is in- 



