482 



has published in the meantime more than twenty-five hundred pages 

 of contributions to its biology ; and it is now rounding this work 

 to a close, and bringing its results to bear, in practical ways, upon 

 the economic problems most pressing and important at the present 

 time. The paper here presented is intended as a preliminary sum- 

 mary only of the principal conclusions, scientific and economic, to 

 be drawn from our studies of the last few years, and it is to be fol- 

 lowed presently by a series of special papers on the various divisions 

 of the investigation. 



Acknowledgments 



Our grateful acknowledgments are due to the U. S. Bureau of 

 Fisheries for financial assistance in carrying out a fairly adequate 

 program of field work in 191 1 and 1912; to the Illinois State Fish 

 Commission for opportunities afforded us to obtain the plankton of 

 the Illinois, Mississippi, and Ohio rivers on long steamer trips which 

 we could not otherwise have made; to the Water Survey of the 

 State, the director of which, Dr. Edward Bartow, has taken virtual 

 charge of the chemical work here reported, and has provided, from 

 his staff, experienced analysts who have made all our determinations 

 of gases from the waters and from the river sediments ; and to the 

 directors of the Missouri Botanical Garden, in St. Louis, Dr. Wm. 

 Trelease, succeeded by Dr. George T. Moore, who have placed the 

 library, collections, and laboratory facilities of the garden at our 

 disposal, and the latter of whom has given us also his personal as- 

 sistance in determining the algre of our river collections. 



Objects of the Investigation 



The Illinois River work of the Natural History Survey, pursued 

 at irregular intervals since 1877, became virtually continuous at 

 Havana for five years, from April, 1894, to March, 1899, after which 

 it seemed expedient, first to diminish, and in 1903 to suspend, field 

 operations in order that our more important scientific results might 

 be organized, reported, and published. This end being largely ac- 

 complished by voluminous papers printed in volumes IV, V, VI, 

 and VIII of the Bulletin of the State Laboratory of Natural History, 

 and by the publication also, in 1908, of an elaborate report on the 

 fishes of the state, active field work on the river problems was re- 

 sumed in July, 1909. 



The opening of the Chicago Drainage Canal in 1900 was a revo- 

 lutionary event in the biological history of the river; and as the 



