488 



Drainage Canal, and to the increase in the volume of the stream and 

 in the extent and continuance of its overflow consequent upon that 

 event. 



Something of the magnitude of this increase may be inferred from 

 a comparison of the river-gage readings at Havana or below Cop- 

 peras Creek dam for two ten-year periods preceding the opening of 

 the canal (on January 17, 1900) and the ten years immediately fol- 

 lowing. The period from 1880- 1889 inclusive was characterized by 

 relatively high water, with an average of 7.4 feet above the low- 

 water base-line of 1879; the second, from 1890-1899, was a low- 

 water period, with an average gage reading of 6.32 feet; and the 

 third or last period of ten years, from 1900-1909, was a maximum 

 period, with an average of 9.72 feet. Taking these last two periods 

 into comparison, separated, as they are, by the opening of the sani- 

 tary canal, we find a difl:'erence of 3.4 feet of average level, attrib- 

 utable in great part to the access of canal waters. There was, how- 

 ever, an apparent difference between these two periods in the rainfall 

 of the upper Illinois basin amounting to 1.9 inches, which may be 

 held to account for a part of the average high water of the later one. 



The River Plankton 



Our Havana plankton collections were made in series, at suffi- 

 ciently frequent intervals between August 30, 1909, and August 29, 

 1 910, from three localities — the Illinois River, Thompson's Lake, 

 and Quiver Lake. Forty collections were made in this period from 

 the river, forty from Quiver Lake, and thirty-nine from Thompson's, 

 distributed as follows by months: 1909 — August, i collection; Sep- 

 tember, 4 collections; October, 4; November, 5; December, 2. 1910 

 — January, i; February, i; March, 2; April, 4; May, 4; June, 3; 

 July, 5 ; August, 4. In December only one collection was made from 

 Thompson's Lake. 



The average plankton product of the Illinois River for each 

 month of this year (August, 1909, to August, 1910), stated in cubic 

 centimeters of plankton to one cubic meter of water,* is shown in 

 the following table, together with corresponding monthly averages 

 for the three years from September, 1895, to August, 1898, inclusive. 



*Equivalent, of course, to parts per million ^3; volume. 



