THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XXXVli 



dred and sixty-five miles (two hundred and five by river) to its 

 union with the Mississippi, twenty-five miles above St. Louis. Its 

 bottom-lands have an average width of 3 . 1 miles, from Utica to the 

 mouth of the river. The immediate banks of the stream are usu- 

 ally higher than the adjacent surfaces, and the same may be said 

 of its tributary streams where they flow through the bottoms of the 

 Illinois. Bayous, lagoons, marshes, and temporary ponds occur 

 along the course of the river, especially in its central portion from 

 Hennepin to Meredosia, all subject to invasion or obliteration by 

 the river in times of flood, but filled, at low water, either from 

 springs or from the general drainage of their basins. Spring-fed 

 lakes are rather common along the eastern side of the river, from 

 Pekin to its mouth, deriving their waters from the rainfall col- 

 lected by the second bottoms, at whose margin they usually lie. 



This large area of marshes, lagoons, and lakes affects the life of 

 the river in many important ways. The flood-plain serves as a 

 storage area for the waters of overflow, greatly delaying the run- 

 off at times of flood. This delay is still further prolonged, in many 

 years, by high water in the Mississippi, which often extends far up 

 the Illinois — in a few instances as much as a hundred miles. As 

 a result of these conditions the average volume of water in the 

 stream throughout the year is greatly increased, and a wider range 

 and breeding ground and a greater food supply are afforded to the 

 fishes of the stream. 



The fall in the Illinois River is but slight — an average of .267 of 

 a foot per mile of its total length. Fifty and seven tenths feet of 

 this fall occur in the first forty-two miles of its course, and from 

 Utica to the mouth of the river the total fall is but 31 feet, or an 

 average of . 137 of a foot to the mile. The effect of this slight fall is 

 seen in the sluggish current of the Illinois, which ranges from 4 of a 

 mile per hour at the lowest water to 1.737 miles when at twelve 

 feet above low-water mark. The usual rate of flow for ordinary- 

 stages varies, however, from 1J to 2\ miles per hour. The differ- 

 ence between low-water and high-water conditions is immense in 

 many ways, especially because of the great expansion of water sur- 

 face resulting from slight changes in level. The annual range in river 

 levels, as recorded at Copperas Creek dam, in the twenty-one years 

 from 1879 to 1899 inclusive, varied from 8 . 9 feet in 1894 to 17 . 7 feet 

 n 1882. It is estimated that the area and volume of the river are 

 not far from a hundred times as great at the highest water as at the 

 lowest, and the conditions of aquatic life are thus enormously 

 affected. The contrasts presented by the Illinois River at high 



