Current 85 



Professor Forbes ('00) has shown that the fine silt 

 from the earlier-glaciated and better weathered soils 

 of southern Illinois, has been a probable cause of 

 exclusion of a number of regional fishes from the streams 

 of that portion of the state. 



It is heavier silt that takes the larger share in the 

 building of bars and embankments along the lower 

 reaches of a great stream, in raising natural levees to 

 hold impounded backwaters, and in blocking cut-off 

 channels to make lakes of them. 



Current — Rate of streamflow being determined 

 largely by the gradient of the channel, is one of the 

 more constant features of rivers, but even this is sub- 

 ject to considerable fluctuation according to volume. 

 Kofoid states that water in the Illinois River travels 

 from Utica to the mouth (227 miles) in five days at 

 flood, but requires twenty -three days for the journey 

 at lowest water. The increase in speed and in turbu- 

 lence in flood time appears to have a deleterious effect 

 upon some of the population, many dead or moribund 

 individuals of free swimming entomostraca being 

 present in the waters at such times. 



With the runoff after abundant rainfall a rapid rise 

 and acceleration occurs, to be followed by a much 

 slower decline. The stuffs in the water are diluted; 

 the plancton is scattered. A new load of silt is received 

 from the land; plant growths are destroyed and even 

 contours in the channel are shifted. 



Current is promoted by increasing gradient of stream • 

 bed. It is diminished by obstructions, such as rocks or 

 plant growths, by sharp bends, etc. It is slightly 

 accelerated or retarded by wind according as the direc- 

 tion is up or down stream. Even where a stream 

 appears to be flowing steadily over an even bed between 

 smooth shores, careful measurements reveal slight and 



