140 
the river continues to rise and it does not greatly expand the overflow 
when at its highest. It may thus materially increase the length of the 
overflow period without adding greatly to the area finally covered. All 
these effects of the canal affluent diminish, of course, down-stream, as 
the normal flow of the river is increased by the addition of tributary 
waters; and at the mouth of the river, in times of flood, less than 6 per 
cent. of the present flow of the stream comes from the sanitary canal. 
The biological importance of this extension of overflow conditions will 
be more clearly seen when we discuss the sources of supply of the river 
plankton. 
We have shown, in an earlier paper, that the average depth of the 
Illinois River at Havana, 173 miles below the mouth of the sanitary 
canal, was about three feet greater during the 10-year period following 
upon the completion of the canal than during the 10-year period imme- 
diately preceding ; and Alvord and Burdick, comparing the gage readings 
for fourteen years after and ten years before the opening of the canal, 
make this difference five feet for Peoria and three feet for Grafton, at 
the river’s mouth.* 
Not quite all this increase in depth can be fairly attributed to canal 
water, for the rainfall was somewhat less in the drainage basin of the 
Illinois during the earlier of these two periods than during the later. The 
average for the part of the state north of the mouth of the Illinois River 
was 34.09 inches per annum for the ten years from 1890 to 1899, and 
35.92 inches for the period from 1900 to 1909. The rainfall for the 10- 
year period just preceding the opening of the canal was 1.83 inches per 
annum less than in the next ten years. 
If, however, we compare Havana levels from 1879 to 1899 inclusive, 
with those of the ten years next following, we find the average for the 
above twenty-one years to be 6.9 feet, and that of the ten years from 1900 
to 1909 to be 9.7 feet—an increase at that point of 2.8 feet fairly attribu- 
table to the canal. : 
2. This greater volume of water throughout the year has also pro- 
duced a greater expanse and depth of the bottom-land lakes, which com- 
monly stand at about the level of the river itself. This fact is well illus- 
trated by the change in Thompson Lake, near Havana—one of the largest 
lakes of the Illinois River series—as shown by two published tables of 
lake levels and corresponding differences of area, for the summer months 
(June to September) of thirty-four years, from 1874 to 1907 inclusive.t 
From the first of those tables we learn that the average level of the lake 
for twenty-six summers preceding 1900 was 429.46 feet above the sea, 
and that for the eight years next following this average was 433.06 feet § 
* Alvord and Burdick’s Report, p, 28. 
+ The data for this comparison were obtained from an article on the ‘Climate cf 
Tllinois,’’ by Prof. J. G. Mosier, published as Bulletin No. 208 of the University of 
Illinois Agricultural Experiment Station. 
t Hepart of the Submerged and Shore Lands Legislative Investigating Committee, 
Vol. I., p. 
¢ Phese levels are equivalent to 5.09 ft. and 8.69 ft., respectively, on the Havana 
gage. 
