131 



The Vermilion and Mackinaw rivers present common fea- 

 tures of drainage. They both drain till plains of compact drift 

 and have a comparatively rapid descent. The run-off is rapid, 

 and floods are sudden and of extreme proportions. Owing to 

 the absence of head-water marshes the flow in the period of low 

 water is very slight. The steep slope, the rapid run-off, and the 

 cultivation of practically the whole of their drainage basins 

 render the amount of sediment carried by their flood waters 

 very large. 



The minor streams, such as Bureau, Clear, Copperas, and 

 Quiver creeks, and the rivulets which course down the bluffs 

 from the adjacent uplands, differ from the streams last described 

 only in the greater steepness of their slopes. This, added to 

 their proximity to the main stream, makes their run-oft' very 

 rapid. The clearing away of the forests and the cultivation of 

 the hillsides also add to the debris which they carry. 



The varying contributions of these tributary streams com- 

 bine to produce the fluctuations manifested in the main stream 

 at the point where the plankton work of the Biological Station 

 has been done. There are also other factors influencing the 

 stage of the river at this point, notably the fluctuations of the 

 larger tributaries below, as Spoon River and the Sangamon. 

 Owing to the slight fall from Havana to the mouth of the river 

 — only 20.4 feet* — the stage of the Mississippi River may mate- 

 rially affect the gage-reading at Havana. High water in this 

 latter streams prolongs the floods in the Illinois, or even turns 

 the current up stream, as in 1844, when, as I am informed 

 by Hon. J. M. Ruggles, of Havana, the up-stream current came 

 within five miles of that place. 



The \7)lu)ne Discharged. — Streams in fertile regions of the 

 north-temperate zone usually discharge but little more than 

 one cubic foot per second per square mile of watershed. Thus 

 the mean discharge of the Great Lakes is about 1| cubic feet 



*Based on Cooley's figures in " Lake and Gulf Waterway," Appendix I. Rolfe's 

 survey makes it 2g feet (RoHe, '94, p. 133), though other data in his possession make 

 it 26.24 feet. 



