418 



Thompson's Lake and presumably other bottom-land wa- 

 ters of similar character, by virtue of their impounding func- 

 tion, are reservoirs in which flood waters are stored for a great- 

 er or less time, permitting the development in general at all 

 seasons of the year of a plankton exceeding in volume from 

 l + -fold to 18- fold that coincidentally developed in channel 

 waters of the adjacent river. Tlie run-off from this and like 

 areas elsewhere thus serves to enrich and maintain the river 

 plankton proper. The slightly developed flood-plain of the Illi- 

 nois and the consequent considerable area of such bottom-land 

 waters — which equalize the floods, prolong the run-off, and favor 

 the production of an abundant plankton in the impounded areas 

 — become, accordingly, factors of great importance in causing 

 the richness, abundance, variety, and long continuance of the 

 unusual plankton production of the Illinois River. 



The similarity in the course of plankton production in 

 Thompson's Lake and elsewhere in our held of operations i^ 

 shown in the following tabular summary, which gives the num- 

 ber of instances of agreement and disagreemeut in the direc- 

 tion of the changes in production in the four localities. 



SIMILARITY IN DIRECTION OF CHANGE IN PRODUCTION I\ THOM 1'SO.n's LAKE 

 AND AT OTHER STATIONS. 



This gives a grand total of 174 instances of similar direc- 

 tion of change in production out of a possible 255, or (58 per cent., 

 for the 5 years included. It is noticeable that the years differ 

 considerably in the degree of agreement detected, the latter 

 years of fuller records exhibiting fullest agreement. They are 



