Fig. 1. — Illinois Ri\< r and its drainam' basin, indicating 

 thr main features discussed in the text. Approximate limits 

 of the drainage basin arc indicated by the heavy broken lines. 



LAKE 

 MICHIGAN 



CHICAGO 



COVER PHOTO: Natural History Survey crew taking dissolved oxygen readings with a galvanic oxygen analyzer. Photo 

 by George W. Bennett. 



