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. 
PuLaTe VIII. 
Seasonal distribution of plankton in Illinois River, Station E, in 1894. Volume 
of plankton in cm.’ per m." 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. Hydrogfaph from Jan.1to 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. 
PLATE XII. 
The same for 1808. 
PLATE XIII. 
The same for 1899. 
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. 
PLatTe XVII. 
Western shore of upper end of Quiver Lake, looking northward, showing rich- 
