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 sight. 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-off 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 Volume 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) cubie feet 
*Based on Cooley’s figures in ‘‘ Lake and Gulf Waterway,’ Appendix I. Rolfe’s 
survey makes it 29 feet (Rolfe, ’94, p. 133), though other data in his possession make 
it 26.24 feet. 
