RIVER CURRENTS 245 



water of the river is in motion, nevertheless there is to be found 

 in each cross-section of a river a point of maximum motion. This 

 is generally a short distance below the surface, and in a symmetrical 

 section, near the center. This fast-moving portion of the river is 

 especially designated the "current," and its course in a winding river 

 is always more curving than that of the river itself. As a result, 

 it impinges alternately upon the right and left bank of the river, 

 which points become the centers of maximum erosion. The bank 

 against which the current impinges will be kept vertical by under- 

 mining, and the river at the same time will be deepest at that point. 

 The opposite side is shallow, the bank sloping, and deposition rather 

 than erosion occurs. 



Velocities of River Currents. The velocity of a river depends 

 on a number of factors, first among which may be mentioned the 

 slope of the river bed, and next the volume of water. The width of 

 the channel is also an important factor, this varying from the in- 

 definite width of the sheet floods of Arizona and Mexico (Mc- 

 Gee-45) to the narrow canyons of a youthful topography. The 

 slope may vary from nearly horizontal to vertical ; in the one case, 

 a nearly stagnant stream results ; in the other, the extreme of a 

 waterfall is produced. The most variable of these factors is the 

 volume, and hence the velocity of a given current may change 

 greatly between low and high water. Sudden changes in volume 

 due to sudden precipitation of vast amounts of water, as in. semi- 

 arid regions, may produce a marked change in the slope or 

 width of the channel, and so affect the strength of the current in 

 a more permanent manner. Within the same stream velocities vary 

 according to slope, or width and depth of the channel. Thus the 

 velocity of the Rhine, during medium height of water, has been 

 found to be 3.42 m. per second at the Bingerloch, 0.63 m. at Wert- 

 hausen and 1.5 m. at Mannheim, while high water at Coblenz gave 

 a velocity of 1.88 m. per second. The Vistula (Weichsel) during 

 high water has a velocity of 1.2 to 1.9 m. per second. The Neckar 

 above Mannheim at medium water has a velocity of 0.9 m., but at 

 high water over 3 m. The Danube at Vienna has a velocity of 1.94 

 m. per second during high water, while the maximum velocity of 

 the Mississippi between the mouths of the (3hio and the Arkansas 

 is 1. 9 1 m. and between Bayou la Fourche and the forking is 1.76 m. 

 per second. (Krummel-42 : 575, footnote.) 



Perhaps the best example of local changes due to change in the 

 bed of the river is furnished by Niagara. This river is placid and 

 calm from its head near Lake Erie to within a half mile or more 

 of the falls, the fall being 14 feet in something over 20 miles, and 



