THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS xli 



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 V/i to 2Yi miles 

 per hour. The difference between low-water and high-water con- 

 ditions is immense in many ways, especially because of the great 

 expansion of water surface 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 in 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 pre- 

 sented hy the Illinois River at high water and at low water 

 respectively are graphically set forth by Kofoid in his report on 

 the plankton work of the Natural History Survey, published 

 in Volume VI. of the State Laboratory Bulletin. 



"A trip by boat," he says, "across the submerged bottom- 

 lands from the Quiver shore fon the east bank, 2^ miles above 

 Havana] to the western bluff in the latter part of May would be 

 far more enlightening than any description that might be given. 

 As we leave the sandy shore of Quiver we traverse the clear, 

 cold, and spring-fed water along the eastern bank with its rapidly 

 growing carpet of Ceratophylhmi [horn wort], and in a few rods 

 note the increasing turbidity, rising temperature, and richer plank- 

 ton of the water which has moved down from the more or less 

 open and slightly submerged bottom to the north. As we cross 

 the muddj^ bank of Quiver ridge and enter the main channel of 

 the river we find rougher water, caused by the wind which usually 

 sweeps up or down the stream with considerable force between 

 the bordering forests. The water also appears much more turbid 

 by reason of silt and plankton, and no trace of vegetation is to 

 be seen save occasional masses of floating Ceratophyllum or iso- 

 lated plants of Lemna, Wolffia, or Spirodela [duckweeds]. Huge 

 masses of cattle-yard refuse, veritable floating gardens, may also 

 at times be seen moving down the channel or stranded in some 

 eddy along shore. As we plunge into the willow thicket on the 

 western shore we have to pick our way through the accumulated 

 drift lodged in the shoals or caught by the trunks of the trees or 

 the submerged underbrush. The surface of the water is one mat 

 of logs, brush, sticks, bark, and fragments of floating vegetation. 



