110 



THE EIVEE. 



As previously stated, the course of the Illinois as compared 

 with that of other streams of 'the Mississippi basin is remark- 

 ably direct. The present stream has not as yet developed in its 

 bottom-lands — the bed of the ancient stream — a meandering 

 course like that of the neighboring Mississippi. The position 

 of its channel in the bottom-lands is often determined largely 

 by the deposits of its tributaries, those made by the streams 

 from the east, as the Mackinaw and the Sangamon, forcing the 

 river towards the western bluff, while those from its western 

 confluents, as Spoon River and Crooked Creek, crowd it against 

 the eastern bluff. The width of the river at low water gradu- 

 ally increases from 536 feet at Peru to 1,040 feet at its entrance 

 to the Mississippi. The expanse in Peoria Lake is over a mile 

 in width, and further down,' in Havana Lake, it is about 2,500 

 feet. 



The undeveloped condition of the flood-plain of the Illinois 

 River and the consequent large areas of the marshes, lagoons, 

 and lakes, affect the plankton of the river most fundamentally. 

 In the first place the flood-plain serves, to an unusual extent, 

 as an impounding area in which the flood-waters are stored, the 

 barriers, natural and artificial, in the bottom-lands combining 

 with the low gradient to delay the run-off. This delay is still 

 further prolonged in most years by high water in the Missis- 

 sippi caused by the run-off from districts of more northerly lat- 

 itudes and higher altitudes and thus occurring after the spring 

 run-off in the basin of the Illinois. In a few instances the 

 backwaters from the floods of the Mississippi have been known 

 to flow up the Illinois for a distance of one hundred miles, 

 and the slope of the stream is such that the impounding 

 effect might, under suitable conditions, extend even farther. 

 The result of this combination of factors is to increase to a 

 marked degree the volume of water, and to add greatly to the 

 diversity of the environment at the time of the maximum de- 

 velopment of spring plankton. It thus profoundly affects both 

 the total product of plankton and its diversification. 



