86 Types of Aquatic Environment 



inconstant fluctuations. The current is nowhere uni- 

 form from top to bottom or from bank to bank. In the 

 horizontal plane it is swiftest in midstream and is 

 retarded by the banks. In a vertical plane, it is swift- 

 est just beneath the surface and is retarded more and 

 more toward the bottom. The pull of the surface film 



retards it a little and 

 when ice forms on the 

 surface, friction against 

 the ice retards it far more 

 and throws the point of 

 maximum velocity down 

 near middepth of the 

 stream. A sample meas- 

 urement made by Mr. 

 Wilbert A. Clemens in 

 Cascadilla Creek at 



Current and Depth in Cascadilla Ithaca in Open Water 

 Creek. Measured by W. A. Clemens, seventeen incheS deep 



gave rate of flow varying from a maximum of 3 .9 1 feet per 

 second two inches below the surface down to i . 7 3 feet per 

 second one inch above the bottom, as shown in the col- 

 umns above. Below this, in the last inch of depth the 

 retardation was more rapid, but irregular. The current 

 slackens more slowly toward the surface and toward 

 the side margins of the stream. 



Mr. Clemens, using a small Pitot-tube current meter, 

 made other measurements showing that in the places 

 where dwell the majority of the inhabitants of swift 

 streams there is much less current than one might ex- 

 pect. In the shelter of stones and other obstructions 

 there is slack water. On sloping bare rock bottoms 

 under a swiftly gliding stream the current is often but 

 half that at the surface. On stones exposed to the 

 current a coating of slime and diatomaceous ooze 

 reduces the current 16 to 32 per cent. 



