626 



Plate VII. 



Hydrographs of Illinois River, 1879-1899, from records of State Canal Com- 

 missioners published in reports U. S. Army Engineers, taken at lower gage on Cop- 

 peras Creek dam, and from records of U. S. Army Engineers, taken at lower gage on 

 LaGrange dam, 1883-1899. Mean hydrograph at the right based on means of monthly 

 averages. 



Plate VIII. 



Seasonal distribution of plankton in Illinois River, Station E, in 1894. Volume 

 of plankton in cm. 3 per m. 3 of water shown by heavy black ordinants, the diagonal- 

 lined apices of which indicate the estimated proportion which silt forms of the total 

 catch. Thermograph in dotted lines, from records of surface temperatures made at 

 the times of plankton collection. Hydrograph in continuous line, plotted from rec- 

 ords at Copperas Creek. Heavy black areas at top of plate indicate the relative 

 number of cloudy days per month at Havana, the vertical space equaling seven days. 



Plate IX. 



The same for 1895. Hydrograph from Jan. 1 to Aug. 8 is that at Copperas 

 Creek, and thereafter in the main from Havana records. Relative thickness of ice- 

 sheet indicated by black area at bottom of plate, 1 mm. equaling 6 cm. of ice. 



Plate X. 



The same for 1896. Hydrograph entirely from Havana records. 



Plate XI. 



The same for 1897. 

 The same for 1898. 

 The same for 1899. 



Plate XII. 



Plate XIII. 



Plate XIV. 



Spoon River near its mouth, looking toward southwest from first bend in the 

 stream. Plankton station (M) located near trestle. Taken at moderately low water. 



Plate XV. 



Quiver Lake in midsummer, 1894, at low-water levels, looking northward from 

 Station C (see PL II.) toward the mouth of Dogfish Lake. Littoral vegetation in 

 foreground. Driftwood indicating high-water margin. Lake rich in vegetation. 

 Plankton station located in narrow strip of open water in middle of lake. 



Plate XVI. 



Quiver Lake, from same location, in low water of 1897. Only a small amount 

 of marginal vegetation visible. Dogfish Lake also largely free from vegetation. 



Plate XVII. 

 Western shore of upper end of Quiver Lake, looking northward, showing rich- 



