449 



months of flood disturbance falls 79 per cent, below the mean 

 for that season, we find in the 10 months of depression in 

 Thompson's Lake a falling off of only 46 per cent. The differ- 

 ence is due to the greater proportion of creek and spring water 

 of recent origin in the former, and to the greater reservoir ca- 

 pacity and consequent longer impounding, as a rule, of the 

 sewage-laden channel waters which predominate in the latter 

 backwater. 



Production in Phelps Lake is 37.34 cm.^ per m.^ or, if all 

 collections are averaged, 36.31 — 67 or 84 per cent, above the 

 means for all years. In keeping with these facts we find that 

 the monthly averages equal or exceed the means for their 

 months in 7 of the 10 months of record, the greatest excess oc- 

 curring during the period of complete isolation of the lake. 



1899. 



(PI. XIII.. XXIV., XXIX., XXXIX., XLII.) 



This year is represented by 13 collections in the Illinois 

 and 3 in Spoon River, and by 7 each in Quiver and Thompson's 

 lakes and 6 in Phelps Lake, all in the first three months of the 

 year. This was a period of a slow rise of the river in January 

 to bank height, with an equally slow decline in the next month 

 followed by an abrupt and well-sustained March flood. The 

 data of comparative production are brought together in the 

 following table. 



PLANKTON PRODUCTION IN l8qq. 



Station 



Illinois River 



Spoon River 



Quiver Lake 



Thompson's Lake 



Phelps Lake j -|- 



Mean of 

 monthly 

 averages 



*The + and — signs for this column refer to the relation of these averages to the 

 mean of all January-March collections; otherwise they are used as heretofore. 



The average production in these winter months in the river 

 is .42 cm.^ per m.^ or, if all collections are averaged, .41 cm.'' 



