540 



plankton of confluent streams, for in quantity and components 

 it is much greater and more diversified than that of the tribu- 

 taries. The tributaries add to the iota! resources of the chan- 

 nel, and increase the total population slightly, while diluting 

 some of the food elements and the plankton per m.* These re- 

 sources are utilized in the channel and backwaters, and the di- 

 lution of the plankton is thus made good. Tributary waters, 

 then do not directly contribute to the channel plankton, and 

 only, indirectly aid in its maintenance. They are in the main 

 diluents of channel resources and of its plankton. 



RELATION OF BACKWATERS TO CHANNEL PLANKTON. 



The discussion of the relation of the backwaters to the po- 

 tamoplankton is one fraught with difficulty both on account of 

 the lack of full data and also because of the complexity of 

 those at hand. The quantitative studies which have been 

 made of Quiver, Dogfish, Thompson's, Flag, and Phelps lakes, 

 each representing a particular type of the bottom-land waters, 

 enable me to compare the amount of plankton present in these 

 various regions with that of the main stream. So, also, the 

 cursory examinations made on many of the collections afford a 

 basis for some general statements as to the nature of the con- 

 tributed plankton, though the data on which these statements 

 rest cannot be given here, and their final verification will 

 come, if at all, when the qualitative analyses of the plankton 

 of these several regions shall have been completed. The ab- 

 sence of any accurate data as to the volume of water discharged 

 into the river from the regions in question adds another ele- 

 ment of uncertainty, while the problem is further complicated 

 by the fact that many bottom-land waters receive tributary 

 streams, of minor importance, as a rule, for the river as a whole, 

 and yet of a size sufficient to affect profoundly the waters 

 which they impound. 



The slight development of the flood-plain, the low gradient^ 

 and the retardation of the run-off due to the floods of the Mis- 

 sissippi River, all combine to accentuate the importance of the 



